Analog Electronic Circuits Lab-manual

Transcription

1 2014 Analog Electronic Circuits Lab-manual Prof. Dr Tahir Izhar University of Engineering & Technology LAHORE 1/09/2014

2 Contents Experiment-1:...4 Learning to use the multimeter for checking and indentifying the electronic components...4 Experiment-2:...7 To study the behavior of a BJT as an amplifier....7 Experiment-3:...11 To study the behavior of the change of BJT parameters on DC biasing of BJT amplifier circuits Experiment-4:...14 To study the Bias stabilization of BJT amplifier circuits Experiment-5:...18 To study the amplifier parameters (voltage gain, current gain, input resistance output resistance) of BJT CE, CB and CC amplifiers...18 Experiment-6:...24 To study the frequency response of BJT CE, CB and CC amplifiers Experiment-7:...28 To study the operation of Direct Coupled two stage amplifier Experiment-8:...31 To Study the Behavior of a Bipolar Junction Transistor (BJT) as a Switch Experiment-9:...35 To Study the Behavior of a MOSFET as a Switch Experiment-10:...39 To study the operation and measure the parameters of a transistor Schmitt Trigger Circuit Experiment-11:...42 To study the operation and measure the parameters of a IC Schmitt Trigger Circuit Experiment-12:...45 To study the operation of transistor Multi-Vibrator Circuits Bistable Multivibrator Circuit Bistable Multivibrator Waveform Sequential Switching Bistable Multivibrator Basic Astable Multivibrator Circuit Astable Multivibrators Periodic Time

3 Frequency of Oscillation...52 Astable Multivibrator Waveforms Monostable Multivibrator Circuit Monostable Multivibrator Waveforms Experiment-13:...63 To study the operation of Multi-Vibrator Circuits using ICs

4 Experiment-1: Learning to use the multimeter for checking and indentifying the electronic components. Apparatus: Analog Multimeter, Digital Multimeter, Transistors 3 numbers resistors 3 numbers capacitors 3 numbers, and diodes 3 numbers. Procedure: Resistor Check 1. Set the selector switch of the analog multi-meter at RX100 range. 2. Short the leads and set the zero with zero-adjust knob. 3. Connect the resistor across the lead and observe the movement 4. Adjust the selector switch such that the pointer is near the half scale deflection 5. Note the value and record 6. Now calculate the value of the resistor from the color code and record 7. Repeat procedure steps 3-6 for two other resistors. Capacitor Check 1. Set the selector switch of the analog multi-meter at RX100 range. 2. Short the leads and set the zero with zero-adjust knob. 3. Connect the Capacitor across the leads and observe the movement. 4. Adjust the selector switch such that the pointer jumps near the half scale deflection. 5. Interchange the leads and observe the deflection. 6. Check all the three capacitors of different values and observe the difference of deflection and record your observation

5 Diode Check 1. Set the selector switch of the analog multi-meter at RX100 range. 2. Short the leads and set the zero with zero-adjust knob. 3. Connect the Diode across the lead. 4. Observe the movement. 5. Interchange the leads, and again observe the movement. 6. Repeat the above procedure for other two diodes 7. Identify the diode terminals and Sketch the diode and mark the observed terminals. Transistor Check Set the selector switch of the analog multi-meter at RX100 range. Short the leads and set the zero with zero-adjust knob. Connect two leads of the transistor across the lead. By interchanging the leads find the base and the type of the transistor i.e. PNP or NPN. (Hint: if black lead is connected to the base and the red to the collector or emitter of NPN transistor, the meter will show deflection.) 5. Identify the collector terminal of the transistor by biasing the base from collector and comparing the deflection. (Hint: connect the collector and emitter across the meter and touch your finger across black lead and base in case of NPN transistor and observe deflection. Repeat this after interchanging the leads. In case of larger deflection the black lead is connected to the collector. In case of PNP transistor bias the base with red lead, in case of larger deflection the red lead is connected to the collector.) 6. Repeat the above procedure of tow more transistors. 7. Sketch the transistors and mark the leads as indentified. 5

6 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. (You can attach more sheets if required) Name Regd. No. Instructor Date 6

7 Experiment-2: To study the behavior of a BJT as an amplifier. Apparatus: Laptop computer Bread Board, Oscilloscope, Regulated DC power Supply, Digital Multi-meter, Connecting leads. BJT and resistors Procedure: 1. Run PROTEUS ISIS on your laptop computer and draw the following circuit. Figure-1 BJT Common Emitter Amplifier Circuit 2. Simulate the circuit shown in Figure-1 using PROTEUS ISIS software and observe the DC voltages at collector of BJT. 3. Change RB so that the collector voltage is nearly half of VCC. 4. Connect signal source at the input and oscilloscope at the output. 5. Measure the peak value of the AC input with the help of oscilloscope and record

8 6. Measure the DC voltages at all the nodes with DC volt meter and record. VB VE VC Draw the input, output voltage observed by the oscilloscope in the following chart. 8. Increase the amplitude of the input signal gradually and observe the change in the output signal. 9. Record your observations in the following chart using multi colours. 8

9 10. Repeat procedure from step-3 to step-9 using actual components and real instruments. 9

10 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your Practical results with the hand calculated results and simulated results. (You can attach more sheets if required) Name Regd. No. Instructor Date 10

11 Experiment-3: To study the behavior of the change of BJT parameters on DC biasing of BJT amplifier circuits. Apparatus: Laptop computer, Breadboard, Regulated DC power Supply, Digital Multi-meter, Required components and connecting leads. Procedure: 1. Simulate the circuit shown in Figure-3 using PROTEUS ISIS software. Figure-3: Fixed bias unstable amplifier circuit. 2. Simulate the circuit of Figure-3 and measure the output voltage VCE and record. VCE = Replace transistor 2N3904 with 2N3055 in Figure-3 and simulate the circuit again and measure the output voltage VCE and record. VCE = Notice the big change in voltage. Why? (This is due to unstable Bias) 11

12 5. Connect the circuit of Figure-3 on bread board. 6. Connect the DC source at the supply terminals of the circuit and set the source to 12V. 7. Measure the DC voltage at the output and record. VCE = Heat the transistor with the help of soldering iron and observe the change in output voltage. (considerable change is expected) VCE = Change the transistor and measure output voltage again and record. (considerable change is expected) VCE =

13 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your practical results with the hand calculated results and simulated results. (You can attach more sheets if required) Name Regd. No. Instructor Date 13

14 Experiment-4: To study the Bias stabilization of BJT amplifier circuits. Apparatus: Breadboard, Regulated DC power Supply, Digital Multi-meter, Required components, Connecting leads. Procedure: 1. Connect the circuit of Figure-4 using PROTEUS ISIS software. Figure-4: Potential divider biased amplifier circuit. 1. Simulate the circuit of Figure-4 and measure the output voltage VCE and record. VCE = Replace transistor 2N3904 with 2N3055 in Figure-4 and simulate the circuit again and measure the output voltage VCE and record. VCE =

15 3. Notice the change in voltage. Why? (The change is negligible. This is due to highly stable potential divider Bias) 4. Connect the circuit of Figure-4 on bread board. 5. Connect the DC source at the supply terminals of the circuit and set the source to 12V. 6. Measure the DC voltage at the output and record. VCE = Heat the transistor with the help of soldering iron and observe the change in output voltage. (negligible change is expected) VCE = Change the transistor and measure output voltage again and record. (negligible change is expected) VCE = Simulate the circuit of Figure-5 and measure the output voltage VCE and record. VCE = Figure-5: Voltage feedback biased amplifier circuit. 15

16 10. Change the transistor of Figure-5 and measure the output voltage VCE and record. VCE = Assemble the circuit of Figure-5 on breadboard and measure the output voltage VCE and record. VCE = Change the transistor in the circuit of Figure-5 and again measure the output voltage VCE and record. VCE = Compare feedback bias and potential divider bias. Which circuit is more stable? Give your comparison in the following section. 16

17 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your practical results with the hand calculated results and simulated results. (You can attach more sheets if required) Name: Regd. No: Instructor s Initial: Date: 17

18 Experiment-5: To study the amplifier parameters (voltage gain, current gain, input resistance output resistance) of BJT CE, CB and CC amplifiers. Apparatus: Breadboard, Oscilloscope, Regulated DC power Supply, Required components, Connecting leads. Procedure: Note: this is a long procedure and you may complete it in multiple sessions. Common-Emitter (CE) Amplifier (Simulation) 1. Connect the circuit as shown in Figure-6 using PROTEUS ISIS software. Figure-6: Common Emitter Amplifier Circuit 2. Connect the signal source at Vin and connect Ch-A of the oscilloscope at the Vout terminal and Ch-B of the oscilloscope at Vin terminal. 3. Draw the input, output voltage observed by the oscilloscope in the following chart. 18

19 4. Calculate the voltage gain from the recorded waveforms graphically Change VCC from 12V to 24V and measure the voltage gain. Is there a change? (Answer the question in discussion and conclusion part of the experiment at the end.) 6. Remove capacitor C3 across R3 and measure the voltage gain. Is there a change? (Answer the question in discussion and conclusion part of the experiment at the end.) 7. Add a resistor R5 as shown in the following circuit. 19

20 Figure-7: Common Emitter Amplifier Circuit for input resistance measurement. 8. Measure the gain with and without resistor. 9. Calculate the input resistance of the amplifier using potential divider rule. 10. Remove the Capacitor C3 and calculate the input resistance of the amplifier again using the above mentioned procedure. Note the change in input impedance and discuss it in conclusion part at the end.) 11. Connect a resistor at the output terminals as shown in following Figure and measure the output with and without resistor. Calculate the output resistance of the amplifier using Potential divider rule. 12. Calculate the current gain using the values of gain, input and output resistances. Common-Emitter (CE) Amplifier (Practical on Breadboard) 13. Connect the circuit of Figure-6 on bread board and repeat the above simulation procedure step-1 to step-6 on practical circuits. 14. Assemble the circuit of Figure-8 on bread board. Figure-8: Common Emitter Amplifier Circuit for practical measurement of input resistance. 15. Set VR1 to zero, connect the oscilloscope across the output, and adjust the signal amplitude so that the output is 2 volts peak to peak. Set the frequency of the signal to 1K Hz. 20

21 16. Increase VR1 gradually and make the output equal to 1Volt peak to peak (half of the previous value). At this point VR1 is equal to the input resistance of the amplifier. 17. Turn of the power, disconnect VR1 and measure the value with the help of digital multimeter. This is the input resistance To measure the output resistance, remove VR1 and connect the signal source directly across the input. Set the amplitude so that the output is 2Volts peak-peak. 19. Set VR1 to maximum value and connect is across the output. 20. Gradually decrease VR1 to make the output equal to 1 volt peak to peak (half of the previous value). At this point VR1 is equal to the output resistance of the amplifier. 21. Turn of the power, disconnect VR1 and measure the value with the help of digital multimeter. This is the output resistance Calculate the current gain using the values of gain, input and output resistances. Common-Base (CB) Amplifier 23. The circuit of Figure-9 is Common-base amplifier. Notice that this is same potential divider bias circuit. The input signal is connected at the emitter through C1 and base is bypassed using C3. Figure-9: Common Base (CB) Amplifier Circuit. 24. Simulate the circuit and practically assemble and test the circuit and measure the gain, input resistance and output resistance as you did for CE-amplifier. 21

22 Common-Collector (CC) Amplifier 25. The circuit of Figure-10 is Common-collector amplifier. Figure-10: Common Collector (CC) Amplifier Circuit. 26. Simulate the circuit of Figure 10 and practically assemble and test the circuit and measure the gain, input resistance and output resistance as you did for CE and CB-amplifiers. 22

23 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your practical results with the hand calculated results and simulated results. (You can attach more sheets if required) Name: Regd. No: Instructor s Initial: Date: 23

24 Experiment-6: To study the frequency response of BJT CE, CB and CC amplifiers. Apparatus: Breadboard, Oscilloscope, Regulated DC power Supply, Required components, Connecting leads. Procedure: Common-Emitter (CE) Amplifier 1. Connect the circuit as shown in Figure-11 using PROTEUS ISIS software. Figure-11: Common Emitter amplifier circuit to study the frequency response of the amplifier. 2. Set the frequency of the input sinusoidal signal to a middle range of 10K Hz. 3. Set the output signal amplitude to 4 volts peak to peak by adjusting the amplitude of the input signal source. 4. Now gradually decrease the frequency so that the output amplitude becomes nearly 2.8 volts peak to peak. This is the lower cut off frequency of the amplifier. Lower cut-ff Frequency: 24

25 5. Now increase the frequency gradually until the output amplitude becomes nearly equal to 2.8 volts peak to peak. This frequency is the higher cut-off frequency of the amplifier. Upper cut-ff Frequency: Band Width: 6. Assemble the circuit on breadboard with real components and repeat step-2 to step Measure and record lower cutoff, upper cut off and Bandwidth of the amplifier. Lower cutoff Frequency: Upper cut-ff Frequency: Band Width: 8. Compare the results of step 4, 5 and 7 and discuss it in the conclusion section at the end. Common-Emitter (CB) Amplifier 9. Connect the circuit as shown in Figure-12 using PROTEUS ISIS software. Figure-12: Common Base amplifier circuit to study the frequency response of the amplifier. 10. Repeat procedure from step-2 to step-8 for common base configuration. Common-Emitter (CC) Amplifier 11. Connect the circuit as shown in Figure-13 using PROTEUS ISIS software. 25

26 Figure-13: Common Collector circuit to study the frequency response of the amplifier 12. Repeat procedure from step-2 to step-8 for the circuit of common collector configuration shown in Figure

27 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your practical results with the hand calculated results and simulated results. (You can attach more sheets if required) Name: Regd. No: Instructor s Initial: Date: 27

28 Experiment-7: To study the operation of Direct Coupled two stage amplifier. Apparatus: Breadboard, Oscilloscope, Regulated DC power Supply, Required components, Connecting leads. Procedure: 1. Connect the circuit of two stage as shown in Figure-14 using PROTEUS ISIS software. 2. Connect the DC voltmeter at each node and measure the DC voltages at each node. VB1= VC1= VE1= VC2= VE2= 3. Perform the approximate DC analysis of the above circuit and calculate the node voltages. VB1= VC1= VE1= VC2= VE2= 28

29 4. Connect the signal source at the input and oscilloscope at the outputs of stage-1 and stage Set the signal frequency to 1kHz and amplitude to 10mV. 6. Observe the outputs and measure the voltage gains of stage-1, stage-2 and overall gain. Avo1= Avo2= AvT= 7. Perform the approximate AC analysis of the above circuit and calculate the gains. Avo1= Avo2= AvT= 8. Change the supply voltage from 24V to 12V and oberve the change in gains if any. 9. Now assemble the circuit of Figure-14 on bread board and practically test the circuits and measure DC and AC parameters. VB1= VC1= VE1= VC2= VE2= Avo1= 29 Avo2= AvT=

30 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment. Compare your results with the hand calculated, simulated and practical results. (You can attach more sheets if required) Name: Regd. No: Instructor s Initial: Date: 30

31 Experiment-8: To Study the Behavior of a Bipolar Junction Transistor (BJT) as a Switch. Apparatus: Laptop computer Bread Board, Oscilloscope, Regulated DC power Supply, Digital Multi-meter, Connecting leads. BJT and resistors Procedure: 11. Run PROTEUS ISIS on your laptop computer and draw the following circuit. Figure-1: BJT as a switch. 12. Simulate the circuit shown in Figure-1 using PROTEUS ISIS software and observe the DC voltages at collector of BJT. 13. Connect Digital Clock signal source at the input and oscilloscope at the output. 14. Set the clock frequency to 1kHz. 15. Run the simulation. 16. Draw the input, output voltage observed by the oscilloscope in the following chart. 31

32 17. Increase the frequency of the input signal to 100K Hz. and observe the change in the output signal. 18. Record your observations in the following chart using multi colours. 19. Measure td, tr, ts and tf from the input and output waveforms. 32

33 20. Improve the switch circuit as shown below using C1 and R3 and observe the change in rise and fall times. Figure-2: Improved BJT switch circuit. 21. Repeat procedure from step-3 to step-10 using actual components and real instruments. 33

34 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment Name Regd. No. Instructor Date 34

35 Experiment-9: To Study the Behavior of a MOSFET as a Switch. Apparatus: Laptop computer, Breadboard, Regulated DC power Supply, Digital Multi-meter, Required components and connecting leads. Procedure: 10. Simulate the circuit shown in Figure-3 using PROTEUS ISIS software. Figure-3: Enhancement MOSFET as a switch. 1. Simulate the circuit shown in Figure-3 using PROTEUS ISIS software and observe the DC voltages at the drain terminal of MOSFET with switch on and off. VDS(on)= VDS(off)= 2. Compare your results with BJT switch and give your comments at the end. 3. Connect Digital Clock signal source at the input and oscilloscope at the output as shown in Figure

36 Figure-4: Enhancement MOSFET as a switch with D-clock and Oscilloscope. 5. Set the clock frequency to 1 khz. 6. Run the simulation. 7. Draw the input, output voltage observed by the oscilloscope in the following chart. 8. Increase the frequency of the input signal to 200K Hz. and observe the change in the output signal. 9. Record your observations in the following chart. 36

37 10. Measure td, tr, ts and tf from the input and output waveforms. 11. Repeat procedure from step-3 to step-10 using actual components and real instruments. 12. Compare Practical results with simulation and record your observations. 37

38 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment Name Regd. No. Instructor Date 38

39 Experiment-10: To study the operation and measure the parameters of a transistor Schmitt Trigger Circuit. Apparatus: Breadboard, Regulated DC power Supply, Digital Multi-meter, Required components, Connecting leads. Procedure: 2. Connect the circuit of Figure-5 using PROTEUS ISIS software. Figure-5: Schmitt Trigger circuit using BJTs. 3. Apply Variable DC voltage using a DC source and variable resistance at the input and connect the DC voltmeter at the output. 4. Gradually vary the input voltage and observe the state change in the output. 5. Measure LTP and UTP. LTP= 39 LTP=

40 6. Connect sine-wave source at the input and oscilloscope at the output, Sketch the waveforms and indentify LTP and UTP. 7. Calculate LTP and UTP using the given values. LTP= LTP= 8. Repeat procedure from step-1 to step-5 using actual components and real instruments. 9. Compare Practical results with simulation and record your observations at the end. 10. Change RE from 2.2k to 1k and observe the change in practical results. 40

41 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment Name: Regd. No: Instructor s Initial: Date: 41

42 Experiment-11: To study the operation and measure the parameters of a IC Schmitt Trigger Circuit. Apparatus: Breadboard, Oscilloscope, Regulated DC power Supply, Required components, Connecting leads. Procedure: 27. Connect the circuit as shown in Figure-6 using PROTEUS ISIS software. Figure-6: Op-Amp based Schmitt Trigger Circuit. 1. Connect sine-wave source at the input and oscilloscope at the output, Sketch the waveforms and indentify LTP and UTP. 42

43 2. Calculate LTP and UTP using the given values in the circuit. LTP= LTP= 3. Make the circuit on bread board using actual components and real instruments. 4. Sketch the waveforms and indentify and measure LTP and UTP. 5. Compare Practical results with simulation and record your observations at the end. 6. Change R1 from 100k to 220k and observe the change in practical results and discuss the effect of increasing feedback resistor in your conclusions at the end. 7. Change from 10 k to 4.7K and observe the change in practical results and discuss the effect of decreasing input resistor in your conclusions at the end. 43

44 Conclusions: Write down the summary, general observation and conclusion about the results obtained in this experiment Name: Regd. No: Instructor s Initial: Date: 44

45 Experiment-12: To study the operation of transistor Multi-Vibrator Circuits. Apparatus: Breadboard, Oscilloscope, Regulated DC power Supply, Required components, Connecting leads. Note: this is a long procedure and you may complete it in multiple sessions. The Bi-stable Multi-Vibrator Bistable Multivibrators have TWO stable states (hence the name: "Bi" meaning two) and maintain a given output state indefinitely unless an external trigger is applied. The bistable multivibrator can be switched over from one stable state to the other by the application of an external trigger pulse thus, it requires two external trigger pulses before it returns back to its original state. As bistable multivibrators have two stable states they are more commonly known as Latches and Flip-flops for use in sequential type circuits. The discrete Bistable Multivibrator is a two state non-regenerative device constructed from two cross-coupled transistor switches. In each of the two states, one of the transistors is cut-off while the other transistor is in saturation, this means that the bistable circuit is capable of remaining indefinitely in either stable state. To change the bistable over from one state to the other, the bistable circuit requires a suitable trigger pulse and to go through a full cycle, two triggering pulses, one for each stage are required. Its more common name or term of "flip-flop" relates to the actual operation of the device, as it "flips" into one logic state, remains there and then changes or "flops" back into its first original state. Consider the circuit below. Bistable Multivibrator Circuit 45

46 The Bistable Multivibrator circuit above is stable in both states, either with one transistor "OFF" and the other "ON" or with the first transistor "ON" and the second "OFF". Lets suppose that the switch is in the left position, position "A". The base of transistor TR 1 will be grounded and in its cut-off region producing an output at Q. That would mean that transistor TR2 is "ON" as its base is connected to Vcc through the series combination of resistors R1 and R2. As transistor TR2 is "ON" there will be zero output at Q, the opposite or inverse of Q. If the switch is now move to the right, position "B", transistor TR 2 will switch "OFF" and transistor TR1 will switch "ON" through the combination of resistors R3 and R4 resulting in an output at Q and zero output at Q the reverse of above. Then we can say that one stable state exists when transistor TR1 is "ON" and TR2 is "OFF", switch position "A", and another stable state exists when transistor TR1 is "OFF" and TR2 is "ON", switch position "B". Then unlike the monostable multivibrator whose output is dependent upon the RC time constant of the feedback components used, the bistable multivibrators output is dependent upon the application of two individual trigger pulses, switch position "A" or position "B". So Bistable Multivibrators can produce a very short output pulse or a much longer rectangular shaped output whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon a second trigger pulse as shown below. Bistable Multivibrator Waveform 46

47 Manually switching between the two stable states may produce a bistable multivibrator circuit but is not very practical. One way of toggling between the two states using just one single trigger pulse is shown below. Sequential Switching Bistable Multivibrator Switching between the two states is achieved by applying a single trigger pulse which inturn will cause the "ON" transistor to turn "OFF" and the "OFF" transistor to turn "ON" on the negative half of the trigger pulse. The circuit will switch sequentially by applying a pulse to each base in turn and this is achieved from a single input trigger pulse using a biased diodes 47

48 as a steering circuit. Equally, we could remove the diodes, capacitors and feedback resistors and apply individual negative trigger pulses directly to the transistor bases. Bistable Multivibrators have many applications producing a set-reset, SR flip-flop circuit for use in counting circuits, or as a one-bit memory storage device in a computer. Other applications of bistable flip-flops include frequency dividers because the output pulses have a frequency that are exactly one half ( ƒ/2 ) that of the trigger input pulse frequency due to them changing state from a single input pulse. In other words the circuit produces Frequency Division as it now divides the input frequency by a factor of two (an octave). Procedure: 1. Connect the circuit as shown in Figure-7 using PROTEUS ISIS software. Figure-7: Bi-stable circuit with triggering circuit. 2. Apply digital clock at trigger input and connect the oscilloscope at the output. 3. Observe and record the input and output waveforms. 48

49 4. Assemble the circuit of Figure-7 on breadboard. 5. Connect the function generator at the trigger input and oscilloscope at the output. 6. Observe and record the waveforms. 7. Compare the results obtained from simulation and practical implementation of the Bistable circuit and discuss your observations at the end of the report. 49

50 Astable Multi-Vibrator Regenerative switching circuits such as Astable Multivibrators are the most commonly used type of relaxation oscillator as they produce a constant square wave output waveform as well as their simplicity, reliability and ease of construction. Unlike the Monostable Multivibrator or the Bistable Multivibrator we looked at in the previous tutorials that require an "external" trigger pulse for their operation, the Astable Multivibrator switches continuously between its two unstable states without the need for any external triggering. The Astable Multivibrator is another type of cross-coupled transistor switching circuit that has NO stable output states as it changes from one state to the other all the time. The astable circuit consists of two switching transistors, a cross-coupled feedback network, and two time delay capacitors which allows oscillation between the two states with no external trigger signal to produce the change in state. Astable multivibrators are therefore also known as Free-running Multivibrator as they do not require any additional inputs or external assistance to osillate. Astables produce a continuous square wave from its output or outputs, (two outputs no inputs) which can then be used to flash lights or produce a sound in a loudspeaker. The basic transistor circuit for an Astable Multivibrator produces a square wave output from a pair of grounded emitter cross-coupled transistors. Both transistors either NPN or PNP, in the multivibrator are biased for linear operation and are operated as Common Emitter Amplifiers with 100% positive feedback. This configuration satisfies the condition for oscillation when: ( βa = 1 0o ). This results in one stage conducting "fully-on" (Saturation) while the other is switched "fully-off" (cut-off) giving a very high level of mutual amplification between the two transistors. Conduction is transferred from one stage to the other by the discharging action of a capacitor through a resistor as shown below. Basic Astable Multivibrator Circuit 50

51 Assume that transistor, TR1 has just switched "OFF" and its collector voltage is rising towards Vcc, meanwhile transistor TR2 has just turned "ON". Plate "A" of capacitor C1 is also rising towards the +6 volts supply rail of Vcc as it is connected to the collector of TR 1. The other side of capacitor, C1, plate "B", is connected to the base terminal of transistor TR 2 and is at 0.6v because transistor TR2 is conducting therefore, capacitor C1 has a potential difference of 5.4 volts across it, v, (its high value of charge). The instant that transistor, TR1 switches "ON", plate "A" of the capacitor immediately falls to 0.6 volts. This fall of voltage on plate "A" causes an equal and instantaneous fall in voltage on plate "B" therefore plate "B" of the capacitor C1 is pulled down to -5.4v (a reverse charge) and this negative voltage turns transistor TR 2 hard "OFF". One unstable state. Capacitor C1 now begins to charge in the opposite direction via resistor R3 which is also connected to the +6 volts supply rail, Vcc, thus the case of transistor TR 2 is moving upwards in a positive direction towards Vcc with a time constant equal to the C1 x R3 combination. However, it never reaches the value of Vcc because as soon as it gets to 0.6 volts positive, transistor TR2 turns fully "ON" into saturation starting the whole process over again but now with capacitor C2 taking the base of transistor TR1 to -5.4v while charging up via resistor R2 and entering the second unstable state. This process will repeat itself over and over again as long as the supply voltage is present. The amplitude of the output waveform is approximately the same as the supply voltage, Vcc with the time period of each switching state determined by the time constant of the RC networks connected across the base terminals of the transistors. As the transistors are switching both "ON" and "OFF", the output at either collector will be a square wave with slightly rounded corners because of the current which charges the capacitors. This could be corrected by using more components as we will discuss later. 51

52 If the two time constants produced by C2 x R2 and C1 x R3 in the base circuits are the same, the mark-to-space ratio ( t1/t2 ) will be equal to one-to-one making the output waveform symmetrical in shape. By varying the capacitors, C1, C2 or the resistors, R2, R3 the mark-tospace ratio and therefore the frequency can be altered. We saw in the RC Discharging tutorial that the time taken for the voltage across a capacitor to fall to half the supply voltage, 0.5Vcc is equal to 0.69 time constants of the capacitor and resistor combination. Then taking one side of the astable multivibrator, the length of time that transistor TR2 is "OFF" will be equal to 0.69T or 0.69 times the time constant of C1 x R3. Likewise, the length of time that transistor TR 1 is "OFF" will be equal to 0.69T or 0.69 times the time constant of C2 x R2 and this is defined as. Astable Multivibrators Periodic Time Where, R is in Ω's and C in Farads. By altering the time constant of just one RC network the mark-to-space ratio and frequency of the output waveform can be changed but normally by changing both RC time constants together at the same time, the output frequency will be altered keeping the mark-to-space ratios the same at one-to-one. If the value of the capacitor C1 equals the value of the capacitor, C2, C1 = C2 and also the value of the base resistor R2 equals the value of the base resistor, R3, R2 = R3 then the total length of time of the Multivibrators cycle is given below for a symmetrical output waveform. Frequency of Oscillation Where, R is in Ω's, C is in Farads, T is in seconds and ƒ is in Hertz. and this is known as the "Pulse Repetition Frequency". So Astable Multivibrators can produce TWO very short square wave output waveforms from each transistor or a much longer rectangular shaped output either symmetrical or non-symmetrical depending upon the time constant of the RC network as shown below. 52

53 Astable Multivibrator Waveforms Example: An Astable Multivibrators circuit is required to produce a series of pulses at a frequency of 500Hz with a mark-to-space ratio of 1:5. If R2 = R3 = 100kΩ's, calculate the values of the capacitors, C1 and C2 required. and by rearranging the formula above for the periodic time, the values of the capacitors required to give a mark-to-space ratio of 1:5 are given as: 53

54 The values of 4.83nF and 24.1nF respectively, are calculated values, so we would need to choose the nearest preferred values of C1 and C2 allowing for the tolerance. In fact due to the wide range of tolerances associated with the humble capacitor the actual output frequency may differ by as much as ±20%, (400 to 600Hz in our example). If we require the output waveform to be non-symmetrical for use in timing or gating circuits etc, we can manually calculate the values of R and C for the individual components required as we did in the example above. Procedure: 1. Connect the circuit as shown in Figure-8 using PROTEUS ISIS software. Figure-8: Astable circuit. 54

55 2. Connect the oscilloscope at the output. 3. Observe and record the input and output waveforms. 4. Measure the times t1, t2 and T and compare it with the calculated values using the relations given above in the example. 5. Assemble the circuit using breadboard. 6. Connect the oscilloscope at the output. 7. Observe and record the output waveforms and measure times t 1, t2, T and the frequency. 55

56 8. Compare the results obtained from simulation and practical implementation of the astable circuit and discuss your observations at the end of the report. Mono-stable or One-shot Multi-Vibrator Multivibrators are Sequential regenerative circuits either synchronous or asynchronous that are used extensively in timing applications. Multivibrators produce an output wave shape of a symmetrical or asymmetrical square wave and are the most commonly used of all the square wave generators. Multivibrators belong to a family of oscillators commonly called "Relaxation Oscillators". Generally speaking, discrete multivibrators consist of a two transistor cross coupled switching circuit designed so that one or more of its outputs are fed back as an input to the other transistor with a resistor and capacitor ( RC ) network connected across them to produce the feedback tank circuit. Multivibrators have two different electrical states, an output "HIGH" state and an output "LOW" state giving them either a stable or quasi-stable state depending upon the type of multivibrator. One such type of a two state pulse generator configuration are called Monostable Multivibrators. MOSFET Monostable Monostable Multivibrators have only ONE stable state (hence their name: "Mono"), and produce a single output pulse when it is triggered externally. Monostable multivibrators only return back to their first original and stable state after a period of time determined by the time constant of the RC coupled circuit. Consider the MOSFET circuit on the left. The resistor R and capacitor C form an RC timing circuit. The N-channel enhancement mode MOSFET is switched "ON" due to the voltage across the capacitor with the drain connected LED also "ON". When the switch is closed the capacitor discharges and the gate of the MOSFET is shorted to ground. The MOSFET and 56

57 therefore the LED are both switched "OFF". While the switch is closed the circuit will be "OFF" and in its "unstable state". When the switch is opened, the fully discharged capacitor starts to charge up through the resistor, R at a rate determined by the RC time constant of the resistor-capacitor network. Once the capacitors charging voltage reaches the lower threshold voltage level of the MOSFETs gate, the MOSFET switches "ON" and illuminates the LED returning the circuit back to its stable state. Then the application of the switch causes the circuit to enter its unstable state, while the time constant of the RC network returns it back to its stable state after a preset timing period thereby producing a very simple "one-shot" or Monostable Multivibrator MOSFET circuit. Monostable Multivibrators or "One-Shot Multivibrators" as they are also called, are used to generate a single output pulse of a specified width, either "HIGH" or "LOW" when a suitable external trigger signal or pulse T is applied. This trigger signal initiates a timing cycle which causes the output of the monostable to change its state at the start of the timing cycle and will remain in this second state. The timing cycle of the monostable is determined by the time constant of the timing capacitor, CT and the resistor, RT until it resets or returns itself back to its original (stable) state. The monostable multivibrator will then remain in this original stable state indefinitely until another input pulse or trigger signal is received. Then, Monostable Multivibrators have only ONE stable state and go through a full cycle in response to a single triggering input pulse. Monostable Multivibrator Circuit 57

58 The basic collector-coupled Monostable Multivibrator circuit and its associated waveforms are shown above. When power is firstly applied, the base of transistor TR2 is connected to Vcc via the biasing resistor, RT thereby turning the transistor "fully-on" and into saturation and at the same time turning TR1 "OFF" in the process. This then represents the circuits "Stable State" with zero output. The current flowing into the saturated base terminal of TR2 will therefore be equal to Ib = (Vcc - 0.7)/RT. If a negative trigger pulse is now applied at the input, the fast decaying edge of the pulse will pass straight through capacitor, C1 to the base of transistor, TR1 via the blocking diode turning it "ON". The collector of TR1 which was previously at Vcc drops quickly to below zero volts effectively giving capacitor CT a reverse charge of -0.7v across its plates. This action results in transistor TR2 now having a minus base voltage at point X holding the transistor fully "OFF". This then represents the circuits second state, the "Unstable State" with an output voltage equal to Vcc. Timing capacitor, CT begins to discharge this -0.7v through the timing resistor RT, attempting to charge up to the supply voltage Vcc. This negative voltage at the base of transistor TR2 begins to decrease gradually at a rate determined by the time constant of the R T CT combination. As the base voltage of TR2 increases back up to Vcc, the transistor begins to conduct and doing so turns "OFF" again transistor TR1 which results in the monostable multivibrator automatically returning back to its original stable state awaiting a second negative trigger pulse to restart the process once again. Monostable Multivibrators can produce a very short pulse or a much longer rectangular shaped waveform whose leading edge rises in time with the externally applied trigger pulse and whose trailing edge is dependent upon the RC time constant of the feedback components used. This RC time constant may be varied with time to produce a series of pulses which have a controlled fixed time delay in relation to the original trigger pulse as shown below. Monostable Multivibrator Waveforms 58

59 The time constant of Monostable Multivibrators can be changed by varying the values of the capacitor, CT the resistor, RT or both. Monostable multivibrators are generally used to increase the width of a pulse or to produce a time delay within a circuit as the frequency of the output signal is always the same as that for the trigger pulse input, the only difference is the pulse width. 59