ELEC3404 Electronic Circuit Design. Laboratory Manual

Transcription

1 School of Electrical and Information Engineering The University of Sydney ELEC3404 Electronic Circuit Design Laboratory Manual Semester Rui Hong Chu

2 LABORATORY TIMETABLE (1st SEMESTER, 2011) Week Monday Date Laboratory/Tutorial 1 28/2 2 7/3 Exp 1- Laboratory Introduction (Op-amp) 3 14/3 4 21/3 Exp 2 - BJT Amplifier 5 28/3 6 4/4 Exp 3 - MOSFET Differential Amplifier 7 11/4 8 18/4 Project Power Amplifier 25/4 Easter Break 9 2/5 10 9/5 Project Power Amplifier (cont.) 11 16/ /5 Project Demonstration 13 30/5 The class will be split into Wednesday (10am-1pm) and Friday sessions (2-5pm) and run in the laboraotry EE440, Building J03. Two students are in each experiment group. The laboratory work of this subject has three experiments and one term project. Lab1: Laboratory Introduction Review of op-amp circuits to be familiar with lab equipment in Lab 440; Lab2: Common mode BJT amplifier; Lab3: Differential amplifier and current mirror based on MOSFETs; Term project: Audio power amplifier. Breadboards will be lent to each experiment group in the first lab session and are required to be returned to the technical officer Ms. Kavitha Jeevanandam (R441), before the end of semester. The term project starts in the week 8 and is due in the week 12. Students can get a project kit from Ms. Kavitha Jeevanandam in the lab440 in the week 8. Details of the project are in the end of this manual or go Any inquiries regarding lab matters please contact Dr.Chu on ruihong@ee.usyd.edu.au. ASSESSMENT Laboratory work directly accounts for 30% of the UoS, in which experiments take 15% and the project takes 15%. The marks are allocated among the three experiments as: 3% marks for Exp1 and 6% for the Exp.2 and Exp.3 respectively. Students should reach a satisfactory standard of reporting laboratory work. Read the detailed requirements for log books below. In the end of each lab session, the experiment is assessed on three parts: 1) your own pre-lab work, 2) observations by lab demonstration staff on your participation in group work in the laboratory and 3) your record of the experiment written in your own log book. For the project, students ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 1

3 are required to record details in their individual log books, including design, calculations and simulations, circuit building, troubleshooting of the circuits, comments, solutions, and conclusions. The project is assessed in two parts: your individual log book and your group demonstration. LABORATORY LOGBOOK Students are required to use a proper logbook (bound book). All preparatory and experimental work and project work must be recorded in it. You should describe, in detail, what you are doing so that years later you could testify under cross-examination in a court of law if necessary these are the skill that you need to develop. Instead of writing your own description you may refer to this lab manual, provided you also add details such as component values and interconnections. Generate descriptive headings such as "mid-frequency input resistance" in addition to any section number For all practical work you should relate your measurements to theory, and make suitable comments on the degree of agreement. All work recorded during the laboratory session must be dated at the beginning, and dated and signed at the end by the student. It must then be also initialled by one of the laboratory staff. Your laboratory notebook is to be a chronological record, DO NOT leave blank spaces for filling in with later results or analysis. If you make a mistake cross out the error with a single line so that it may still be read. DO NOT use liquid paper or delete supposed errors in such a manner that they may not be read, as you may later find the material to be useful. PREWORK There are pre-lab works for each experiment. The prework must be completed in the logbook before entering the laboratory. The prework usually consists of some mathematical analysis that is closely related to the experimental work and is intended to prepare you for the lab. The labs are designed so that a student who has done the prework should be able to complete the lab in the allotted time. If you find that you are having difficulties completing labs then it is probably a good idea for you to do all of the theoretical work (in addition to the assigned prework) for the experiment before entering the lab. To ensure that you can complete the experimental tasks within the allocated lab session, you could collect all the parts and components from the Lab440 and build the circuits on breadboards before starting the actual lab. Therefore you have more time on testing in the lab session. SAFETY RULES OF LABORATORY To be responsible for your own safety and keep the laboratory in a good order, you must comply with the rules below. Solid footwear must be worn by all students inside the laboratory. Staff are required by the university to ensure that everyone in the laboratory is wearing solid footwear. Students with bare feet, thongs, sandals, or other forms of open footwear will not be allowed into the laboratory. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 2

4 No smoking, drinking, or eating is permitted in the laboratory (this includes chewing gum and confectionaries). Act sensibly and tidy up benches after you complete your experiements. You should not take equipment from another bench. If something is faulty (or missing) ask a tutor for assistance. There is an emergency stop buttons in the lab. It is to be used in an emergency to cut power to the entire lab. No components and equipment are allowed to take out of the laboratory without the permission of the technical staff. You will be asked to leave the laboratory at any time if you offend the rules above. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 3

5 Experiment 1. Laboratory Introduction - Operational Amplifier AIMS Get familiar with the equipment in the 3 rd year electronic laboratory Lab440, by reviewing simple op-amp circuits. Learn how to use an oscilloscope to measure frequency response and voltage gain for an amplifier. PRE-LAB WORK 1. For an inverting op-amp circuit given below, calculate the voltage gain. Typically with this circuit you would use an input capacitor to set a Low Frequency 3 db point somewhere between Hz. What value of capacitor is needed in this case? 10K 1K - Vout Vin 2. In the Figure below, the open/close loop frequency response for an op-amp is illustrated. For the close-loop frequency response, estimate its high frequency 3 db point and marked on the graph. 3. Apart from the voltage gain and frequency response, another important specification for an op-amp is the slew rate. How does the slew rate affect the performance of an op-amp? Find out how to calculate the slew rate and the value for µa741 in the datasheet [1]. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 4

6 EXPERIMENTAL WORK You are going to use a breadboard to set up the circuits. Fig.1 illustrates a simple breadboard, and the connections of holes and example components [2]. The lines in Fig.1 (b) mean that the holes are internally connected. A thumb rule of setting up a circuit on a breadboard is that, using leads short with all components as flat on the board as possible. Power rail Fig.1 (a) A simple breadboard (b) holes and components connections 1. Build a non-inverting amplifier shown in Fig.2 on a breadboard. Set up V in as 1kHz sinusoidal wave from a function generator and leave the output open-circuit. By increasing V in, measure V out before it gets saturated (waveform of V out starts to clip off at top or bottom). Sketch the characteristics of V out versus V in and work out the voltage gain. Calculate the theoretical voltage gain and compare with the measured one. The specification of 741 [1] says that DC supply voltages should not exceed 15V. What happens in your measurement when the DC supplies are increased to 20V? Examine it in the time domain and x-y mode on the CRO. offset N1 1 8 NC IN- IN 2 3 ua741 7 Vcc 6 OUT Vcc- 4 5 offset N2 Fig.2. Non-inverting input op-amp pin configuration of ua Connect the non-inverting op-amp with a 1k load shown in Fig.2. Measure voltage gain (V out /V in ) versus frequency and sketch the frequency response. To do this, firstly measure the 3dB cut-off frequencies by the means of 7/5 ratio. Then measure the gain at center frequency and the frequency bigger and smaller than the higher 3dB and lower 3dB frequency respectively to quickly plot the frequency response. 3. Build an inverting amplifier shown in Fig.3 on the breadboard. Measure the voltage gain (V out /V in ) versus frequency and sketch the frequency response. Mark out the 3-dB cut-off frequencies on the plot. Calculate the theoretical voltage gain and compare with the experimental one. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 5

7 10K Vin 1K 15V V Vout 1K Fig.3. Inverting input op-amp 4. Build a voltage follower circuit as shown in Fig.4 on the breadboard. Input a square wave of about 1 khz from the function generator. Observe the slope of the transitions at the output and compare it with that of the input square wave. That will give you a "slew rate" in volts/microsecond of the op-amp. The specification of 741 [1] says to expect 0.5 volts/microsecond - what happens in your measurement? Fig.4 Voltage follower or unity-gain buffer Reference: [1] Data sheet of 741, available on: [2] How to use a breadboard, available on: ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 6

8 Experiment 2. BJT Amplifier AIMS Learn to design and construct a BJT common emitter amplifier. Be able to test performances of an amplifier, such as input/output impedance, voltage gain and frequency response, and interpret testing results. PRE-LAB WORK Design a common emitter amplifier shown in the Fig.1 using a BC 547 to have an emitter current of about 1 milliamp. Vcc=20V R1 Rc Cc Cb BC547 BC547 R2 Re Ce 10uF c b e Fig.1 BJT common emitter amplifier Leg configuration of BC547 [1] Design Notes Aim for about 10% of VCC across Re. Then choose an E12 value [2] for Re to set the emitter current of 1mA. The base voltage then needs to be at about 2.7 Volts. Ensure that the current flow through the voltage divider R1 and R2 is at least 10 times the base current (which also flows through resistor R1) and this tends to minimize the possibility that the base current (when varying with AC) will effect the biasing arrangement. Make V CE about the same as the voltage drop across Rc for a largest output swing. Design Tasks Select E12 values for R1, R2, Rc, and Re. Choose values for the input and output DC blocking capacitors Cb and Cc so that they do not affect the circuit performance at 10 khz. Develop the AC equivalent circuit of the amplifier from which you can predict the voltage gain, and input and output resistance. These predictions will be compared with corresponding measurements later on. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 7

9 EXPERIMENTAL WORK Visit the laboratory prior to your scheduled lab session and get hold of all components that you need to set up your designed circuit shown in Fig.1 on the breadboard. Think about your layout (maybe even sketch out some layouts!!), aim to make your leads short with all components as flat on the board as possible. You may place a bypass capacitor across the power rails. Be careful to insert the electrolytic capacitors with the correct orientation or they may get hot and explode. You can do a lot of troubleshooting before the formal lab starts you can even apply a 9 Volt battery (or even two in series which will get you close to your starting design of 20 Volts) and test DC bias conditions. There are three parts in this experiment: DC biasing, small signal performance, and large signal performance of the amplifier. 1. Amplifier Operating Point - DC Biasing Conditions The amplifier only works when the operating point is biased properly. Therefore it is important to check the DC biasing of the amplifier before inputting an AC signal from a function generator. Measure the DC voltage at the base, emitter, and collector (all with respect to earth or zero volts) by the CRO or DMM. Deduce the emitter current and calculate DC voltages at base, emitter and collector, respectively to compare with the calculation in the pre-lab. All of those measurements should be close to your pre-work predictions or else there is something wrong! 2. Small Signal Measurements Choose an input signal level which causes no distortion your output should appear to be a GOOD sine wave and you should be able to spot the onset of distortion of the output voltage with your eye certainly the onset of clipping (flat top or bottom) should be very obvious. You also can check the distortion by using X~Y mode on the CRO. Connect V in and V out with two channels of the CRO and make it displayed in X~Y mode. You should a straight line. If not (i.e. it is curved on either end of the line), you need to reduce the input level to make it linear which indicates no distortion of V out. In this small signal (normally v be <20mVpp) region measure the following items. AC voltage gain at mid-band frequency Set up the input at a small signal level and with the mid-band frequency. You can find the mid-frequency by placing the output voltage in the Y-channel of the CRO and the input voltage on the X-channel with the CRO in X~Y mode. Adjust the frequency of the input signal until the ellipse becomes a straight line (no distortion and no phase shift). If it is a curve then reduce the input amplitude till it is substantially straight. Measure the output and calculate the voltage gain. Compare with the predictions in pre-work. Record the waveforms of the output and input in the same coordinate and comment on the phase relationship. Frequency response Here you need to use/develop common sense to use enough measurement points so that you are able to sketch frequency response which will fall of (drop) at both the low frequency and also the high frequency ends. Three points are crucial to locate the frequency response: mid-band frequency, and lower and higher 3-dB cutoff frequency, which are corresponding to the max gain and the gain with 3dB drop respectively. You can use 5/7 approximation to measure lower and upper 3 db cutoff frequencies: start at mid-band frequency, make the output cover 7 divisions (peak to peak) on CRO ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 8

10 by adjusting the vertical scale; increase or decrease frequency until the output drops to 5 divisions; the frequency are corresponding to the higher or lower 3dB cutoff frequency. The geometric mean of the upper and lower 3dB cutoff frequencies is the mid-frequency. You can also measure the output voltage at the frequency lower and higher than low- 3dB and high-3db, respectively. With these five points you can sketch the frequency response. Z in (at mid-band frequency) It is important to know the input and output impedance of an amplifier when designing a preceding or following stage for it. In order to measure the input impedance, a schematic circuit diagram in Fig.2 is used, where a resistor R is inserted externally between the input of the amplifier and the function generator. The block in dashed line is the amplifier, which is represented by the input impedance Z in. The value of R is to be chosen properly based on the predicted input impedance of the amplifier. You need to measure the voltage on both sides of R (with respect to the ground), that is V1 and V2, and calculate the input impedance Z in with the value of R. Compare the measured value with the predictions in the pre-lab and comment on the discrepancy. Note: To measure the current flowing through the R accurately, you may increase the amplitude of the signal from the function generator depending on the resistance of R. However the voltage at the input of the amplifier must be remained as small signal level, i.e cannot be bigger than 20mVpp. Fig.2. Input impedance measurement of the amplifier Z out (at mid-band frequency) Fig.3 shows the schematic circuit diagram for measuring the output impedance Z out. The block in dashed line refers to the amplifier, represented by a voltage source (opencircuit voltage of the amplifier) in series with the output impedance Z out. Connect the output terminal of the amplifier with a resistor which is typically in the vicinity of the value of the Z out. Measure the open circuit voltage at the output of the amplifier (without connecting the R). Then measure the voltage at the same terminals with the R connected. You can find out the current flowing through the Z out, and further calculate the Z out. Compare the measured value with the predictions in the pre-lab and comment on the discrepancy ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 9

11 Amplifier Zout output Open Circuit Vout R Fig.3 Output impedance measurement of the amplifier 3. Large Signal Measurements A large input signal can cause the amplifier saturated and distorted. In this part of experiment, you will find out the thresholds to cause the distortion. For a sinusoidal input at mid-band frequency, increase the amplitude until the collector voltage just clips in one direction. The voltage from the near clipping peak to the ground is the maximum output without distortion, also called MOL (max. output level). Increasing the input amplitude further will eventually cause clipping in the other direction, giving the other MOL. You also can clearly check the occurrence of the voltage clipping in x~y mode of the CRO, where the straight line starts to curve at one end. Measure these two maximum outputs, that is MOL and MOL-, and sketch the waveforms. What are the corresponding inputs for the MOL and MOL-, respectively? Reference: [1] Datasheet of BC547, available on: [2] E12 series resistor, check related information on: ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 10

12 Experiment 3. Differential Amplifier and Current Mirror AIMS To understand the important specifications of a long tailed differential amplifier, such as differential gain, common mode gain, and common mode rejection ratio (CMRR). PREWORK 1. The circuit below shows a simple MOSFET differential amplifier. Choose values for Rs, Rg, and Rd to provide a leg current of about 1mA. 2. Design notes: As the input impedance of a MOSFET is very large, to provide a DC potential of 0V at the gate, Rg should be more than several mega ohms (MΩ) or even directly grounded. Remember that this is essential to allow DC bias to actually happen! Assume the MOSFET Q1 and Q2 are exactly matching, half of this 'leg' current then flows through each MOSFET so choose an E12 resistor value for R d which sets the drain voltage about half way between 0 volts and 10 volt supply. Find out the threshold voltage V t of 2N7000 from the datasheet [1]. Assume the overdrive voltage V ov is about 1 volt, from the drain current of half of 1mA and 1 W 2 1 W 2 I D kn ( VGS Vt ) kn V, you can work out V ov GS and further R S. Use E12 series 2 L 2 L resistor value for R S to set it close to the calculated one. 3. To improve the common-mode rejection ratio (CMRR), in practice a current mirror is normally utilised instead of R S to provide the leg current. In the current mirror circuit shown below, the current flowing through the resistor R is I ref =1 ma. As the gate current is zero, so the resistance of R can be worked out the drain current of the MOSFET Q3 VDD VSS VGS I D3 Iref [2]. If Q3 and Q4 are exactly matching, I D4 =I D3 =I ref.choose R the closest E12 series resistor for R and verify the current. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 11

13 VDD To source of Q1 and Q2 R Iref ID3 0 ID4 Q3 VGS - Q4 VSS EXPERIMENTAL WORK 1. Classic MOSFET Differential Amplifier Measure DC bias Build a circuit shown in Fig.1 on a breadboard. Power on the circuit by 10V (DC supply set up as in Fig.2) and measure the DC biasing at the gate, source and drain and the leg current to prove that the MOSFET is biased in the saturation region. Compare the measurements with the calculations in pre-lab work. The pin arrangement of 2N7000 is also illustrated in Fig.2. 10V Rd 1uF Vout 1uF 2N7000 Q1 Q2 Vin Rs 1mA -10V Fig.1. The circuit for measuring differential voltage gain Fig.2 DC power supply connection Measure differential voltage gain Apply an AC signal from a function generator (FG) to the gate of Q1. The amplitude of the input should be in the region of "small signal" for the MOSFET (the output is linearly proportional to the input) and the frequency is at the mid-band (check by x~y mode on the CRO). Measure the differential voltage gain A d (=V out /V in ). Record the waveforms for V in and V out on the same graph and measure the phase relationship. Phase relationship between the inverting and non-inverting amplifier To appreciate the inverting and non-inverting input of the amplifier, there is another very useful test you can perform in Fig.1. Apply the FG output to the gate of Q2 while ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 12

14 the gate of Q1 is grounded. Record the waveforms of V in and V out and compare the phase relationships with the measurement above, which should really convince you about the words used for inverting and non-inverting terminals. Measure common mode rejection ratio - CMRR Apply a common mode input signal as shown in Fig.3 To do this connect the negative sides of two 1µF capacitor together and then connect with the FG output. Measure the Common Mode gain A c (=V out /V in ). Calculate the common mode rejection ratio CMRR(= A d /A c ). Note: Due to the common mode gain A c <1, you may increase the input accordingly to make the output measurable by the CRO. 10V Rd 1uF Vout 1uF Q1 2N7000 Q2 1uF Vin Vin Rs 1mA -10V Fig.3. The circuit for measuring common mode voltage gain 2. Differential Amplifier with a Current Mirror Build and measure the current mirror Construct the current mirror designed in pre-work on the breadboard and use it to replace the source resistor Rs in Fig.1. The circuit of the differential amplifier with a current mirror is illustrated completely in Fig.4. Adjust R to get exactly the same DC current as you had before (you can do this by measuring and getting the same voltage drop across the drain resistor Rd). Measure A d and A c and calculate CMRR Following the procedures in the Part 1, measure the differential voltage gain A d and common mode voltage gain A c (the input signal connected in Fig.4 is for measuring A d only) respectively. Calculate the common mode rejection ratio CMRR and compare it with the CMRR from the Part 1. This comparison should convince you why a current mirror is used for a differential amplifier in practice. Note: You may have this all designed and wired in place on your breadboard and DC performance tested at home or prior to the scheduled lab session so that you can make best use of your lab time making useful measurements. ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 13

15 10V Rd 1uF Vout 1uF 2N7000 Q1 Q2 Vin 10V R 1mA Q3 2N7000 Q4-10V Fig.4. Differential amplifier with a current mirror and differential input Reference [1] Datasheet of 2N7000, [2] Sedra/Smith, Microelectronics Circuits, 5th edition, Oxford University Press, ELEC3404 Lab Manual 2011, School of EIE, University of Sydney 14