University of North Carolina, Charlotte Department of Electrical and Computer Engineering ECGR 3157 EE Design II Fall 2009

Transcription

1 University of North Carolina, Charlotte Department of Electrical and Computer Engineering ECGR 3157 EE Design II Fall 2009 Lab 1 Power Amplifier Circuits Issued August 25, 2009 Due: September 11, 2009 In this assignment, you will build some basic amplifier circuits. Many of these amplifiers will make use of a common circuit building block known as the op-amp. We ll also start to use devices known as transistors. You ll learn a lot more about the physics of these devices in your other classes. In this course, you ll be learning how to use them to build a complete system. Exercise 1: Basic op-amp circuit design In this exercise, you re going to analyze, design, and build some basic op-amp circuits. Let s first look at the non-inverting amplifier. Your design specifications for this amplifier are pretty simple: Please answer or do all of the following. Specification Value Voltage Gain ~ 15 R 1 + R 2 At least 50kOhm 1. Design the amplifier to meet the specifications shown above. Start by choosing a value for the total resistance R 1 + R 2. Then, chose a value for R 1 that allows you to meet the gain spec. Finally, chose a value for R 2 that lets you meet the resistance spec. In your report, please include all of your design calculations. Your gain does not need to be exactly 15, but it should be close. Try to use standard resistor values. In other

2 words, don t try to create wacky combinations of series and parallel resistors in order to get an exact gain of Now, let s try to build the amplifier using an LF356 op-amp. The datasheet for this part is available on the course web site. Here is a pinout for the chip: To make this chip work, you ll need to power the chip. In this case, do the following: Apply +12V from the power supply to the V + pin (Pin 7). Apply -12V from the power supply to the V - pin. (Pin 4). These voltages can be provided by the power supply on the bench. Be sure to place a bypass capacitor between Pin 7 and ground and place another between Pin 4 and ground. These capacitors should have a value between 0.01μF and 1μF. We ll discuss the need for these later in the semester. Please note that the LF356 chips are in the lab. We won t be distributing these. Now, assemble the amplifier using the resistor values that you chose in Question 1. The source v S will be the Tektronix function generator at the bench. Apply a sine wave from the function generator. This sine wave should have the following characteristics: Amplitude: 100mV Frequency: 100Hz Type: Sine Wave Sketch the input and output voltage. 3. Now, increase the amplitude of the sine wave that you are applying at the input. At some point, the output signal should become flat-topped. Explain what s happening in terms of what we ve learned in class. 4. Decrease the amplitude back to 100mV. Measure the voltage at the negative terminal of the op-amp. How does this value compare the voltage at the positive terminal?

3 5. Let s think about why we may have set the resistors to be so large. Examine the negative current limit graph on Page 5 of the LF356 data sheet. Assume that the circuit is sitting at 25 degrees C (a pretty reasonable temperature) and that the output voltage is reaching to -10V. If so, how much current can the op-amp supply at its output? How much total resistance can be connected to the op-amp output under these conditions? What happens to the maximum output current if the temperature increases to 125 degrees C? 6. Using the datasheet, determine the maximum value of the voltages that can be used to power the op-amp. 7. Returning to the circuit, reset the amplitude of the input sine wave to 100mV. Slowly increase the frequency from 100Hz to 1MHz. What happens to the output voltage as the frequency increases? What does this result imply about the op-amp? What type of filter do we have (i.e. low pass, high pass, etc)? Now, let s look at a different kind of inverting amplifier. This circuit is very similar to the inverting amplifier that we examined in class. The only difference is the insertion of the capacitor in series with R 1. Please answer or do all of the following: 8. At high frequencies, does the capacitor act like a short-circuit or an open-circuit? Based on your answer, please draw an equivalent schematic that describes the circuit at high frequencies. In this schematic, the capacitor should be replaced with either an opencircuit or a short-circuit. What is the gain of the circuit at high frequencies? 9. Determine an expression for the gain of the high-frequency equivalent circuit that you drew in Part 8. This expression is often referred to as the high-frequency gain of the circuit. 10. Chose components so that the magnitude of the high-frequency gain is approximately 10. Use R 1 = 16kOhm. 11. At low frequencies, does a capacitor act like an open-circuit or a short-circuit? Based on your answer, please draw an equivalent schematic that describes the circuit at low frequencies. In this schematic, the capacitor should be replaced with either an opencircuit or a short-circuit. 12. Determine an expression for the gain of the low-frequency equivalent circuit that you drew in Part 11. This expression is often referred to as the low-frequency gain of the circuit. 13. Using the expression for capacitor impedance 1/(jωC), provide an expression for the overall gain v OUT /v S of the circuit. To approach this problem, first determine the combined impedance of R 1 and C and then use the two Golden Rules of op-amps.

4 14. Your expression for the overall gain should look very similar to the transfer function of a high-pass filter. The cutoff frequency of this filter should be expressed in terms of C and a single resistance. Choose a value for C so that the cutoff frequency is approximately 1000Hz. Try to use standard capacitor values, i.e. 0.1μF, 0.33μF, etc. 15. Construct the circuit using the resistor and capacitor values that you determined in the previous parts of this exercise. Once again, use the LF356 op-amp and power it using +12V at Pin 7 and -12V at Pin4. Remember to use bypass capacitors. 16. Once again, apply a sine wave from the Tektronix function generator as the input v S. This sine wave should have the following characteristics: Amplitude: 100mV Frequency: 10kHz Type: Sine Wave Record the gain of circuit at this frequency and compare to the expected value. 17. Now, slowly decrease the frequency of the sine wave input and try to find the cutoff frequency of the circuit. At the cut-off frequency, the amplitude of the output signal should be approximately 70% of its value at high frequency (i.e. 10kHz). Use this fact to find the cutoff frequency. At that frequency, sketch the input signal and the output signal. What is the cutoff frequency of your circuit? If the measured frequency is different than the expected frequency, why do you think this is? 18. Use your expression from part 13 to show that the value of the overall gain of the circuit at the cutoff frequency is approximately 70% of its high-frequency value. 19. At the cutoff, the input voltage and the output voltage should have a phase shift. Based on your measurements, what appears to be the value of this phase shift? Calculate this using the following formula: ϕ = 360 o Time between a peak of the input and peak of Period the output Note that this calculation will require you to make two measurements. Exercise 2: Basic Transistor Amplifier Design As we discussed in class, most op-amps provide a reasonable voltage gain, but they do not provide serious power gain for devices like antennas and speakers. In this exercise, we ll explore the transistor amplifiers that we ll need to combine with our op-amps in order to achieve serious power gain. First, consider the following circuit, which is known as a Class A amplifier:

5 Please answer or do the following: 1. Construct a class A amplifier using a TIP31 NPN transistor. These transistors were given to you. Place a heat sink on the transistor. Use the transistor datasheet to determine which pin is the collector, which pin is the emitter, and which pin is the base. Choose the value of R L so that the output current is approximately 10mA when the output voltage is 1V. 2. Connect the function generator as the source v S. Set the function generator as described below: Type: DC Measure the output voltage. Increase v S in increments of 0.25V and record the output voltage at each setting. Sketch your results on a set of axes such as the following: v OUT v S 3. At a particular value of the input voltage v S the output voltage should stop increasing. Why?

6 4. At what approximate voltage V BE, ON does the transistor turn on? 5. What is the voltage gain of this circuit? Hint: Consider the slope of the v OUT v S curve. 6. When experimenting, you should notice that the DC voltage value on the function generator display screen is approximately ½ of the actual value that is output. Disconnect the function generator from the circuit and place an approximately 50 Ohm resistor across the function generator output. Measure the voltage across the resistor. How does the value of this voltage compare to the value on the display screen? Provide a schematic of a possible Thevenin equivalent circuit for the function generator. Now, combine your LF356 with the Class A amplifier in order to create the following circuit. Replace the load resistor R L with a 10 Ohm power resistor. Choose a value for R POT so that the power dissipation in R POT is less that 5mW. Connect the positive power terminal of the op-amp to +12V and connect the negative power terminal to -12V. Make sure to place a heat sink on the TIP31 transistor. Make sure to connect the collector of the transistor to +12V. Please answer or do the following: 7. Using one of the two op-amp Golden Rules, determine the relationship between v S and v OUT. 8. Once again, vary the DC value of the input v S and measure the output voltage. In this case, the source v S is provided by the potentiometer. Use your observations to describe how the op-amp has improved the circuit. You don t need to draw any plots, etc. for this problem. 9. How much current is being drawn by the load resistor when the output voltage is 5V? Compare this output current with the maximum output current of the LF356. Use this result and your answer to part 7 in order to explain why the combination of the op-amp and the transistor is useful as a power amplifier. 10. Why does the transistor require a heat sink in this case? Now, consider the following circuit, which is known as a Class B amplifier. This transistor amplifier stage is more efficient than the Class A designs shown previously. It is commonly used in stereo amplifiers and small RF systems, such as cell phones.

7 Please answer or do the following: 11. Construct a class B amplifier using the TIP31 and TIP32 transistors available in the lab. Use the transistor datasheets to determine which pin is the collector, which pin is the emitter, and which pin is the base. Choose the value of R L so that the output current is approximately 8mA when the output voltage is 8V. 12. Connect the function generator as the source v S. Set the function generator as described below: Type: DC Measure the output voltage. Increase the offset in increments of 0.25V and record the output voltage at each setting. Repeat for negative input voltages. Sketch your results on a set of axes such as the following: v OUT v S 13. Using your results from part 11, determine the voltage gain of the Class B amplifier. 14. What is the base-emitter voltage V BE, ON at which the transistor Q 1 turns on? What is the base-emitter voltage V BE, ON at which the transistor Q 2 turns on?

8 15. Now, change the output of the function generator so that it provides a sine wave with the following characteristics: Type: Sine wave Amplitude 8V Frequency: 100Hz Sketch the output voltage and input voltage. Why does the output voltage appear as it does? 16. The class A circuit and class B circuit are two different types of audio power amplifiers. Which one would be preferred if a high quality sine-wave output is needed? Why? Which one would be preferred if an efficient circuit is needed? Why? Exercise 3: Building the complete stereo power amplifier Now, we re going to combine the class B power amplifier from above with the op-amp stages studied previously. The op-amp will provide us with a voltage gain, and the transistor stage will provide us with a current gain. The combination of these elements will allow us to make a stereo amplifier that you will demonstrate at your check-off. First, consider the circuit shown below:

9 In this circuit, the LF356 should be powered to +15V and -15V. R L should be the power resistor that you used with the Class A amplifier in Exercise 2, and the 4.7Ohm resistors are the white, high-power components that you received from the TA. Be sure to place heat sinks on your TIP31 and TIP32 transistors. Please answer or do the following: 1. Construct the circuit shown in the figure. Calculate values for R 1 and R 2 so that the overall gain of the op-amp circuit is approximately 30. Make sure that R 1 is at least 10kOhm. 2. Use the function generator as the input source v S. Setup the function generator to output a sine wave with the following characteristics: Amplitude: 0.2V Type: Sine Frequency: 100Hz Sketch the corresponding input and output voltages. Using the oscilloscope, determine the overall gain of this circuit. An easy way to do this is to compare the peak value of the input signal to the peak value of the output signal. How does the transistor stage affect the overall voltage gain? 3. With the input described in Part 2, what is the peak value of the current that is delivered to the load? How does this compare to the current that can be delivered by the op-amp? Think back to Exercise 1, Part 5. Now, let s modify our previous amplifier to the following: +15V +15V R B1 Q 1 0.1μF 4.7Ω R 1 + v S + 4.7Ω R L v OUT + - R 2 Q 2 R B2-15V -15V 0.1μF

10 The diodes are the 1N4148 diodes that were given to you. Please answer or do the following: 4. The purpose of the diodes in this circuit is to ensure that the transistors are always turned on. In order to do that, you must make sure that the diodes are always turned on. This means that you need to choose values for the resistors labeled R B1 and R B2. Before designing, remember the following: When a DC current is passed through a diode (as is the case here), the diode will turn ON. When the diode is ON, it can be modeled as a voltage source as shown below: To choose values for R B1 and R B2 use the following steps. Assume that both the input voltage v s and the voltage at the output of the op-amp is 0V. Replace each diode with a 0.7V source. Assuming that a negligible amount of current flows into the base of Q 1 and the base of Q 2, select values for R B1 and R B2 so that the current through R B1 and R B2 is between 7mA and 10mA. 5. Construct the complete amplifier using the 1N4148 diodes and the transistors and op-amp that you used previously. Note that the cathode, or negative terminal, of the diode is marked with a black line on its package. 6. Use the function generator as the input source v S. Setup the function generator to output a sine wave with the following characteristics: Amplitude: 0.2V Type: Sine Frequency: 1kHz Using the oscilloscope, determine the overall gain of this circuit. An easy way to do this is to compare the peak value of the input signal to the peak value of the output signal. 7. What is the peak value of the output current? 8. Apply a pure DC input from the function generator and adjust the value from -0.1V to 0.1V by adjusting the Offset value. Record the output voltage at several different input voltages. Sketch your results on a set of axes such as the following: v OUT v S

11 Compare the input-output relationship of this circuit to that of the Class B stage with no diodes. What was the effect of adding the diodes? 9. Now, replace the power resistor with the capacitor and the speaker as shown below. Speakers are available from the TA office. Please return the speaker as soon as you re finished because we have a limited number! Please note the polarity of the capacitor. 10. Test your circuit using the function generator as set below. Amplitude: 0.2V Type: Sine Frequency: 1kHz What happens to the sound as you adjust the amplitude of the function generator output? We will bring an ipod to your check-off in order to test your circuit. You can demonstrate this circuit before check-off, but you must do so during office hours. 11. What is the purpose of the 220μF capacitor in series with the speaker? Hint: Think about the way in which capacitors deal with DC currents. 12. Using your answers to Parts 6 and 7, describe why the combination of the op-amp and the transistor stage make a nice audio power amplifier.