Download University of North Carolina, Charlotte Department of Electrical and Computer Engineering

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:
Specification
Voltage Gain
R1 + R2
Value
~ 15
At least 50kOhm
Please answer or do all of the following.
1. Design the amplifier to meet the specifications shown above. Start by choosing a value
for the total resistance R1 + R2. Then, chose a value for R1 that allows you to meet the
gain spec. Finally, chose a value for R2 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
words, don’t try to create wacky combinations of series and parallel resistors in
order to get an exact gain of 15.
2. 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 vS 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
Offset: 0V
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?
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 R1. 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 R1 = 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 vOUT/vS of the circuit. To approach this problem, first determine the
combined impedance of R1 and C and then use the two Golden Rules of op-amps.
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 vS.
This sine wave should have the following characteristics:
•
Amplitude: 100mV
•
Frequency: 10kHz
•
Type: Sine Wave
•
Offset: 0V
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 the output
Period
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:
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 RL so that the output current is approximately 10mA when the output voltage is
1V.
2. Connect the function generator as the source vS. Set the function generator as described
below:
•
•
Type: DC
Offset: 0V
Measure the output voltage. Increase vS 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:
vOUT
vS
3. At a particular value of the input voltage vS the output voltage should stop increasing.
Why?
4. At what approximate voltage VBE, ON does the transistor turn on?
5. What is the voltage gain of this circuit? Hint: Consider the slope of the vOUT – vS 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 RL with a 10 Ohm power resistor. Choose a value for RPOT so that the
power dissipation in RPOT 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 vS and
vOUT.
8. Once again, vary the DC value of the input vS and measure the output voltage. In this
case, the source vS 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.
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 RL so that the output current is
approximately 8mA when the output voltage is 8V.
12. Connect the function generator as the source vS. Set the function generator as described
below:
•
•
Type: DC
Offset: 0V
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:
vOUT
vS
13. Using your results from part 11, determine the voltage gain of the Class B amplifier.
14. What is the base-emitter voltage VBE, ON at which the transistor Q1 turns on? What is the
base-emitter voltage VBE, ON at which the transistor Q2 turns on?
15. Now, change the output of the function generator so that it provides a sine wave with the
following characteristics:
•
•
•
•
Type: Sine wave
Offset: 0V
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:
In this circuit, the LF356 should be powered to +15V and -15V. RL 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 R1 and R2 so that the
overall gain of the op-amp circuit is approximately 30. Make sure that R1 is at least
10kOhm.
2. Use the function generator as the input source vS. Setup the function generator to output
a sine wave with the following characteristics:
•
•
•
•
Amplitude: 0.2V
Type: Sine
Frequency: 100Hz
Offset: 0V
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
0.1μF
RB1
Q1
+
R1
vS
4.7Ω
−
+
−
4.7Ω
RL
R2
+
vOUT
-
Q2
RB2
-15V
-15V
0.1μF
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 RB1 and RB2. 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 RB1 and RB2 use the following steps. Assume that both the input
voltage vs 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
Q1 and the base of Q2, select values for RB1 and RB2 so that the current through RB1
and RB2 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 vS. Setup the function generator to output
a sine wave with the following characteristics:
•
•
•
•
Amplitude: 0.2V
Type: Sine
Frequency: 1kHz
Offset: 0V
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:
vOUT
vS
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
Offset: 0V
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.