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Design and bread-board an audio frequency amplifier
with the ability to supply at least 6 watts to an 8ohm load.
To power the amp you will first use the supplies in the lab
with rails at .Design your amp to have a volume control.
The input to your amp will be a CD player.
The load will be the bookshelf audio speakers that will
be provided in the lab.
You want the response to be flat over the entire frequency
range. The use of feedback will help achieve a flat frequency
response.
The amp will be tested and judged according to how good your
design is.
The only active devices you can use are transistors
and op-amps.
You can use power transistors which will be made available
in the lab.
You'll be amazed how good your hi-fi can sound!
- 1.
- Begin with the preamp. Use two op-amps like in the pre-amp stage
of Fig. (3.3).
Set up the first
op-amp as a unity gain follower which will isolate the finite
output impedance of the CD player from the audio amp. Set up the
second op-amp as a voltage amp, with variable gain ranging from
0 to 20. Use a potentiometer in the feedback loop to control the
gain, and standard op-amp range resistor values.
- 2.
- Measure the preamp input and output voltages. Is the desired gain
achieved without distortion?
- 3.
- Connect the preamp output directly to the speaker. What happens
to the maximum wave form? What is the maximum power that the pre-amp
can provide. How loud can it get? (NOTE THAT THE 741 CAN SOURCE ONLY 20mA.
If MORE THAN
20mA ARE ATTEMPTED TO BE DRAWN FROM THE 741, CIRCUIT PROTECTION
AUTOMATICALLY GETS TURNED ON TO KEEP THE OP-AMP FROM BURNING UP.
MOST CIRCUITS DO NOT HAVE SUCH NICE FEATURES, SO CIRCUIT DAMAGE CAN OFTEN OCCUR
WHEN SHORTING OUTPUT TO GROUND OR LOW IMPEDANCES.)
- 4.
- Breadboard the power stage independently. Use the 2N3904 and 2N3906 BJT's
for Q1 and Q2 respectively. For Q3 and Q4 use the power transistors supplied
by your lab instructor. For R1 and R2 use 5K resistors, and for
R3 and R4 use resistors.
For RL use a 10K resistor.
Connect the negative feedback as shown, and for now, ground the input to IC3.
The following questions refer to the power stage.
-
What is the voltage gain of the circuit? Recall, the large open loop gain of the
op-amp combined with feedback will cause the output to adjust itself so
V+=V-.
-
What is the current gain of the circuit. From the manufacturer's specification
sheet, you will see that the of the 3904/3906 is approximately 200,
while that of the power transistor is 100.
-
Check the DC bias conditions. Does Vo=0 for Vin=0?
Are the DC voltage levels symmetrically distributed around zero volts?
Are all the transistors in forward active, and the diodes appropriately on?
-
Apply a signal from your function generator directly to IC3.
Does the measured voltage gain agree with your predicted value?
- 5.
- Connect the two stages together while still using only the 10K load.
Apply a 1kHz 0.5V signal from the
function generator to the circuit. Observe the output while varying
the gain of your amplifier. Vary the input frequency between
100Hz and 15kHz. Make sure your amp is performing to your
satisfaction.
- 6.
- Connect the CD player to the input of your circuit. Compare the output
and input signals. Vary your voltage gain. Is the output an
enlarged replica of the input? What is the maximum output voltage before
clipping occurs?
- 7.
- After you are satisfied with the performance of your amp with the
10K load, turn down the volume and power and
connect the speaker to the output. Gradually turn up the
power and then the volume. Observe and listen to your signal.
Comment on its quality.
- 8.
- Replace the lab power supply with one similar to the one your made in
Lab 1. Now your amp is totally built by you!
How does it work? Congratulations!
- 9.
- Measure the frequency response of your amp.
Turn the volume to a moderate level.
Use the signal
generator as your input and sweep it over the audio range.
Is the gain constant for all frequencies?
- 10.
- What is the maximum power conversion efficiency of your circuit?
Does your theoretical value agree with your measured value?
Next: Tone Controls
Up: Experiment: Building the Hi-Fi
Previous: Experiment: Building the Hi-Fi
Neil Goldsman
10/23/1998