Basic Audio Amp

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 $\pm15V$.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 $1\Omega$ resistors. For R_L 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 $\beta$ of the 3904/3906 is approximately 200, while that of the power transistor is 100.
• Check the DC bias conditions. Does V_o=0 for V_in=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?