# Common Emitter for audio amplifier

Discussion in 'Homework Help' started by Ryouko, Apr 26, 2009.

1. ### Ryouko Thread Starter New Member

Apr 26, 2009
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Here is the situation, I am taking an electronics course and have to design a audio amplifier. There are a few restrictions on it though, we have to use BJTs(Q2N2222) and we will be supplied +13.8V rails. The amp must output 12W across 8 ohm speakers. We have decided to use a differential amplifier on the input, going into a common emitter, going into a push pull amplifier. We started by modeling them in Pspice, the differential amp is working fine, but now we are stuck on the common emitter. We need a gain of a little under 30, with an input signal of 1V(peak to peak).
Here is a model of it in pspice with some values plugged in, and I need some help with it. I can't seem to get the gain anywhere near 30, at most 2.5, and it doesn't seem to be following the equations we went over in class. I would like to change as little as possible about this circuit, or at least keep the supply voltages the same, as well as the BJT and input.

It either gives me a small gain, or something that would sound horrible.

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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Have you studied RC circuits? Your capacitors are much too small.

3. ### Ryouko Thread Starter New Member

Apr 26, 2009
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I thought they had to be very small so that they wouldn't affect the AC signal at higher frequencies? What would larger capacitors do to the circuit. I was under the impression that their only purpose is to block the DC. Are you saying that they are small enough to affect the signal differently at different frequencies since their impedance is frequency dependent?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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That's exactly right. Xc=1/(ωC).

5. ### Audioguru Expert

Dec 20, 2007
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I simulated it and it starts having some voltage gain at 200kHz and has a max gain of 27dB at 10MHz. It has a huge voltage loss at audio frequencies because its capacitor values are much too low.

12W into 8 ohms is a voltage swing of 27.7V p-p. But this transistor will have a max voltage swing of only 2.8V p-p and the push pull output stage will also need some extra voltage swing.

6. ### Ryouko Thread Starter New Member

Apr 26, 2009
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0
Thank you for your input, we got it working this morning in lab.