Electrical – Single Stage Transistor Amplifier

What is wrong with this circuit?

Typically a circuit like this has a gain that is roughly numerically the collector impedance divided by emitter impedance. So if you had 2k2 in the collector and 220 ohms in the emitter you'd get a voltage gain of ten.

This type of circuit is usually pushed to have much higher gains by bypassing the emitter resistor with a capacitor. This makes the emitter impedance very low at audio frequencies but also significantly increases distortion.

Then if you really want things to go wrong you make collector impedance something really low by hanging a 4 ohm speaker on it. Collector impedance (AC) falls from 2k2 to about 4 ohms and you have virtually no gain and lots of distortion.

Reducing the value of emitter bypass capacitor reduces distortion but also reduces the gain. How do I find the correct value for getting good performance at audio frequencies?

Use a resistor in series with C1 to control the AC gain.

Using this single stage I am able to get very less audio output. Its almost inaudible. If I use a second stage, I get highly distorted output. However it is audible to certain extent. What should I do to reduce the distortion without affecting the gain?

See my observations above - don't hang a speaker on the collector - use an emitter follower or better still get an audio amplifier IC like the LM386.

Can you please provide the design steps involved in choosing the capacitor values?

I think this has been done.

Your design is a very basic and might work but since it has no feedback it is not so predictable in how it will behave in practice.

The "proper" engineer's way of designing this is by use of feedback. You basically make a crude amplifier with a high gain and enough bandwidth and use feedback to get the gain you actually want.

Unfortunately this design procedure is not something which can be explained in a few sentences. I learned this in a course which took several days and included a design assignment.

I found a University course here that should explain this method, that's 70 slides to get you started ;-)

If this is too much of a stretch in the time you have then just remember this and come back whenever a more challenging amplifier design task pops up.

Another option altogether would be to use an opamp in a feedback configuration. If you want to know more about opamp circuit design look here: Opamps for everyone