design audio amplifier circuit

[NASK]

I wish to design audio amplifier circuit. Hear I use two stage of amplifiers. 1st stage amplifies 100mv to 1.2V peak @ 2KHz.

Q1. When I change the frequency to 50Hz output becomes to 210mv peak signal. At that time when I changed c3 cap to 100uf again it amplify 1.25V peak signal.  I need to amplify audio signal therefore amplitude is changing according to frequency. What can I do for it?

Q2. 2nd stage is power amplify part. Output of at that stage 180mv peak shows @ 2 KHz signal. That mean 2nd stage amplitude is decreased how it happened and solution for it.
 
circuit it attached

[Zero999]

1) C3 was too small. Now you've fixed it by increasing the value to something sensible.

2) The transistors probably aren't getting enough base drive. Did you measure it with the speaker connected to the output?

Why not use the LM386?

[mikerj]

Q1. When I change the frequency to 50Hz output becomes to 210mv peak signal. At that time when I changed c3 cap to 100uf again it amplify 1.25V peak signal.  I need to amplify audio signal therefore amplitude is changing according to frequency. What can I do for it? :pal

The input coupling capacitor and the the input impedance of the amplifier forms a high pass filter which determines the lower cut off-frequency of your amplifier. If you need it to operate at a lower frequency then use a larger value capacitor, but since you have already done this I'm not sure what you are asking?

Q2. 2nd stage is power amplify part. Output of at that stage 180mv peak shows @ 2 KHz signal. That mean 2nd stage amplitude is decreased how it happened and solution for it.

This is because you are biasing the output stage using ridiculously low value resistors. The two 470 Ohm resistors are effectively paralleled so your first stage is trying to drive an impedance of 470/2 = 235Ohms (ignoring the impedance of the output transistors).  Your first stage has an output impedance of approximately 1k

235/(1000+235) * 1.25 = 237mV, very close to what you are measuring.

You don't need to AC couple the first stage to the output stage.  If you DC couple it, and correctly bias the first stage then the output stage will also be correctly biased since it is a follower, and won't need separate biasing.  Also why have you used three capacitors to couple the stages together?

[Zero999]

Q1. When I change the frequency to 50Hz output becomes to 210mv peak signal. At that time when I changed c3 cap to 100uf again it amplify 1.25V peak signal.  I need to amplify audio signal therefore amplitude is changing according to frequency. What can I do for it? :pal

The input coupling capacitor and the the input impedance of the amplifier forms a high pass filter which determines the lower cut off-frequency of your amplifier. If you need it to operate at a lower frequency then use a larger value capacitor, but since you have already done this I'm not sure what you are asking?

Yes, that's true. I forgot about the input capacitor.

Q2. 2nd stage is power amplify part. Output of at that stage 180mv peak shows @ 2 KHz signal. That mean 2nd stage amplitude is decreased how it happened and solution for it.

This is because you are biasing the output stage using ridiculously low value resistors. The two 470 Ohm resistors are effectively paralleled so your first stage is trying to drive an impedance of 470/2 = 235Ohms (ignoring the impedance of the output transistors).  Your first stage has an output impedance of approximately 1k

235/(1000+235) * 1.25 = 237mV, very close to what you are measuring.

You don't need to AC couple the first stage to the output stage.  If you DC couple it, and correctly bias the first stage then the output stage will also be correctly biased since it is a follower, and won't need separate biasing.  Also why have you used three capacitors to couple the stages together?

That's true, but he said:

I wish to design audio amplifier circuit. Hear I use two stage of amplifiers. 1st stage amplifies 100mv to 1.2V peak @ 2KHz.

That must have been with the first stage not connected to the second.

[mikerj]

That's true, but he said:

I wish to design audio amplifier circuit. Hear I use two stage of amplifiers. 1st stage amplifies 100mv to 1.2V peak @ 2KHz.

That must have been with the first stage not connected to the second.

His attached pdf shows that the first stage is disconnected from the output stage whilst he is measuring the voltage.

[Zero999]

That's true, but he said:

I wish to design audio amplifier circuit. Hear I use two stage of amplifiers. 1st stage amplifies 100mv to 1.2V peak @ 2KHz.

That must have been with the first stage not connected to the second.

His attached pdf shows that the first stage is disconnected from the output stage whilst he is measuring the voltage.

Yes, you're right. I missed that. You're right, that's what you should expect to happen.

The output stage should be DC coupled with no unnecessary coupling capacitors.

There should be some emitter resistors to prevent thermal runaway.

How much gain is required? This is a crappy circuit with not much power output and lots of distortion, at high output levels. The capacitors could be larger for better low frequency response, but there's no point going lower than couple of hundred Hz, because small speakers have no bass.