Electrical – Audio Amplifier Design (CE Amplifier – Class AB Power Amplifier)

The basic answer to this is to start by making sure all the signal paths have only high pass filters with rolloff no higher than 20 Hz, and low pass filters with rolloff no lower than 20 kHz. There will still be some uneveness in the frequency response, but it shouldn't be drastic. Take a close look at what filters are formed by any caps, and make sure they meet the above criteria.

Then make the open loop gain about 10x or more of what you want the closed loop gain to be. That leaves room for some negative feedback to do its job. Don't go crazy with gain, as too much gain will make it difficult to keep the amp stable, and too much feedback to get to the desired gain introduces transient intermodulation distortion (TIM).

Then put feedback around the whole amp. Again, this hopefully doesn't need to be more than 20 dB or so.

You don't need the Class AB external BJT amplifier at all. Notice in the datasheet for the LM386 that the output stage is already a Class AB amplifier. Given the voltage you want to work at you do not need any external add on amplifier for the LM386.

There are several mistakes in your circuit:

1. Q3 is inverted. You will draw excessive current connected this way (and it won't work).

2. You only need one capacitor coupling the LM386 to your output stage. Change the 250 uF (C2) to 47 uF and eliminate C3. The plus sign goes to the LM386 output because this will sit at approximately VCC/2.

3. R3 need to have the lower end of the pot grounded, it won't act as a volume control connected as shown. It will also upset the bias in the LM386.

4. You need a capacitor to isolate the input signal source. I'd recommend a 10 uF in series with R3.

5. There is no feedback around both amplifiers so you have two independent amplifiers. The gain for your LM386 is 20 so overall gain is going to be in the hundreds, which is way too high for your application.