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I have designed an AB Audio Amplifier, but even though the gain is acceptable, the frequency response is terrible.As seen in the schematic, there is a feedback/preamplifer stage and an output stage, I have used MULTISIM to perform AC Sweep of the only the preamplifier stage, and the frequency response there is acceptable, but the response at the output isn't.

What can I do to improve the frequency response while allowing only a reasonable fall in gain?

Below are the input and output stages respectively.

Preamplifier Stage

Output

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  • \$\begingroup\$ Where's the frequency response plot? What response are you trying to achieve? And where on earth did you find such a weird circuit? Start by deleting C3b and increasing C7b... \$\endgroup\$ – Brian Drummond Jan 26 '16 at 21:27
  • \$\begingroup\$ I don't have the reputation to post more than 2 pictures, and this was an assignment.I've deleted C3b and there's a massive improvement in frequency response.Goes way beyond audible! Thank you! \$\endgroup\$ – Ahmed Ali Abbasi Jan 26 '16 at 21:35
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    \$\begingroup\$ I think you need another couple of series diodes to bias your output stage if they are indeed darlingtons \$\endgroup\$ – Neil_UK Jan 26 '16 at 21:45
  • \$\begingroup\$ OK there are many (ahem) "departures from accepted practice" in it but that was the first and biggest. C7b is deleting most of your bass... If you linked to the frequency response pic, someone will come along and edit it into the question. \$\endgroup\$ – Brian Drummond Jan 26 '16 at 21:55
  • \$\begingroup\$ C7b should be at least ten times the size. Do the math. C7b and R2 form a high pass filter. Turnover frequency 90Hz. \$\endgroup\$ – user207421 Jan 26 '16 at 22:25
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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.

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