Design a power amplifier

Question

I am required to design a power amplifier with these specifications:

Input signal, Ui in range: 540 [mV];
Output load, RL: 90 [ohm];
Input resistance Ri >150 [kohm];
Output resistance Ro < 0.9 [ohm];
Amplification in voltage, Av: 10;
Operating temperature range: 00
-700C (verifiable by testing in
temperature);

Signaling the presence of supply voltages with LED type diode. The problem is that I started the design but the documentation given by our supervisor were lacking at best and I don't know if I am going the right way and how to continue.

Until now I tried to finish two different designs: the first one is based on a book and the second one is based on the documentation given.

What can I do now? What books or other kinds of documentation should I be looking for regarding my inexperience in the domain?

Answer 1

Your first circuit diagram is closer to a sensible design but quite a few of the component values need to be changed to be closer to what they should be, the feedback resistor is missing and the feedback doesn't appear to be attached to the amplifier's actual output between R10 and R11. Also better to use a Vbe multiplier rather than the two diodes (D3 & D4) to get a good adjustment of the output transistors' bias and good control of the quiescent current.

Below is an amplifier which I designed myself which is largely based on Douglas Self's Blameless Amplifier but has three major differences. Mine is a single voltage supply amplifier with Darlington output transistors and I have added a capacitance multiplier.

This circuit should give you a reasonable idea about the required component values.

For a dual supply amplifier, when a polarised electrolytic is used for C3 (your top diagram) you sometimes see a diode across this capacitor to prevent the capacitor being reverse biased with a larger voltage under fault conditions.

I've added a photo of the finished stereo (dual mono) amplifier to give you an indication of what's possible.

The red capacitors near the centre of the amplifiers are the all important compensation capacitors (C5 in your top diagram) required to make the amplifier stable.

The blue trimmer pot (centre right of each amplifier) is used to set the bias between the output transistors' bases thereby setting the output stage quiescent current.