Class A Audio Amplifier

Question

I am trying to design a Class A Audio Amplifier with the following specs:

• 9V DC Power Supply, Maximum withdrawn current 15mA
• Used with a Low Impedance (8 Ohm) Speaker
• Provide a minimum variable gain of 10
• Can work with frequencies from 20 Hz - 5 kHz
• Input signal is 0.2 V(peak to peak)

I tried several designs and I faced the following problems:

1. Is it possible to actually drive that low impedance load with only 15mA, given this input signal? All the designs I did drew about 500mA from the DC Supply
2. The signal always had some sort of clipping.
3. I had some problems choosing coupling capacitors for the design.
4. While choosing a Q point for each transistor, I had some problems figuring out which Ic should I assume to start working with

The design is mainly built on a common emitter stage then a common collector stage to act as a buffer. I will attach two design concepts I tried (none of which actually worked).

I would like to know if one can clarify the correct procedure of calculating the correct Q point, and then walk me quickly through the steps of making the required calculations of designing such an amplifier.

EDIT So I have been working for some time on this, and I reached this final design, but we neglected that 15mA current limitation.

I am not sure how this performs on real life, I have just been testing that on simulators.

Answer 1

Just based on your specs you're going to have current draw issues.

You are driving the circuit with a 0.2 V signal and want a maximum gain of 10, meaning a peak output voltage of 2 V.

$$ \frac{V}{R} = I,\quad \frac{2}{8} = 250\ \text{mA} $$

This would also assume a 100% efficient system which you won't get anywhere close to in a class A amp.

I would recommend starting with a minimum working example, i.e. instead of starting with a multistage amp try and get a common-emitter amplifier working to your specs then see if you can improve it by extending it to a second stage.