Designing Multistage Amplifier for 200mW Output to 8Ω Speaker From 20mV Mic Input

I am attempting to build a multistage amplifier that meets the following requirements:
Delivers an average power of 200 mW to an 8 Ohm speaker from a microphone that produces a 20 mV peak-peak sine wave
Source resistance is 500 Ohm
Lower cutoff frequency needs to be below 50 Hz
Upper cutoff frequency needs to be above 20 kHz
The attached circuit is what I have currently. The gain is only 21 V/V right now, but I'm pretty sure I know how to make it higher. Upper and lower cutoff frequencies seem to be good, and the oscilloscope outputs seem to be stable and undistorted, but I'm worried about this not working when I go to breadboard it. In particular, someone said that the base resistances are too low and that the transistors would just fry, and I don't think the 92 uF capacitor is really feasible.

Your diagram did not show. Please send the diagram to Maurizio at
maurizio.dipaolo(_at_)aspencore.com
ans he will be able to put it in, then we can reply.

Schematic

Rather than relying on emitter resistor feedback to set the gain of the individual stages, it is usually advantageous to use overall negative feedback as well. This offers improved performance in a number of areas.
An example of the technique is here, https://www.eeeguide.com/two-stage-ce-amplifier-using-series-voltage-negative-feedback/
Don't worry about capacitor values, pick the next standard value up, and they are normally plus or minus 10 to 20 percent anyway.
Not sure what the story is about the base resistors and frying the circuit, the values don't seem too untoward, without doing any maths and checking it all out myself.
The highest power dissipation will be in the emitter follower, so you need to check that.
I appreciate that you are possibly trying to learn a bit about BJT amplifier circuits, but there are Integrated Circuits that are designed for this sort of application.
Cheers,
Richard

Hi.. OK, had a look at your diagram and you are more or less on the right track.
A couple of general comments
1. Learn about preferred values of components. You will not be able to get most of the resistors in your diagram. Instead of 48K use 47K, instead of 163 ohms use 150 or 180 ohms - 150 is probably closer. Google "preferred values" to find out more. For capacitors - your 92 uF output capacitor - go up to 100 uF. Sure, you work out the values and they are never preferred values, but learn to go up or down to the nearest value. Unless your circuit is super sensitive it won't change much.
2. Your base resistors, while maybe a bit low, will not cause any of the transistors to fry. If you need a low base resistance for gain, but a higher one to get your DC bias right, then bypass all or part of the resistor with a capacitor. The capacitive reactance should be less that 1/10 the value of the resistor is is bypassing at the lowest frequency you are interested in. Here's a great article on the subject:
http://www.learningaboutelectronics.com/Articles/What-is-a-bypass-capacitor.html
3 In general, the point in any stage where the signal is output should be biased at around 1/2 the supply voltage. This gives you maximum signal swing + and -. Try putting a virtual DC voltmeter on your simulation at Q1 and Q2 collectors and Q3 emitter and see what you get. And unless you've been told to use 18V for your Vcc I'd use a lower value, 12V or even 9V. You should still get 200mW output with no problems.
Specifically, for your output stage, I would be using a complimentary (NPN/PNP) 2-transistor output stage. Here's one that does almost exactly what you want:
http://www.zen22142.zen.co.uk/Circuits/Audio/250mW_amp.htm
That whole circuit could probably replace your Q2 and Q3. Note that there is an error, T3 in their diagram should be a BC557 PNP type. You will notice that R5 / R10 / C3 in the emitter of T1 is a good example of bypassing to get different AC and DC conditions. You will also notice that this circuit is driven from 9V!
You will also notice that there is always either R6 or R7 in series with the speaker, so you're very unlikely to blow anything. Your circuit on positive output half cycles has only the transistor Q3 in series with the loudspeaker - so on large signals it would be easy to overdrive it and maybe blow it (the instantaneous impedance of the capacitor would be almost nil).
Sorry, quite a lot to take in here, and I don't have time to analyse the specifics of your circuit, biasing etc, but see how you go with this.
In the real world you'd probably use a small IC to drive your speaker (like an LM386) but doing a design like this is very instructive and you'll learn a lot more.
Good luck and let us know how you get on. Cheers - David

A multistage amplifier using two or more single stage common emitter emitter amplifier is called as cascaded amplifier. A multistage amplifier with common emitter as the first stage and common base as second stage is called cascode amplifier.

The multistage amplifier are constructed with the series connection of more than one amplifier in a single casing the output of one amplifier is used input for others
known as one stage. So as single multistage amplifier has more than one stage.
Each amplifier in this arrangement performs its separate process of amplification.
So the output of this circuit is amplified much time according to a number of amplifiers in circuits.

I appreciate that you are possibly trying to learn a bit about BJT amplifier circuits, but there are Integrated Circuits that are designed for this sort of application.

Thanks everyone. Was just reading through old posts and found this rather helpful.