How to design a schematic diagram of amplifier that can drive 8 ohms 0.25 Watts speaker? [closed]

Question

I have here a schematic diagram for amplifier that drives a 0.025watts 8ohms speaker. But what I need is an amplifier that drives a 0.25 watts 8 ohms speaker.

So, here are my concerns:

1. How can I possibly raise the Av to 7?
2. What is the function of R1? Is it really that necessary?
3. What are the biases and configuration use in the schematic?
4. What parameters do I need to consider in designing an amplifier?

Answer 1

I'm going to focus on the points in the question: -

• Power output 0.25 watts into an 8-ohm load from a 3 volt supply
• Input is a sinewave at 1kHz
• AC gain is about 7
• Only BJTs, diodes, and passive components are allowed

If you have a 3 volt supply, in a simple circuit, the biggest voltage you can produce is 3 Vp-p. This is a sinewave, as per the detail in the question and therefore the RMS of the sinewave is going to be: -

\$\dfrac{3 Vp-p}{2\sqrt2}\$ = 1.06 Vrms.

Into 8 ohms this produces a power of 141 mW i.e not enough to satisfy the professor. This likely means using a transformer output. You could design it using a H bridge configuration bet this is probably more complicated.

Here is a class A transformer amplifier: -

Using a 1:1 transformer will do the job because the maximum swing on the collector is twice the supply rail so immediately you can generate something in the order of 5.6 Vp-p. I'm not saying 6 Vp-p because the BJT will likely saturate at 0.2V and this gets doubled when analysing the ac peak to peak voltage the circuit could produce.

5.6 Vp-p gives you a potential sinewave power of about 0.49 watts.

As you are a relative novice, I'd consider going down this route as you can get gain and sufficient drive to the loudspeaker (\$R_L\$ in the diagram) from this circuit.

As this is a homework question I'll not go into any more detail and leave it to you to do some more research on the circuit. Good luck.