amplifierbjtcomponentsschematicstransistors
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:
None of the options is good. The speaker produces a sound pressure level for a certain wattage inputted and this is the criteria you need to look at. Some speakers are more effective than others and this effectiveness is largely determined by their enclosure. Luckily, for any given size and shape of speaker the enclosure is roughly constant.
Decide what SPL you need and look at speaker specs to shortlist a handful then design your enclosure to suit the shortlisted items then do comparative studies. You are obviously looking for bandwidth, loudness and distortion as the key ingredients of the selected speaker. Choose also, the speaker that is capable of withstanding the power output that can be thrown at it. Choose an amplifier that won't melt down ie has protection built in.
Can someone please explain to me, why is that there is a resistor after c1?
With this amplifier topology the global gain is defined by \$A=\dfrac{R4}{R1}\$, provided that the open loop gain from the transistors is much higher than that.
In that the base of Q1 will act as virtual ground for the AC signal. Notice that for DC the base is biased to approximately 0.6V. With R1 at (virtual AC) ground, your input impedance equals R1.
A simplified opamp circuit would look like this (notice that it will not work in practice due to low voltage).
simulate this circuit – Schematic created using CircuitLab
Best Answer
I'm going to focus on the points in the question: -
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.