Help Me With This Amp

[nightmare13]

Hi Guys, I have a problem with this project. the instructor wants us to make an audio amp using only BJT transistors with Beta = 50 (forward current gain), and the output should have 2 watts and 8 ohms resistance. the input has 0.1 mV peek-to-peek and 10kOhms output resistance. the sources available to make the bias are 24 V. thanks in advance.

 

[audioguru]

You don't have a spec for the audio amplifier to have low distortion or wide bandwidth so it is very easy to design for only 2W into 8 ohms. Only 3 transistors are needed.
How much of the design have you completed? Attach your schematic and we'll help you.

 

[Roff]

Assuming 2W rms into 8 ohms, this requires 4V rms, or 11.3V p-p. With 100uV p-p input, the gain needs to be 113,000. You can't do that with 3 trannies.
Can you?

 

[nightmare13]

Thanks for replying. Here is what i could reach.

 

[nightmare13]

Hi guys, our prof. told us to look at it as a current gain not a voltage gain, (i guess he meant the darlington pair by that) and the output is supposed to be 500 mA and 4 V Peek (the input is 0.1 mV Peek and not peek-to-peek) myproblem is that the output is always in nano or pico volts. I feel like i'm going nowhere. Please guys my deadline is tomorrow, i really need your help.

 

[audioguru]

2W into 8 ohms is 4V RMS or 5.66V peak. Therefore the voltage gain must be 5.66/0.1= 56.6
Your amplifier has a few problems:
1) It should not feed DC to the 8 ohm speaker, the speaker should be fed through a coupling capacitor.
2) The bottom transistor Q2 with its extremely high value emitter resistor is doing nothing. It should be a PNP emitter-follower transistor so it can pull the output down.
3) An audio amplifier that produces 2W into a speaker should use medium power transistors like TIP31 and TIP32 for the output so they don't melt like little 2N2221 transistors would. You should be able to make only 2W without darlington transistors.
4) R8 and R3 are reducing the power to the speaker to nearly nothing.
5) The 1st transistor Q1 does not have a base bias voltage so it is turned off.
6) The 1st transistor Q1's collector resistor has a value much too low and is the same value as its emitter resistor so it has no voltage gain.

You weren't taught anything about transistors? I just looked in Google for Simple Audio Amplifier Circuit and many links have a circuit that will work.

 

[audioguru]

Hi Ron,
I don't believe that the input signal is only 0.1mV Peek. I think it is 1000 times higher, at 0.1V peak.
Also, I don't believe that the output resistance must be 10k ohms. I think it is the input resistance.

These guys weren't taught anything about transistors. Maybe they skipped class or the prof forgot to teach it.

 

[Roff]

Hi Ron,
I don't believe that the input signal is only 0.1mV Peek. I think it is 1000 times higher, at 0.1V peak.
Also, I don't believe that the output resistance must be 10k ohms. I think it is the input resistance.

These guys weren't taught anything about transistors. Maybe they skipped class or the prof forgot to teach it.

You're probably right on all counts. I was just taking him literally.
I thought he was saying the source resistance is 10k.