Distortion in Output Signal (BJT Amplifier)

[LanceProspero]

So we were asked to create a cascaded BJT Amp with a minimum gain of around 100v with a ac input of 100mv rms and a freq of 1khz. this was one of my past designs but same problem occurs even thou i achieved the gain of 100v. what seems to be the problem in my output signal?

 

[ronsimpson]

Please remove BC1 and BC2 and tell me how is the distortion.

 

[AnalogKid]

Your circuit has WAY too much gain. The signal at the Q1 collector is is a squarewave, and CC3 and RL are differentiating this into the spike waveform. Put a scope probe on the Q1 collector or emitter to confirm.

ak

 

[LanceProspero]

ic

Your circuit has WAY too much gain. The signal at the Q1 collector is is a squarewave, and CC3 and RL are differentiating this into the spike waveform. Put a scope probe on the Q1 collector or emitter to confirm.

ak

if it does do that, then what will i need to change?

 

[LanceProspero]

Please remove BC1 and BC2 and tell me how is the distortion.

once i remove the bypass caps the signal becomes a clean sine wave

 

[AnalogKid]

At audio frequencies, those capacitors "short out" the emitter resistors because their impedance is much lower. This makes each transistor try to run at it's full gain rather than the gain set by the circuit's resistors. You can look up the gain on the datasheet, multiply the two, and see that with a 0.1 V input the output was trying to make a huge signal, and clipping it to a square wave.

ak

 

[audioguru]

The datasheet of a transistor shows its current gain, not its voltage gain.
Here is a simulation showing that each transistor has a voltage gain of about 32 times so their total gain is much too high.
The values for the coupling capacitors are much too small to pass 1kHz at its full level.