Hi, this will be my first post here. I've been working on a class ab amplifier for my finals project in my electronics lab class. I attached the schematic:



It works perfectly fine on PSPICE simulation but once I build the circuit on my breadboard, the output and input signals both get a really high frequency and high amplitude signal. Even with no input signal.

I found that if I remove the 1000uf and 0.001uf capacitors, it works but my gain is ruined ]=


I also noticed if I just I touch (or hook up a wire) at the node between the 2 diodes (where the 0.001uf capacitor was), it reduces the high frequency oscillations by a bit.

I have also tried physically moving the output and input signals farther apart from each other but it didn't help.

Any idea what's going on and how to fix it? Much appreciated.

 

Probably not the definitive answer, but...

You have designed C1 and C3 for about 2.5 Hz as the low frequency limit. Not the cause of the oscillation, but not in the audio range, either.

C3 allows Q1 to operate at the highest gain that is inherent in the transistor, and they vary significantly from batch to batch. Whatever the simulator said is only a guess or an average.

You have absolutely nothing to limit the high frequency range of the amplifier.

You have no negative feedback at any frequency, not even a resistor in series with C3 to limit the broadband gain.

You have designed the output transistors in the range of 500 ma, but they are specified as 200 ma max in their datasheet.

The physical layout of an amplifier with zero limitations on its high frequency gain can also cause problems.

Just a few suggestions. Not like I'm really an expert.

 

Your "-12V" supply is configured as a +12V supply.

 

You do not show any bulk bypass caps on the power supplies. The simulator does not care but the real circuit does. I would use a few hundred microfarads on each of the 12 volt supplies.

 

@SniperFox
You need a small resistor between Q3 and output and Q2 @ and output. Something in the range of 0.22 and 1.0 ohms.

Also, input impedance is pretty low and you could design something with a differential input (long tail pair) and add some feedback.

 

Sorry guys I'm still a noob at this stuff. It's my first electronics class I'm taking. I still don't know design processes on negative feedback and stuff.

#12 - I thought the amp was designed for about 100mA output, because I'm getting +/-2v output and 2/22=90ish.

Alec_t - the -12v value is actually for the supply, so if I turned it backwards, I'd have to change the value to 12v. They're equivalent.

RichardO - I tried to replace the 0.1uf capacitors with 100uf ones (the best ones I had at the moment) and wow! It helped a lot! Why weren't the 0.1uf ones good enough and how would I go about choosing the right values?

GopherT - sorry I'm pretty noob (as I said earlier), could you explain how/why those small resistors would help? I'm also not exactly sure what a differential input (long tail pair) is?

 

#12 - I thought the amp was designed for about 100mA output, because I'm getting +/-2v output and 2/22=90ish.

RichardO - I tried to replace the 0.1uf capacitors with 100uf ones (the best ones I had at the moment) and wow! It helped a lot! Why weren't the 0.1uf ones good enough and how would I go about choosing the right values?

The reason for the large value capacitors is to keep the power supply low impedance at low frequencies. If the power supply has some of the signal on it then that can cause positive feedback and oscillation. Note that you need both the 0.1uf capacitors and the large caps. The small caps have less inductance than the large ones to keep the power supply low impedance at high frequencies. Otherwise you can get high frequency oscillation. You might not hear the high frequency but it can cause problems such as hot output transistors.

I noticed that the output transistors a shown as being 2N3904 and 2N3906. These parts can only dissipate about half a watt. Also, the do not have a very high gain when driving a 22 ohm load at a few volts. They should be replaced with Darlington or Sziklai pairs in higher power packages such as a TO-220.