# Class-AB Power Amplifier not working in LTSpice

So we were asked to create an audio amplifier simulation in LTSpice. We decided that the total gain of the circuit should be around 600, having four stages in total: differential amplifier, CE amplifier, bandpass filter and a class-AB power amplifier (this is how we were asked to do it for the project). The simulation has done obtained as follows:

The blue wave is the voltage just before the power amplifier and the green step-like signal is the output after the power amplifier. The waveform itself is getting disturbed

But when I simulate the circuit without the power amplifier it seems to run fine.. This is the circuit without the power amplifier

This is how the simulation is being obtained without the bandpass filter. The green signal is the voltage after the bandpass filter and the blue line is the input AC voltage

Can someone help me with the mistake? Also, does using buffer help, if it is at all related to the problem in any way? (If you did help, thanks for you help in advance!)

• Q5 PNP is backwards: its collector should go to -VEE Mar 4 at 14:52

Besides the backwards transistor you're also going to find that the base resistors in the output stage are too high in value. The output in your last image only looks okay because you have no load connected.

You have 12 V rails and an 8 $$\\Omega\$$ load, that's 1.5 A. The transistors are specced as having a minimum beta of 25 at 1 A and 10 at 3 A so let's say 20, that means you need 75 mA base current, figure $$\frac{V_{CC}-V_{BE}}{I_B} = \frac{12~V-0.6~V}{75~mA} = 152$$

so you're looking at roughly 150$$\\Omega\$$ resistors.

Here's a quick simulation with 150$$\\Omega\$$ (green), 1k (blue) and 5k (red) resistors. You can see the value rather dramatically affects the point where the amp clips.

I am the one who posted the question ;)

I found out that the problem was that the right way of connecting the power amplifiers is that thier emitters should be connected as follows:

Observe the difference in the emitter direction of PNP in the question and the above diagram..

The output waveform (after the power amplifier) has been obtained as follows: