# class AB amplfier help

as a class project, we were given a circuit schematics of a low distortion audio power amplifier. and we were asked to simulate it, i did that no problem. however I am trying to understand the circuit but the professor didn't tell us anything about it except that it is a class AB amplifier.

I know that class AB is a combination of both class A and B amps, but after i looked at the circuit i couldn't find the class A or the class B amps.

can you guys explain to me what the different parts of the circuit do.

thank you and have a nice day.

this is the exact circuit the professor gave us

this is my attempt at drawing it.

this is the output vs input, the input is the green the output is the blue

• Can you add reference designators to all the components? It would make it much easier to explain if you could do that. Dec 11, 2017 at 1:47
• @mkeith thank you for your help but i don't know how to do that or what does that mean i am sorry, is it the name of the components? Dec 11, 2017 at 1:49
• @mkeith i don't need full explanation, just in general like which part does what. Dec 11, 2017 at 1:54
• Notice that the transistors have labels such as Q12, Q15, etc? The op-amp also has a label, U1. Those labels are called reference designators. Many of the components don't have them. If you want someone to explain a circuit, you should put reference designators on every component. Dec 11, 2017 at 1:59
• I believe there are also several errors in the schematic. It may give low distortion with a high impedance load, but it appears to me that it is not capable of driving an 8 Ohm load. Is this the exact schematic your teacher gave you, or did you draw it? Maybe you should carefully compare the two. Dec 11, 2017 at 2:08

## 3 Answers

Let's say that Q9 gets removed, and the bases of Q1 and Q16 get tied together to the op amp output. This is called a class B amplifier. The two halves (upper and lower) of the output circuitry get turned on or off depending on the polarity of the op amp output.

There's a problem, though. For op amp voltages near zero, neither half is conducting. (Why? hint - think about base voltages) So during the period that the op amp is transitioning from plus to minus or vice versa the output voltage won't respond at all. This is called crossover distortion.

Now add Q9 back in. It's just a current source, and it keeps both halves just a little bit on, so there is no dead zone during crossover. Technically, you could make a case for the amp being class A, since both halves are always conducting, but the "always on" current is very low compared to the peak current, so this is not done. Instead, the circuit is considered a hybrid of class A and B, or just a class AB amplifier.

Please refer to your teacher's circuit. I used those reference designators. In future it would be a good idea when copying a circuit to also copy the reference designators to avoid confusion.

Q13 is a current source.

Q9 is called a Vbe multiplier. It maintains a constant voltage drop between the bases of Q11 and Q16, and helps set the steady state bias current. Without Q9, it would be class B, not class AB. The 10k pot allows you to fine tune the bias current.

In a classic amplifier, Q11 and Q16 would be the output devices. But in order to improve performance under heavy load, this amplifier added the other 4 output transistors, Q14 and Q10 (which function kind of like a single high gain transistor) and Q15 and Q12 (which also function kind of like a single high gain transistor).

I am pretty sure the 100 Ohm resistor at the emitter of Q10 is supposed to just be a short (or maybe a much smaller resistor). I am also pretty sure that the 1k resistor connected to the emitter of Q15 is supposed to be 1 Ohm.

In your simulation, you should add an 8 Ohm resistor from output to ground to represent the speaker.

There are probably other errors that I did not notice. These just kind of jumped out at me. After you simulate with 8 Ohms, I think it would be reasonable to discuss the results with your teacher. Depending on your relationship with the teacher, you can be diplomatic, or just show that it doesn't work.

In your version of the schematic, Q2, D5, D6, R17 form a current source. Q1, R3, R4, R5, R16 maintain a voltage difference between the bases of Q8 and Q3 to make sure at least one of them is always conducting, as WhatRoughBeast noted above. Q6, Q7, Q8 form one half of the power output stage: they turn on when current must be supplied to the load; the op amp drives the (presumably) low-power Q8, which then supplies base drive to Q7, which in turn drives Q6. Read up on Sziklai pairs to understand what's going on. Q3, Q4, Q5 turn on when current must be sunk from the load; otherwise they work in a very similar fashion. As for the jumble of resistors in between, I believe there are a few errors in the original schematic (I agree with mkeith here); for instance, I see no low-impedance path from the emitter of Q5 to the load. Also, note that no current will flow through the load — the other end of, marked Voutput, isn't connected to anything! Basically, the voltage at Voutput is the output voltage of the amplifier without a load.