# Need a little help understanding the way my class AB power amplifier works.

#### tuna

May 25, 2023
4
I have an assignment to simulate the given class AB power amplifier. However, it would be nice to get a grasp of what's going on in the circuit (atleast conceptually, no need for 50 equations with 50 variables). I understand the simplest diode biased AB power amp, but this is really too big a step for me. I know the circuit should be divided into smaller parts and see how they interact, but still it's too complicated for me. Any help would be most welcome.

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#### crutschow

May 7, 2021
661
It's difficult to discuss the circuit without transistor part designations (Q1, Q2, etc.).
Could you add those to the schematic and repost it?

#### tuna

May 25, 2023
4
Sure, my bad. Tn is npn and Tp is pnp.

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#### crutschow

May 7, 2021
661
Okay.
Tp1 and Tp2 form a differential input stage to allow AC and DC negative feedback to stabilize the output.
Tn1 and Tn2 are a current mirror load to provide a high-impedance load for that stage to give a high differential voltage gain.

Tn3 provides a DC drop (similar to that often done by diodes) to bias the output transistor slightly on for AB operation.

R8 provides DC feedback to bias the output to the input ground through the input differential stage so there is no DC bias on the output speaker load.
This is fed back from Tn1 to Tn4 for this purpose.

R9 and C1 attenuate the negative AC feedback through R8 to the differential input, to set the AC circuit gain (to about 10) and reduce the output distortion.

Anything else you want to know?

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#### tuna

May 25, 2023
4
Thank you so much, kind stranger. I would just like to know when did you start being able to analyze these circuits so fast. How much hours in the field do I need to be as well versed in electronics as you?

#### crutschow

May 7, 2021
661
I would just like to know when did you start being able to analyze these circuits so fast. How much hours in the field do I need to be as well versed in electronics as you?
That's a difficult question as it depends greatly on your ability to develop an understanding of basic analog electronics.
I have a BS in Electrical Engineering and retired from doing circuit design work in the industry after over 40 years, so I'm not a good judge to remember when I reached the point to readily analyze a new circuit.
Certainly you need to be able to recognize basic circuit functions, such as a transistor amplifier stage (using BJTs and/or MOSFETs), a differential circuit stage, a bias stage, a push-pull output stage, a current-mirror stage, etc., which comes with learning and practice.

Basically you look at each small block in the circuit and try to determine what it is doing in the complete circuit.
Sometimes it's apparent and other times it is not.

Feb 19, 2021
410
field..... Build / proto circuits, you are building up your AI memory and skills.

Regards, Dana.

#### AnalogKid

Jun 10, 2015
2,739
R10, R11, and Tn3 perform as a zener diode. This replaces the diode-bias technique you are familiar with. The temperature tracking is not as good as four diodes in series mounted on the power transistor heatsink, but you can adjust the resistor values so the voltage drop across the transistor more closely matchs the total voltage drop through the four output stage base-emitter junctions. This is a way to reduce the amount of static current through the output transistors needed to eliminate crossover distortion (the "A" part of Class AB).

ak

#### bertus

Moderator
Nov 8, 2019
2,919
Hello,
R10, R11, and Tn3 perform as a zener diode.
This combination is sometimes called a "rubber diode":

@tuna : You might find a lot of info in the following book:

Bertus

#### tuna

May 25, 2023
4
Thanks to everyone for the clarification. I discovered a great book on the topic from Bob Cordell: Designing Audio Power Amplifiers. Really learning a tonne here.
Regards, tuna

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