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# 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
787
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
787
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
787
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.

#### danadak

Feb 19, 2021
673
Read constantly, read about what you dont know, read diverse areas within the
field..... Build / proto circuits, you are building up your AI memory and skills.

Regards, Dana.

#### AnalogKid

Jun 10, 2015
2,843
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
3,096
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

#### hamidch

Oct 17, 2023
4
A class AB power amplifier operates by combining the features of both class A and class B amplifiers. It's designed to provide efficient amplification with reduced distortion. In a conceptual sense, you can think of it as having two transistors (one for positive, one for negative half of the signal) that operate in a push-pull fashion.

Each transistor conducts during a portion of the input signal cycle, ensuring that there's an overlap where both transistors are active, minimizing crossover distortion. The diode biasing method used helps maintain a small quiescent current through the transistors even when the input signal is low, preventing them from turning off completely. By breaking the circuit into smaller sections and understanding how these components interact, you can appreciate how a class AB power amplifier efficiently amplifies audio signals.

#### Delta Prime

Jul 29, 2020
1,615
By breaking the circuit into smaller sections and understanding how these components interact, you can appreciate how a class AB power amplifier efficiently amplifies audio signals
I am addressing the human being behind the artificial intelligence (AI) facade.
Like most things electronics it's about minimizing disadvantages and maximizing advantages it is why when using NPN, PNP transistors in a "push-pull" configuration ,AB amplifiers you get their individual A & B amplifier advantages without their disadvantages but still cannot fully
reproduce audio signals & their efficiency is good for approximately 58% to 78%.
if you can appreciate that!
Then you're lowering your standards

Last edited by a moderator:

#### hamidch

Oct 17, 2023
4
A class AB power amplifier is a hybrid electronic amplifier that combines the virtues of class A and class B amplifiers. While class A amps offer linearity but are inefficient, and class B amps are more efficient but prone to crossover distortion, class AB amplifiers mitigate these issues by biasing the output transistors slightly on even at zero input signal. This configuration divides the circuit into three main stages: input, driver, and output.

The output stage employs complementary NPN and PNP transistors, and its biasing uses a diode circuit to eliminate crossover distortion. This amplifier's operation involves signal amplification through these stages, ensuring both halves of the input signal are amplified without distortion, resulting in improved efficiency and fidelity. Simulations using tools like SPICE can illustrate its performance under various conditions.

#### Delta Prime

Jul 29, 2020
1,615
A class AB power amplifier is a hybrid electronic amplifier that combines the virtues of class A and class B amplifiers. While class A amps offer linearity but are inefficient, and class B amps are more efficient but prone to crossover distortion, class AB amplifiers mitigate these issues by biasing the output transistors slightly on even at zero input signal. This configuration divides the circuit into three main stages: input, driver, and output.

The output stage employs complementary NPN and PNP transistors, and its biasing uses a diode circuit to eliminate crossover distortion. This amplifier's operation involves signal amplification through these stages, ensuring both halves of the input signal are amplified without distortion, resulting in improved efficiency and fidelity. Simulations using tools like SPICE can illustrate its performance under various conditions.
Gotcha! I was setting you up brother.Hook line and sinker!
But it's all cool; you were just trying to help.Catch and release...

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