# How to calculate the bias resistors of a class AB amplifier ?

Discussion in 'The Projects Forum' started by Michael George, Mar 7, 2015.

1. ### Michael George Thread Starter Member

Feb 8, 2015
52
2
Hello everybody,
I've read about class AB amplifier and I understand How it works but I don't know how to calculate the values of R1 and R2 resistors. Would you tell me the simplest way to calculate them ?

One more thing, I've read that the characteristics of Q1 must mach the characteristics of Q2.
What is meant be characteristics ? Is it hfe or something else ?

2. ### bance Member

Aug 11, 2012
315
34
Try this tutorial.

HTH Steve

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3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,948
924
The voltage across R3 sets the quiescent current, the voltage is the sum of the base /emitter on the transistors which is 1.4v, ideally the output at the emitters is half the supply voltage,R1,2 are chosen to give a biasing just above the minimum cut off point.

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4. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,178
1,474
For lowest distortion, the two transistors must have identical current gain curves (current gain as a function of collector current). This is harder than it sounds because complementary devices, even ones designed for audio circuits, rarely are truly complementary. And your circuit is more dependent than most on device matching because it has no negative feedback.

Also, silicon transistors have a negative temperature coefficient. That means that the value of Vce decreases with temperature. In your case, it means that once R3 is adjusted for a particular static current through Q1 and Q2, that static current will increase as they warm up, to the point that the transistors might overheat and fail. This is caused by thermal runaway, a common problem with early low cost transistor amplifiers. The standard solution is to place a diode physically against each output transistor for best thermal contact, and incorporate the two diodes into the base bias network. The voltage drop across the diodes tracks the drop across the two base-emitter junctions, and keeps the static current relatively static.

Sorta like this: http://electronics-diy.com/4x4.php

ak

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5. ### Sukrit Virmani New Member

Feb 5, 2016
1
0
HI ANALOGKID,

THE LINK YOU POSTED, DOES IT HAVE A NEGATIVE FEEDBACK IN THE CIRCUIT TOO?

6. ### dannyf Well-Known Member

Sep 13, 2015
2,196
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Find the maximum current your transistors need to deliver and pick a reasonably conservative beta.

From that, you get the the maximum base current.

Those two resistors should be sized so they can comfortably deliver such base current. I would pick the resistors so they idle at least 2x of the base current and likely more.