crossover distortion in class B amplifier

Thread Starter

xaxxa

Joined May 13, 2022
7
Hello, I have a question about the crossover distortion that occurs in a class B amplifier.

I know that it happens when the input voltage is higher than Vth of the PMOS and lower than the one of the NMOS. Does it depend on whether we have symmetric or non-symmetric power supplies? If I have 10V as Vdd and -5 as Vss, will it still occur around 0 volts? Or would it be shifted to the common mode of my amplifier, so it would happen around 2,5V?*


Thank you in advance
 

Ian0

Joined Aug 7, 2020
5,504
An amplifier design with negative feedback will eliminate most of the crossover distortion. Please post your circuit diagram.
 

BobTPH

Joined Jun 5, 2013
5,227
MOSFETs are not the best choice for a class B or AB amplifier since the thresholds are are not very well controlled. BJTs always turn on at about 0.6V, so they are easier to deal with.

Bob
 

crutschow

Joined Mar 14, 2008
30,102
I know that it happens when the input voltage is higher than Vth of the PMOS and lower than the one of the NMOS. Does it depend on whether we have symmetric or non-symmetric power supplies?
Not if the output stage bias is unchanged.

But you should use an Class AB design, which eliminates most of the crossover distortion.
It's difficult for negative feedback to totally eliminate crossover distortion, due to the high frequency feedback needed.
 

jlm1948

Joined May 19, 2014
15
I know that it happens when the input voltage is higher than Vth of the PMOS and lower than the one of the NMOS.
Actually it happens around the point where current in both the P and N is close to zero.
Does it depend on whether we have symmetric or non-symmetric power supplies?
No.
If I have 10V as Vdd and -5 as Vss, will it still occur around 0 volts?
It really depends where the output rests and if the load passes DC.
Or would it be shifted to the common mode of my amplifier, so it would happen around 2,5V?*
It would if the load is capacitively coupled, but the load passes DC to ground, the upper MOS would be in class A.
A pretty common trick in pro audio is adding a resistor at the output connected to one of the rails, so at rest, the output stage is in class A, and goes progressively into class AB and then class B, so x-over distortion happens at a level that is loud enough to mask distortion.
 
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