Audio Amplifiers and my problem?!!!

Thread Starter

vngn10

Joined Jun 5, 2021
5
Hi everyone, can I ask about what class is this circuit? I thought it was class AB amplifier but then I read more things and I started confusing about class B amplifier and class AB. Thank you and have a nice day!!!
1622900330320.png
 

MrChips

Joined Oct 2, 2009
23,515
I am going to hazard a guess and say Class B.
The opamp supply current is 2-3mA. Hence Q1 and Q2 base bias is between 400-600mV.
 

BobTPH

Joined Jun 5, 2013
3,308
I was going to suggest they were on at 600mV, but then I realized they are Darlingtons, so class B. Without DC feedback, I think this will have terrible crossover distortion.

Bob
 

Ian0

Joined Aug 7, 2020
2,189
It's a dreadful design - there's nothing to stabilise the bias as the temperature changes. If it were to be biassed into Class AB, it would blow up. It's only stable against temperature changes in temperature if it's firmly in class B, in which case it's quality would barely be good enough for announcing "This vehicle is reversing".
The typical LF351 supply current is 1.4mA, so only 280mV across base and emitter, it appears to have been designed so that it doesn't get into class AB even with the LF351 supply current at the max. value on the datasheet of 3.4mA.
 

Thread Starter

vngn10

Joined Jun 5, 2021
5
It's a dreadful design - there's nothing to stabilise the bias as the temperature changes. If it were to be biassed into Class AB, it would blow up. It's only stable against temperature changes in temperature if it's firmly in class B, in which case it's quality would barely be good enough for announcing "This vehicle is reversing".
The typical LF351 supply current is 1.4mA, so only 280mV across base and emitter, it appears to have been designed so that it doesn't get into class AB even with the LF351 supply current at the max. value on the datasheet of 3.4mA.
Thank you sir, it really helps
 

bertus

Joined Apr 5, 2008
21,364
Hello,

I have seen simelar designs.
In those the resistor on the output of the opamp (R6 in the OP schematic) was much lower, to force current in the power lines of the opamp.
In the designs I have seen the value was between 47 and 100 Ohms.

Bertus
 

Thread Starter

vngn10

Joined Jun 5, 2021
5
Hello,

I have seen simelar designs.
In those the resistor on the output of the opamp (R6 in the OP schematic) was much lower, to force current in the power lines of the opamp.
In the designs I have seen the value was between 47 and 100 Ohms.

Bertus
My professor took this for us as a term project for final. And he didnt teach us about class of amplifiers so that's why I'm confuse a little bit
 

Ian0

Joined Aug 7, 2020
2,189
Hello,

I have seen simelar designs.
In those the resistor on the output of the opamp (R6 in the OP schematic) was much lower, to force current in the power lines of the opamp.
In the designs I have seen the value was between 47 and 100 Ohms.

Bertus
But at idle, that doesn't force any current through the power pins. The output is at zero, so no current flows through R6 no matter how low it is.
I've seen it with a bias pot between pin 4 and pin 7 of the op-amp so the current could be adjusted to bias the output stage into conduction (but with no thermal compensation).
 
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