quick high level audio amplifier question

Thread Starter

MikeJacobs

Joined Dec 7, 2019
186
So obviously a common emitter because of its efficiency is not a good idea for an output stage.
However, just a quick question as im just playing around in the simulator.

Can you get a little BJT amp common emitter to drive a low impedance load like an 4,6,8 ohm speaker?

Also, what makes an output stage an "output stage"? The fact that it can drive low impedance loads or the fact that it is powerful or both?


Thanks in advance.
 

Ian0

Joined Aug 7, 2020
1,113
Can you get a little BJT amp common emitter to drive a low impedance load like an 4,6,8 ohm speaker?
Yes, but generally output stages are common-collector because biassing is easier. A push-pull common emitter stage is difficult to bias.
Common emitter is no less or more efficient than common-collector
Also, what makes an output stage an "output stage"? The fact that it can drive low impedance loads or the fact that it is powerful or both?
It's an "output stage" simply because it's the bit that connects to the output socket.
An audio amplifier output stage must have the ability to drive 4 ohm loads, regardless of its power.
 

MisterBill2

Joined Jan 23, 2018
7,689
There are some very good audio amplifiers with push-pull common emitter output stages, even though the biasing will be a bit more complex and the efficiency may not be the ultimate.
Sometimes efficiency is not the most important parameter for an amplifier. The inherent negative feedback of the voltage follower output does tend to reduce distortion, in addition to providing a lower impedance output.
 

Ian0

Joined Aug 7, 2020
1,113
Efficiency depends mainly on class:
Class A with constant current drive <25%
Class A with an inductor <50%
Class B <78%
Class D - theroretically 100%
and very little on whether it's common-collector or common-emitter, bipolar or MOSFET.
 

Ian0

Joined Aug 7, 2020
1,113
PNP transistor with its emitter connected to the positive supply, and NPN transistor with its emitter connected to the negative supply, biassed up so that there is a small amount of standing current.
As we said before, the biassing isn't easy. . .
 

MisterBill2

Joined Jan 23, 2018
7,689
PNP transistor with its emitter connected to the positive supply, and NPN transistor with its emitter connected to the negative supply, biassed up so that there is a small amount of standing current.
As we said before, the biassing isn't easy. . .
A push-pull common emitter circuit can also have the emitters tied together and the collectors tied to the power sources. Then the bases are tied with 2 diode drops between them , and driven with a driver centered at themidpoint voltage. My Marantz 2235 receiver uses that output stage configuration. The huge problem is that the bias diodes fail open and so the transistors get really hot.

And the class of operation has not much to do with the configuration, much more with conduction angle.
 

Ian0

Joined Aug 7, 2020
1,113
Have you read chapter 29 of Self on Audio? He argues that there is no difference between common collector and common emitter.
 
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