Class (A)B output stage: why 2 emitter resistors, why not 1?

[deleveld]

Hi everyone,

Just a quick question that i havent seen addressed elsewhere.

Consider a class B output stage for a design where simplicity is very important and and quality isnt. Every design I have seen uses a separete emitter resistor for the top and bottom half. Is this really necessary?

Wouldnt just using 1 for either the top -or- bottom half be sufficient? The temperature stabilization issue concens the total emitter resistance so that shouldnt be a problem.

Any comments?

thanks for the discussion...

Doug

[richie00boy]

If you only put one in the top half, where does the degeneration/feedback for the bottom half come from?

[deleveld]

If you only put an emitter resistor in the top half then the feedback point can come from directly the emitter of the bottom half. That shouldnt be a problem, should it?

It does make the gains of top/bottom asymmetrical, but we are considering a design with emphasis on simplicity and not on quality.

Doug

[Wavebourn]

Narrow mind think that the more symmetrical the amp is the better it works.
Narrower mind think "Because it is always drawn such a way".
Narrowest mind don't think, it copies designs of guys who give interviews.

[GK]

I've done this before, but not in HiFi amplifiers.

[deleveld]

Did you only use emitter for top or for the bottom output transistor?

I assume you had no particular problems with only one emitter resistor?

thanks,

Doug Eleveld

[ilimzn]

Regarding the use of a single emitter resistor, as long as your bias generator and drivers do not have any connection to the output point, it will work fine regarding the bias itself. In general, the bias generator will satisfy this, and for the drivers you need the modified EF version (where the emitters of the drivers connect to each other through a resistor, and do not connect to the output).
However, regarding performance, only one side will see degeneration WRT the output NFB pick-off point so you will get elevated second harmonic - which might even be received positively by the listeners
I have seen this done on a MOSFET amp but with the intention of matching the transconductance of the N-ch and P-ch FET better - the resistor was in the source line of the N-ch MOSFET.

[deleveld]

Thanks very much for the info.

My intended application is a high quality amplifier but I am not so worried about the increase in 2nd harmonic distortion because the amp has a H-bridge output stage. I think the bridge output stage will cancel at least some of the 2nd harmonic.

I think you are right about the MOSFET output stage. I could certainly see using unequal resistors there in an effort to equalize transconductance of the P and N channel devices.

Doug

[ilimzn]

Originally posted by deleveld
I think you are right about the MOSFET output stage. I could certainly see using unequal resistors there in an effort to equalize transconductance of the P and N channel devices.
Doug

It's not a perfect way to do it but you can tailor transconductance to something more symetrical for a load and output power it is most likely to be used in.

Regarding H-bridge, yes this will do. However, since youa re going to have a larger voltage drop from one power rail to output than from the other power rail (Iout x Re), you may want to make your ground reference truly floating and offset it a bit to get maximum output voltage.

[deleveld]

Yes I agree that symmetrical clipping is a good idea!