emitter resistors - how to calculate for discrete opamp.

[robmil]

Hi

I'm looking at building another discrete opamp preamp and while looking at a few designs I have seen quite different emitter resistor values ranging from - 3.9ohms on a bryston design to 33ohms on the JE990 design. Samual Groner
uses anything from 2.7ohms to 18ohms.

Is there a calculation for this resistor, can you scope the difference in values - is there a big difference between say 3ohm, 6ohms and 10ohms in the same circuit? does it make an audible difference going from 30ohms down to a lower value ?

thanks

Rob

 

[godfrey]

Emitter resistors where - input stage or output stage or...?
Either way, yes it makes a difference. Have you got a schematic for reference?

 

[DF96]

Emitter resistors depend on required stage and total open-loop gain, distortion, quiescent current etc. There isn't a quick answer to your question, except to suggest you read a good book on op-amp internal design.

 

[robmil]

thanks for the replies

It's the output emitter resistors.

 

[DF96]

Output emitter resistors help ensure a clean handover between the two halves of the push-pull. The value depends on the circuit used, and affects the correct quiescent current.

 

[CBS240]

Assuming push-pull class AB bias, one place to start might be Zout~= (re+Re)/2.
're' being the intrinsic resistance of the transisor used. There is an audiable effect in that there is an optimum value that results in lower crossover distortion.