Push-Pull output resistance Value

[mirror_pole]

Hello guys,

Simple question,: if i use a push pull output stage with additional cascode transistors what is the range of the output resistance i can expect? So far i mostly analysed circuits without having a feeling for the values i can expect. Is it possible to get a 600k ohm output resistance?

 

[KlausST]

Hi,

No, not exactly.

Maybe you could refine your question.

Klaus

 

[Audioguru]

Why do you want such a high output resistance? If you want a current source then make a current source.
Most outputs have a very low output resistance so that there is very little voltage loss and very little high frequencies loss.

 

[mirror_pole]

Im sorry, i forgot to mention that its part of a one stage OTA Topologie. Since OTAs have a high output resistance they act as a voltage controlled current source at the output, so i thought this value of 600kohm is somewhere normal. I build an OTA in cadence and just want to get a feeling for the values i can get. Its my second design so far.

 

[crutschow]

Since OTAs have a high output resistance

A high output resistance current-source is usually generated by negative feedback, not a series resistance.

 

[Dominik Przyborowski]

Output impedance depends to output stage current, used process, and topology.
Negative feedback decrease output impedance by open loop gain (to value which is inverse OTA's transconductance).

In general the answer for your question is "yes, it is possible" like any other value.

 

[crutschow]

Negative voltage feedback reduces output impedance, but negative current feedback increases output impedance.