Long Tailed Pair - Swapping BJTs for FETs

[aziltz]

Long Tailed Pair - Swapping BJTs for FETs

Anyone have any insight on this?  I want to use a LTP as a Clipper with jFETs.

http://en.wikipedia.org/wiki/Differential_amplifier#Long-tailed_pair

[davidallancole]

You would just put your JFET in and adjust the current source (or resistor on the source side) so that the current is balanced between each JFET.

Do you have a schematic you are wanting to use?

[aziltz]

You would just put your JFET in and adjust the current source (or resistor on the source side) so that the current is balanced between each JFET.

Do you have a schematic you are wanting to use?

Kinda, but the current through the BJT's is intentionally mismatched. 

I'm also comparing it to the Discrete OpAmps in the Boss BD-2.

[davidallancole]

What is this schematic snippit from?

If you follow the LTP's like in the BD-2, they have a ballanced current and they use JFETs as well.  Which one would you prefer to follow more?  The schematic snippit or the BD-2?

[aziltz]

I'm going for a FET version of the mismatched one above, using it as a gain stage/clipper.  The LTPs in the Boss design had the same biasing topology as the text-book balanced BJT versions, so I was wondering if swapping the semiconductors would give similar results or if the resistors would have to be changed.  I suppose I could analyze the quiescent setup of the BJT and adapt it to a FET setup.  I wasn't sure if there was an easier way or if some had already done that.

[davidallancole]

With the text book LTP, you can switch the devices and all you would have to do is adjust the single source/emitter resistor.  You could probably do that with the mis-matched setup as well.  If you could figure out what the dc voltage is at the R11/Q3c junction, I am sure you could put in the FETs and adjust R10 to get the same voltage.

You might have to change the R5/R12 reference points to ground instead of half supply.

[aziltz]

Thanks for your help

[davidallancole]

Not a problem.  Let us know how it works out.

[JDoyle]

If you bias the gates to 1/2V+ (in a single supply circuit, ground for dual supply) you should be able to swap BJTs out for JFETs without major problems. The minor problems are always going to be there, namely: lower gain, higher offset and worse CMRR and PSRR.

But it should work fine if the gates are biased correctly...