Bipolar op amp retrofit with JFET preamp?

[Jim Patchell]

    It would be a lot easier to replace the little bugger (LM324) with a
TL084, or TL074, etc.  Now, if you are using the LM324 in a single supply
application, you have to be careful.  The common mode input range of the
LM324 includes the negative (pin 11) supply rail.  This means that the
input of the amplifier will still function as such down to this voltage.
If you are using split supplies (say + and - 15 volts), then there should
be no trouble, just replace it.

    Now if you insist on using the fet follower, you are probbly going to
have to use split supplies anyway in order to get the voltage range you
want.  The best was is to make a follower using a pair of fairly closes
matched jfets, using one as a follower and the other as a current source
load.  But like I said, this will require split supplies.

    The TL0x4 part, by the way, does not have a common mode input range
that will go to the negative supply rail and the outputs won't get close
either (1 to 2 volts).  So either way, I would say you are going to be
stuck with split supplies.

Scott Gravenhorst wrote:

> Is it possible (even desirable) to retrofit a bipolar opamp with
> a JFET preamp to increase the input impedance?
>
> I realize that there are ready made JFET input opamps.  What I
> am looking to do is increase the input impedance of one of the 4
> amps in an LM324.  The application is a unity gain control voltage
> amplifier whose range is from about +.03 volts to about +6 volts.
> Can I simply pass the CV thru a source follower with say a 1 meg
> gate bias resistor to ground and then into the opamp input?  I look
> at that and think that the JFET will be forward biased and won't work.
> A 1 meg resistor to -v for gate bias instead?
>
> -- Scott (who is obviously not an EE)
>
> -- Scott Gravenhorst                    | Linux Rex, Linux Vobiscum
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