On 11/6/2014 2:02 PM, David Platt wrote:
In article , rickman wrote:

I don't follow on this. How does a low output impedance drive the
current drain?

Well, the current to pull up the output against the low impedance has
to come from somewhere! It has to accounted for as part of the budget.

I never said the load is low impedance. I said the output should be
low impedance.


Consider the current used only by the amp, not the load.

You might want to experiment with using a one- or two-stage
common-drain amplifier using "electrometer grade" JFETs such as the
2N4117-2N4119 family (obsolete, but still available if you hunt around
a bit). They have extremely low gate leakage (i.e. very high input
impedance). Even with 0 volts on the gate (relative to source) they
have a low Idss, and of course you can add a resistor between source
and ground and reduce the standing current as low as you want.

I'll take a look. The input impedance does not need to be *huge*. My
concern is the drain current. This needs to be operated in an analog
range rather than digital and provide gain.

--

Rick