Mike Keitz wrote:
> You can get around the bias current limitation by providing another path
> for the bias current to flow rather than through the input terminating
> resistor.

Nobody EVER does this kind of thing at the input stage of
oscilloscope-type instruments. It's important to devise a circuit that
doesn't send a voltage or current back into the source; you risk
upsetting the circuit being tested or even stopping it working.

Even worse is a 'scope that lets digital switching signals leak back
into the circuit being scoped, which can upset the circuit very
seriously and produce an entirely misleading display on the 'scope
screen.

> I'd be tempted to try JFET source followers between the input and the
> voltage gain stage.
This is commonly done. Such a stage attenuates signals travelling in the
reverse direction - that is, back into the source. Very beneficial.

>  DC offset may go out the window, etc...
Not if it's done properly. There is a circuit that uses a matched pair
of high-transconductance junction FETs (both in the same package) that
has low voltage offset, low tempco, unity gain, wide bandwidth, high
input impedance and low output impedance. In my opinion God sent this
circuit for jobs like this. Or was it Siliconix? Anyway, the FETs are
used in series; the signal goes into the gate of the top FET, and the
bottom FET (powered from a -5V rail) acts as a matched current source
for the top FET.
The upper FET has a 1 Megohm (1% tolerance) resistor from gate to
ground, so the input resistance of the stage is 1 Megohm exactly. A X10
scope probe can then be used if the capacitance of the FET input circuit
is adjusted to be within the range of adjustment of the probe.
The remaining problem is then overload protection, which isn't hard.
And, of course, stopping it oscillating - this circuit has a very wide
bandwidth; I've used it up to 100MHz.

Siliconix used to make these FETs with numbers like E300 and E301. I
think it's called TEMIC or something now... Anyway, it's a great
circuit.

John Blackburn,
South London UK.