[Test-Equipment] Re: Re: tube input impedance (increasing thereof)

[J M Noeding]

On Saturday, December 14, 2002 9:53 AM "Barry L. Ornitz" =
<ornitz@tricon.net> wrote:


> There are some tricks with bootstrap circuits where the AC=20
> input impedance is raised considerably but the DC input=20
> impedance is increased only slightly.  Essentially you=20
> capacitively couple the "ground" end of the resistor to a=20
> replica of the input signal.  But for a physiology experiment,=20
> I suspect you will need a frequency response down to DC.  Even=20
> if you do not, the capacitor will have to be an exotic PTFE=20
> unit or its leakage will cause problems.

You could feed a cathode follower from a constant current source=20
(2x 2N2222) with a series cathode resistor for the grid bias,=20
but DC-stability is always a problem and usually sets
the limit for thermionic valve applications

>=20
> Your best bet is to use a solid-state instrumentation=20
> amplifier.  You can build these from conventional operational=20
> amplifiers or buy them prefabricated.  Your big concern here=20
> is what is the common mode voltage they have to withstand. =20
> Getting a terra-ohm DC input impedance with decent frequency=20
> response is not that difficult with modern IC parts.
>=20
> However with an input impedance this high, it takes great care=20
> to eliminate surface leakage effects.  In a previous job, I=20
> designed some equipment that made resistance measurements=20
> routinely up to 10E15 ohms.  Glass is no longer a suitable=20
> insulator here, and is marginal beginning at the 10E10 ohm=20
> level.
>=20
>         Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net
>=20
probably the best, but is it worth to implement in an old equipment?

73,
Jan_Martin, LA8AK=20
N4623-Kristiansand (tel.: +47-380-87178)
http://www.laud.no/la6nca/LA8AK/=20



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