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October 13, 2011


george storm

Regarding bootstrapping the input resistance: when do such corcuits have a known and non-removable input resistance? Otherwise we do better either to buy a large-value resistor or to bootstrap using a follower with a potential divider at the output.

BTW, the AC-mode time constant of most 'scopes is set not by technology, but so that the user does not perceive an excessive delay before the signal settles. (Sorry to be a spoil-sport here)

John Dunn

That R1 was an RN55 or RN60 style (I forget which) and that style of film resistor didn't come in values of high enough ohms for the purpose at hand. Bootstrapping was the answer to that.

Fred Floru

Below is a link to an IC that uses a similar technique to boost just the common impedance of a differential amplifier.


Sergio Franco

Rewriting as Rin = R2/(R2/R1-R3/R4) better evidences that Rin depends on the difference between two resistance ratios. In applications where these ratios are designed to be very close to each other, a slight mismatch may alter Rin significantly, making it even infinite or negative, depending on the direction of the mismatch. So, even though the circuit is elegant in theory, one has to be aware of this feature when applying it in practice. Regards,

Sergio Franco


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