Bootstrapping the input or output impedance of OPA

[Patrick Yang]

Hi guys,
I have some questions on bootstrapping technique in improving the OPA input impedance, as the figure shown below, how to explain the input impedance is bootstrapped by the CF? Thanks in advance!

 

[Audioguru]

The opamp has negative feedback that produces an output level exactly the same as the input level.
Cf feeds the output level to the junction of R1 and R2. Then R1 has the same signal level at both ends then has no current in it so the input impedance is extremely high.

 

[LvW]

Perhaps it is worth to mention that without Cc (dc coupling of Vin) the whole bootstrap circuitry is not needed (makes no sense).

 

[FvM]

Perhaps it is worth to mention that without Cc (dc coupling of Vin) the whole bootstrap circuitry is not needed (makes no sense).

Yes. But if you imagine Cc as part of the signal source, the bootstrap construct becomes meaningful. It's e.g. used for some capacitive sensors.