AoE photodiode boostrap

[nihial]

In the AoE in the photodiode section there's a circuit where a photodiode is bootstrapped by a jfet. It is mention in the text that there is a emitter follower not shown for simplicity but I don't understand where you put it. I'm trying to upgrade a frontend of a photodiode stage I'd like to test the circuit with that mentioned emitter follower especially if it improves the overall circuit.



Thanks in advance !

[PartialDischarge]

Turn to the next page and there you have your emitter follower in 8.83!

[David Hess]

Adding an emitter follower in series with the source of an FET is done because the transconductance of the FET is so much lower.

[nihial]

Turn to the next page and there you have your emitter follower in 8.83!

Oh my god I feel so dumb to not have seen it

[nihial]

Adding an emitter follower in series with the source of an FET is done because the transconductance of the FET is so much lower.

Thank you for the explanation

[David Hess]

Adding an emitter follower in series with the source of an FET is done because the transconductance of the FET is so much lower.

Thank you for the explanation

I should have also said that it is very common circuit configuration when you want an FET source follower to have a gain close to 1 and the ability to drive any significant load resistance.  The impedance buffer for oscilloscope inputs and active probes almost always uses the same configuration.

An alternative with better performance at low frequencies is the FET equivalent of a Sziklai pair with the input transistor replaced with an FET.  Local feedback keeps the gain closer to 1:

https://en.wikipedia.org/wiki/Complementary_feedback_pair