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Please help design discrete JFET preamplifier

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magic:
I only suggested it as a way to get flat input impedance, not more bandwidth.

And beware that I can't find links so it could be my own invention never tested in practice. Beware that I also invented a perpetual motion machine at age of seven ;)

Someone:

--- Quote from: Yansi on March 06, 2020, 09:35:05 am ---
--- Quote from: Zero999 on March 06, 2020, 09:24:29 am ---Designing discrete amplifiers is fun but why not use a J-FET op-amp? What's wrong with the TL072?
--- End quote ---
Noise. I need to work with signals 20mV full scale with this preamp.

Also, TL072 is 3MHz GBW, so with gain of 11, we are talking 270kHz unity gain, if I am not mistaken.
--- End quote ---
Ok, so what noise are you getting from the circuit you are using? What is the source input impedance? There are many opamp choices applicable.

Kleinstein:
The TL072 is not very good noise wise and DC performance is also limited. There are considerable better modern OPs available (e.g. OPA140, OPA827, ADA4625 and similar). These also have pretty good DC performance.  With the right JFETs one may get lower noise with discrete version, but the BF245 shown in the plan would already be higher noise.

To also get good DC performance with discrete JFET input, I would consider a 2nd JFET as current source or in a differential configuration and combining it with an OP. There are few different options, depending on wether DC performance or white noise is more important.

1 M impedance up to 100 kHz would need a very low input capacitance (less than some 1.5 pF !) - this would be main difficulty. Chances are one would need drain side bootstrapping or at least a cascode input stage.

Yansi:
TL072 was given due to being already in the BOM and available. No interest in switching to other types.

Who did say BF245?  My schematic says 2SK170, which is I think a pretty low noise floor FET.

1M is critical especially at low frequencies, compensated dividers are what is there to fix the HF response.

DC performance is not critical, I am not interested in DC signals.  20Hz or so cutoff is fine. All following and preceding stages are AC coupled anyway.

Kleinstein:
The BF245 was is he first plan shown. The 2SK170 is in deed low noise, but also relatively high input capacitance.  So the input divider has to take the capacitance into account.  Other types can have lower capacitance (e.g. about half with 2SK3557) .

If really this noise noise level is needed, an OP is indeed not a real choice.

With discrete JFETs one kind of has to adjust the DC operating point. One could add a automatic DC adjustment from the output: So have an OP do extra DC feedback. So one can get away without extra loading to the input, more like virtual AC coupling at the output side.

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