Re: Capacitor-feedback for low noise

Ken Smith wrote...
>
> Winfield Hill wrote:
>[...]
>> Now let's speak specifically. Jeroen Belleman has an input stage
>> using one of the low-voltage-noise JFETs I was talking about. The
>> issue we'll address here is voltage noise. When you add a voltage
>> feedback path to the source of the JFET, the lower divider resistor
>> contributes Johnson noise. Keeping the contribution much lower than
>> Jeroen's JFETs would involved resistor values under about 20 ohms.
>> So if you wanted a gain of 10, your feedback resistor would be 180
>> ohms, an awkwardly-low value, requiring high current at high output
>> levels.
>
> You can also take the feedback from the output of a second stage.
> An over simple example:
>
> +V
> !
> / R3
> \ +V
> / !
> ! !/e
> +----! Q2
> ! !\c
> ! !
> Q1 ! +------- to next stage
> !-- /
> -------->! \ R1
> !-- /
> ! !
> +------
> !
> /
> \ R2
> /
> !
> VX
>
> Note +V must be very quiet.

I must not have been very clear, this is the JFET configuration I was
talking about, thanks for the drawing! Except of course, you meant Q2
to be a PNP, as I marked above. To see the problem with this circuit,
consider the value for R2, and the 0.65nV spot noise of Jeroen's JFETs.
R2's noise adds (sqrt sum of squares) to the JFET noise. If we choose
say 0.5nV as the noise budget for R2, to avoid too much noise increase,
then R2 has to be no more than 15 ohms! Do you see the problem now?
To get a wide 75MHz bandwidth Jeroen specified a gain of 10x, but this
would mean R1 would only be 135 ohms! You see the problem now, right?
Well, Jeroen's problem anyway; We don't need a 75MHz bandwidth and can
use a higher preamp gain. For 100x, R1 = 1.5k, which is fine.

> In real life, the circuit ends up a lot more complicated than this
> but this drawing shows the concept. The overall feedback of the
> transimpedance amplifier trys to make the output of this section
> have zero swing so the output swing of this section only really
> has the high frequency components on it.

Not necessarily true at the highest frequencies, where the post-JFET
stage gain is low, yet full output may be desired. The gains can be
setup differently, with less JFET-stage gain and more gain in the
output stage at high frequencies, but then you end up with increased
high-frequency noise, e.g., as in the LT1028 opamp above 200kHz.

> If your input is such that the circuit always stays in the linear
> mode, the VX of the JFET stage can be servoed to bring the gate
> voltage to run at exactly zero. This lets you get a DC accurate
> and low noise input at the same time.

Yes, exactly, but Jeroen's ac-coupled transformer version can't.

>[....]
>> Note, you could create an input stage without any internal voltage
>> feedback to the JFET, getting as much gain as it gives, and thereby
>> avoid a feedback-resistor Johnson-noise problem. But unpredictable
>> gain might create other problems elsewhere in a closed-loop design.
>
> Make that "might" into "is almost certain to at some time". The circuit
> bandwidth ends up different from unit to unit and depends on temperature.

Right, that's why I like to run the JFET preamp stage in its own
feedback mode, even though it's inside a larger feedback loop.


--
Thanks,
- Win
.

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