Re: Capacitor-feedback for low noise

In article <dfcqe202cj3@xxxxxxxxxxxxxxx>,
Winfield Hill <Winfield_member@xxxxxxxxxxx> wrote:
>Ken Smith wrote...
>> You can also take the feedback from the output of a second stage.
>> An over simple example:
>>
>> +V
>> !
>> / R3
>> \ +V
>> / !
>> ! !/e
>> +----! Q2 <- corrected by Winfield to PNP
>> ! !\c thanks
>> ! !
>> 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.

Yes the PNP is the correct transistor in this case. The circuit is also
an over simplified version of the final version.


> 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?

No, not exactly. R2 does have to be 15 Ohms. That is correct. See
below:

> 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?


No, this where I don't see that there must be a problem.

First off, R1 = 135 Ohms does mean that Q2 is running at a large current
if we assume something like (oh, lets say) 13.5V across it, we will have
100mA flowing in Q2.

Yes that is a lot of power but that is why Gawd invented the heat sink. I
don't remember the power budget being limited. Was it? 1.35W is not that
hard to get rid of.

If we assume the JFET is running at an ID of about 10mA it only implies a
HFE in Q2 of 10 or more. Q2 doesn't need to be a Darlington to do that.


[ .. low swing on the output ..]
> Not necessarily true at the highest frequencies, where the post-JFET
> stage gain is low, yet full output may be desired.

Does the OPs case require full swing at the high end? I don't remember.

> 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.

Yes, a prpblem that bit e before LT added information about that to the
data ***.

>
>> 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.

It may be posible to arrange things so that there is a DC path and an AC
path provided the thing doesn't ever go non-linear. Harris used to make
some fast op-amps like this. They had two amplifiers inside. The specs
looked very nice but never mensioned the overload recovery which was
dreadful. We tried them in a charge sensitive amplifier the results were
not good :<


[.. JFETs make your circuit oscillate ...]
> 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.

I learned the hard way with a pair of 2SK170s leading into an op-amp.

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