Re: Capacitor-feedback for low noise
- From: Winfield Hill <Winfield_member@xxxxxxxxxxx>
- Date: 2 Sep 2005 05:52:18 -0700
Zigoteau wrote...
>
> I've looked at your webpage and at the paper whose reference you
> gave - they're both for voltage amplifiers.
Yes, but you want a voltage-amplifier input stage, see below.
>> Hola now! I have no time now to work out if your line of reasoning
>> is correct, alas. Can we pick this up again next Wednesday?
>
> I would appreciate your comments on the idea that the best
> configuration for a transformer-feedback transimpedance amplifier
> using low-noise op amps would have two stages, the first being a
> current amplifier with the transformer primary connected between
> the output and inverting input.
OK, let's speak generally. Transimpedance amplifiers are made with
high-gain high-impedance opamps, etc., inside them. If you can find
an ideal JFET opamp to make your transimpedance amplifier, fine. As
I've discussed many times here on s.e.d., there's a straightforward
way to analyze the issues involved in selecting parts. Sometimes
you're forced to use a kind of "composite opamp," which has several
stages in series. The first stage is optimized for low input noise,
high input impedance, and provides some voltage gain (note, it's a
voltage amplifier), the later stages provide wideband voltage gain.
For the input stage, you're trading off the primary factors of the
JFET's gate-leakage current, voltage noise and capacitance, as you
select the JFET. Sometimes you're pushed toward an input stage
with less voltage noise than is available in commercial JFET opamps
(low voltage noise and low capacitance are two very critical aspects
for current amplifiers at higher frequencies, where they combine to
create a current noise, i_n = 2pi f e_n C).
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. Jeroen elegantly solved this vexing problem with a feedback
transformer, http://jeroen.home.cern.ch/jeroen/tfpu/LNA.shtml
When using such JFETs I have solved this issue with a higher gain
of 100, which allows using reasonable-value feedback resistors,
like 20-ohms and 2k. It also allows DC gain. But my application
did not require the high 75MHz bandwidth that Jeroen's did. If he
had used a gain of 100, he'd have suffered a reduced bandwidth.
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.
--
Thanks,
- Win
.
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