Re: Low noise mosfet amplifier

John Larkin wrote:
On Fri, 16 May 2008 07:52:31 -0700 (PDT), Winfield Hill
<hill@xxxxxxxxxxx> wrote:

On May 16, 7:33 am, TWil...@xxxxxxxxxxxxxx wrote:
John Larkin wrote:
TWil...@xxxxxxxxxxxxxx wrote:
I've been experimenting with a high voltage MOSFET amplifier, (Vs
approx. 350V, so noyt that high), and, whilst the results are not bad,
there is an unnacceptable level of audio noise generated in the output
stage.
The output devices I'm using, (IRFBF20), are not characterised for low
noise and were chosen for their voltage rating and power handling. Can
anyone recommend a suitable low-noise alternative.
Ted Wilson
Yes, mosfets are very noisy. I don't know that a "low-noise,
low-voltage mosfet" exists, or would help a lot.
Cascoding isn't a bad idea, but use an opamp or a bipolar transistor
for lower noise. Possibly you can do the trick where the opamp V+
supply pin drives the mosfet source.
The other thing to do is close a feedback loop on the output, through
an opamp. That's a little trickier, as you have loop stability issues
to deal with.
How much output current do you need into what sort of load? At what
speed? Maybe my HV optocoupler amplifier circuit would be useful.
I want to avoid BJTs for reasons I'll not go into at the moment. I've
observed the output on a decent scope and seen no evidence at all of
any instability, but I've isolated the output stage and its driver and
that's where a significant amount of the overall noise originates.

Peak output requirement in thie prototype is about 150mA into the
primary of a push-pull output transformer, though subsequent versions
could be several times that, but still well within the MOSFET's
current capability. Frequency response of 20kHz ideally, although
could live with less for the application. Not sure how an optocoupler
would help remove the noise if it originates in the output devices
themselves.
Ted, it seems you're seeing more noise in the
MOSFET stage than theory predicts, and more
than measurements of MOSFET performance
predict. Despite your measurments, it's likely
this is due to RF oscillation. Sadly, high-voltage
MOSFETs love to make self-contained oscillators
at high RF frequencies (15 to 40MHz) whenever
they are operated in the linear mode with Vds
greater than say 25 volts, and Id above 0.25mA.

It's not always easy to see this RF oscillation
externally, unless you look at the right place
with the right tools. But it's often easy to fix,
by placing small ferrite beads on the leads.
It's possible the exact type of bead may matter,
but once I had good results with a certain bead,
I bought a pile of them and stuck with it, so I
can't advise you there. My beads have about
20 ohms of loss resistance above 20MHz.

The disappearance of your excess noise will
be your best evidence that the MOSFET was
in fact oscillating.all along. Let us know what
you learn.

Yesterday we were testing the first article of a new 8 channel
arbitrary waveform generator. It appeared to have low-frequency
channel-channel crosstalk through the power supplies of the output
amps; each channel uses a pair of THS3201's. The crosstalk behavior
was truly bizarre, between strange combinations of channels, dependent
on loading and cable lengths and such. Young Engineer spent some
optimization of a couple of Cuk/Sepic switchers (useful detour) but
things just got stranger. Old Engineer starting poking things with
fingers, and it was obvious that something was happening that the 250
MHz scope couldn't see. So we cranled up the spectrum analyzer and,
lo, a rainforest full of birdies around 700 MHz. A crosscheck with the
trusty 7104 (1 GHz analog scope) showed multiple oscillations. We have
invented the 16-opamp distributed RF oscillator.


A client once blew the EMC. So I flew out there. After having to go to their hardware store and place chicken wire over the Agilent analyzers noisy LCD (!) I found a small stand of redwoods near 260MHz. Came right out of a SMPS module. Not el cheapo but one of the major brands. Called them up. "Oh really? We'll check and call you right back". Ring, ring .... ring, ring. "Well, ahm, what can I say? S..t!" They had never noticed that the feddback stuff was singing.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
.

Relevant Pages

  • Re: Low noise mosfet amplifier
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