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Re: Gate resistor for lateral power MOSFET?

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>Early testing looks very good!  Stability seems fine with a few different
>loads, distortion (at least as viewed on the 'scope) is negligible, and
>BW and slew rates are at least twice as fast as I need.  This is with gate
>resistors around 240 ohms (no beads, at least not yet).
>
>It would still be nice to have a better understanding of why these
>resistors are needed.  For now I'll content myself with a fuller testing
>with more reactive loads.  Hopefully that will shake out any weaknesses.

My recollection/belief is that you're dealing with the equivalent of
the "grid stopper" resistors that were used with many vacuum-tube
amplifier circuits.

MOSFETs, like tubes, have a significant amount of capacitance at their
gates/grids (both gate-to-source, and the Miller capacitance).  The
leads/traces connected to the gates have a non-zero inductance... so
you've got a series-resonant circuit, with the gate right in the
middle of it (maximum-voltage-excursion point).  This can be a recipe
for instability, up to and including parasitic oscillation at the
resonant frequency.  A term I recall from the vacuum-tube days was
"snivets" (these were thin vertical lines on a TV screen, caused by
parasitic oscillation of this general sort in the output tube).

Putting a stopper resistor in series with the gate or grid (ideally,
close to the device) both rolls off the bandwidth (resistor R
interacting with gate C) and kills the Q of the L/C resonant circuit.
It quiets the shrieking and screaming no end and can help keep the
Magic Blue Smoke where it belongs :-)

Anyhow, that's my (possibly-faulty) recollection and analysis.

I had to deal with a somewhat-related problem some years ago,
trouble-shooting a simple twin-audio-tone oscillator designed for
doing IM analysis of single-sideband ham transceivers.  Very simple
circuit, which a guy built based on an article in QST - two twin-T
audio oscillators using 2N2222 transistors.  It did oscillate, but the
frequencies and amplitudes were unstable and it'd misbehave if you
brought your fingers near the circuit.  The owner couldn't figure
out the cause, and gave me the box as a "use this for parts if you
like" gift.

A bit of poking around with a spectrum analyzer showed that this
"audio" oscillator was breaking into RF parasitic oscillation at
upwards of 100 MHz!

Sticking a ferrite bead around the base and collector leads of each
2N2222 killed the Q of the resonances and fixed the problem.  A
resistor of a few tens of ohms in the base would probably have done
just as well.

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Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-28 17:46 +0000
  Re: Gate resistor for lateral power MOSFET? John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-28 11:35 -0700
    Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-28 22:22 +0000
      Re: Gate resistor for lateral power MOSFET? John Larkin <jjlarkin@highlandtechnology.com> - 2016-09-28 17:20 -0700
  Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-28 19:16 -0700
    Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-28 23:07 -0700
      Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-29 12:51 -0700
  Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-28 19:52 -0700
    Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-28 20:13 -0700
      Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-29 12:52 -0700
    Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-29 16:41 +0000
      Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-29 18:52 -0700
        Re: Gate resistor for lateral power MOSFET? boB <boB@theresnoplacelikehome.com> - 2016-09-30 00:54 -0700
          Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 15:57 +0000
        Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 15:53 +0000
          Re: Gate resistor for lateral power MOSFET? George Herold <gherold@teachspin.com> - 2016-09-30 09:19 -0700
          Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-09-30 20:04 +0000
            Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-09-30 14:07 -0700
              Re: Gate resistor for lateral power MOSFET? dplatt@coop.radagast.org (Dave Platt) - 2016-09-30 14:52 -0700
              Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-01 17:29 +0000
                Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-10-01 12:19 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-02 02:59 +0000
                Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-10-01 21:10 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-03 00:20 +0000
                Re: Gate resistor for lateral power MOSFET? Winfield Hill <hill@rowland.harvard.edu> - 2016-10-02 09:43 -0700
                Re: Gate resistor for lateral power MOSFET? Frank Miles <fpm@u.washington.edu> - 2016-10-03 00:33 +0000
          Re: Gate resistor for lateral power MOSFET? Phil Allison <pallison49@gmail.com> - 2016-09-30 22:40 -0700

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