Dual Gate MOSFET Revelations from W7ZOI

Some days I feel like I can’t see the forest for the trees. After submitting my last dual gate MOSFET experiment to the EMRFD group for review, I got a nice note from Henning suggesting that I needed to bypass gate 2 directly. Of course, like a three-year old who has to constantly ask “Why?”, I questioned how necessary it was to get the bypass capacitor right at gate 2. Wes, W7ZOI kindly gave me an education on the pitfalls of an unbypassed gate 2:

Yes, it can make a profound difference. The capacitor really need
to be right at the gate. If you don’t bypass gate 2, you will dump
the noise voltage from the resistor driving gate 2 right into the
FET. With 100K, that voltage will be high. This could really trash
the amplifier NF performance, turning a stellar performer into
something that is pretty bad for NF. But use a really good bypass
cap that is going to do a good job at VHF and UHF. A 1000 pF with
really short leads is good. This is a good place to use a chip cap
even if you are among the folks who don’t like SMT parts.

Many thanks to Wes for putting up with such a stupid question. One thing that I really appreciate about his writing is that he can explain things to you in a way that make it seem completely obvious, yet without talking down to you. He also goes on to explain the rationale behind two commonly seen resistors in dual gate MOSFET amps:

There are two resistors that we often see in the drain circuits.
One is a swamping resistor that is directly across the primary (drain
winding) of the output transformer. This merely constrains the gain
to a smaller value. It also serves to provide a clean output R.
This loading R will help to stabilize the amplifier, but will not do
a lot at UHF.

This is an area worthy of further investigation. Specfially how much it degrades IMD and NF compared to the benefits

The other resistor is one that is right in series with the drain.
This is often in the region of 20 to 100 Ohms. This serves to
provide a wideband load that kills UHF oscillations. The utility of
this resistor can be studied with a microwave stability analysis,
easily done with numerous programs, or from scratch if you are
willing to do some analysis. You will see examples of the small
drain R in some of the amplifiers in emrfd.

This looks like a good project for LTSpice. I figured that looking into these MOSFETs would be pretty straightforward once I understood the biasing, but this investigation is leading into all kinds of interesting side roads. I can tell that I have my work cut out for me!

Leave a Reply