RE: Osc/buffer for HexFET PA?

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From: David Taylor (david@rmit.edu.au)
Date: Mon May 22 1995 - 09:09:58 EDT


JCoote@aol.com asked:

>> I am thinking of building a small VXO companion transmitter for a
>>>>shortwave portable.
>> I'm considering using commonly available IRF-511 HexFET(s) in the PA.

... and myers@bigboy73.West.Sun.COM (Dana Myers) replied:

>The input impedance of the FET is essentially a large value resistor
>(on the order of > 1G) in parallel with a couple of fairly large
>value capacitors....

I also want to build a MOSFET PA, and have obtained copies of a number of
articles by Drew, VK3XU, published in the Australian WIA Journal, 'Amateur
Radio'.

Drew's approach is to parallel a low value resistor of 100ohm across the
reactive input Z of the FET. The driver has a wideband transmission line
transformer in its collector to match the collector Z to the 100ohm
resistor. A 2N3053 driver is used for 80m. The MOSFET drain is fed DC via
an RF choke and output is taken via a 'pi' low pass filter using T68-2
toroid inductors.

It looks like Drew has designed the PA to be stable and easy to get going.

I have been wondering about, and would appreciate comments on, how a MOSFET
PA would work with tuned input and output ccts. The output would be as
described, but the input would use a 'pi' section that would resonate with
the gate capacitance. It should reduce the drive power required
substantially. Obviously neutralisation would be necessary to be stable,
and bandwidth would be limited, possibly requiring a 'drive tuning'
function.

My problem is that I have more time to dream about these possibilities than
to implement them ;-) Will keep the list posted as to successes and
failures!

72/73

David Taylor Amateur Radio: VK3JKP
Computer Centres CW Ops QRP Club (VK, No 423)
RMIT (Bundoora Campus) Internet: david@rmit.edu.au
Melbourne 3083 Australia


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