Steve,
It is not a good idea to leave the gate of a MOSFET with no static drain
on it.
When the board is removed from the K2 the gate at the input is exposed -
thus the 100k
The 100k between the 2 devices might be omitted if you wish.
The first link you mentioned is Eric Swartz work - the second is from
Tom Hammond (SK).
I would recommend Tom's version. The 2nd FET could be a 2N7000 if it
will handle the current and voltage that you have to deal with.
73,
Don W3FPR
On 5/16/2016 10:01 AM, Steve Kavanagh via Elecraft wrote:
I'm planning to add an external amplifier/transverter keying circuit to one
of my K2s. I found two versions on the web, both apparently designed by N0SS
(SK). The one on the Elecraft site
(http://www.elecraft.com/Apps/Amp_keying_ckt.htm ) appears to be his original
circuit, using a 2N7000 driving a 2N2222. The archive of Tom's later kit
offering (http://www.mmccs.com/mmarc/n0ss/k2_t-r_rly_drvr.pdf ) uses two
MOSFETs and is capable of keying somewhat higher voltage and current. But also
he added a 100k resistor from the input gate (8R) to ground and another from
the gate of the second MOSFET to ground.
I am trying to figure out if these two 100 k resistors serve any useful purpose
or if I can happily leave them both out of the 2-MOSFET circuit. There doesn't
immediately appear to be any need to pull these gates down as there are several
loads on the input 8R voltage (on the K2 RF board) which will do that for the
1st MOSFET, and the 1st MOSFET will pull down the gate of the 2nd one when
needed. Maybe something odd will happen on power-up or power-down?
Any thoughts?
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