[Elecraft] K3 keying an amplifier

[Paul Christensen]

>I don't think the diode isolation will work in this case.
>
> Scott   K9MA

If the Icom used an amp key line from an NPN transistor with an open 
collector and a high value (e.g., 10K) pull-up resistor to a positive supply 
rail, then all one needs is a steering diode added to the Icom key line 
configured with the cathode to the junction of the Icom's collector and 
pull-up resistor.  The diode's anode points to the Alpha 9500.  The K3 with 
its MOSFET output simply parallels the 9500 key line.  It doesn't care.

If the Icom is unpowered or powered, a Schottky steering diode will isolate 
the pull-up resistor while still allowing the driving transistor's junction 
to pull low enough to ground.  An unbiased or unpowered NPN open-collector 
switching transistor should not pull to ground potential if the rig is 
powered down.

The only remaining obstacle is that when the Icom is powered down, the Alpha 
9500 key line would see 10K resistance through the pull-up resistor to the 
power supply positive rail.  Powered down, that rail may be close to ground 
potential.  So, whether a steering diode alone will work is really a matter 
of: (1) the value of the pull-up resistor; and (2) whether the Alpha 9500's 
key line circuitry is sensitive to high-impedance switching.  No problem if 
the Alpha uses a photo-transistor on the input key line, for example.  But 
if say a pair of both pull-up and pull-down resistors are used ahead of a 
CMOS gate, then the value of the resistors need to be known.

If a steering diode still doesn't work, then a separate switching transistor 
can be used, using an open-collector without a pull-up resistor -- or a 
MOSFET without a pull up on the drain.

Paul, W9AC