[ARC5] SS Cathode modulator.

[Kenneth G. Gordon]

On 12 Oct 2015 at 20:49, Dennis Monticelli wrote:

> 
> This four transistor circuit from 73 is very primitive. As others have noted
> there is poor control of the operating point and non-linear conversion of voice
> to plate current.  In addition the output transistor would need to have
> excellent SOA in order to survive because the base is looking into a relatively
> high resistance.  However, if built I do believe it will function (as long as
> it survives). 

Hmmm...very interesting....

> 
> There is a better way.  Years ago I designed a SS circuit to function as a
> cathode modulator for a typical class C stage.  This circuit has a well-defined
> operating point, SOA protection, and uses feedback for linearity.  The circuit
> operates as a V to I converter.  Because it does not simulate a voltage
> controlled resistor like the other circuits, neither the grid voltage nor the gm
> of the power tube has any effect on the linearity or tube operating point.  The
> V to I converter represents an ideal current source that accurately follows the
> audio input regardless of cathode voltage, while the action of the grid drive is
> to simply chop the tube's conduction at the carrier rate.  The only place where
> non-linearity enters the picture is the plate/screen current split, which is not
> perfectly constant with cathode current level.

You wouldn't happen to have a schematic of this device, would you?

Op-amp V-I converters are fairly common and routine these days.

I wouldn't be sure how to modulate one, though.

Ken W7EKB