Hello Andy,
Monday, August 10, 2015
> Forgot to mention, although I hope it is more than self-evident to
> readers here, the diode is orientated so it points 'upwards';
> allowing a positive drive voltage to appear on the gate and clamping the
> negative to ground.
> Andy G4JNT
> On 10 August 2015 at 13:31, Andy Talbot <[email protected]> wrote:
> Yes there is. Or , are: several.
> Don't drive the MOSFET directly from the IC. Instead use a
> capacitor and DC restorer circuit. That guards against both the
> driver staying high permanently if drive is removed, and also
> against IC damage if the FET fails
> Capacitor (typically 100nF for 137/475kHz) from IC output to gate
> Diode like IN914 , 1N4148 etc from gate to ground. Shunt the diode
> with a resistor of a few kohms to stop it floating
> The capacitor decouples the chip output at DC. The diode clamps
> the waveform at the cap output / gate so it's most negative
> excursion is forced to sit near enough at ground.
> Or use transformer coupling, as in my QRO 137kHz transmitter.
> http://www.g4jnt.com/137tx.pdf
> Andy G4JNT
Thanks for the replies gentlemen! Would I need to make any other
circuit changes to capacitively couple the gates? With the gates
individually driven from separate 6R8 resistors in the G3YXM circuit, do I
leave those resistors in? Or replace them with the caps and the diode
/ resistor shunts? Thanks Andy.
--
Best regards,
Chris mailto:[email protected]
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