[ARC5] SS Cathode modulator.

[Dennis Monticelli]

Well, the Safe Operating Area curve defines the safe and unsafe regions.
Secondary breakdown occurs in the linear region between hard on and hard
off.  Destruction due to avalanche occurs at high voltage, even if the
transistor is off.  The transistor is safe when it's hard on and the
collector to emitter voltage is low.

Dennis AE6C

On Tue, Oct 13, 2015 at 11:44 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> Perhaps 50 %, but I have no experience with CC modulation and
> others here are better qualified to confirm or deny. In any case,
> the practical limits for Q4 are: turned off (high voltage liable to
> destroy Q4) and hard on (0.6 V collector-emitter, with silicon
> transistor for Q4)..
>
> 73 de Neil ZL1ANM
>
>
> On Wednesday, October 14, 2015, J Mcvey <ac2eu at yahoo.com> wrote:
>
> > I think there has to be an active carrier for linear am, so the cathode
> > current current would have to be about 50%?
> >
> >
> >
> > On Tuesday, October 13, 2015 3:08 AM, AKLDGUY . <neilb0627 at gmail.com
> > <javascript:_e(%7B%7D,'cvml','neilb0627 at gmail.com');>> wrote:
> >
> >
> > I'm pretty sure the 4 transistor modulator will work, with two caveats.
> >
> > Firstly, when the cathode resistor Q4 is lightly turned on, the voltage
> > at its collector will rise to a high enough value that may destroy Q4.
> > There really needs to be a high value resistor from Q4's collector to
> > emitter to keep this voltage down to 50 or so.
> > Normally there is a 51K ? resistor fitted in the transmitter to prevent
> > cathode-heater breakdown when the relay opens the cathode circuit.
> > The article doesn't mention this resistor. Intending constructors
> > should ensure that a resistor is fitted, and fit one if not present.
> >
> > Secondly, there's a great likelihood of RF feedback with this circuit
> > when Q4 is partially conducting since RF flows in the cathode circuit
> > and Q4 lifts the cathode above ground by at least 0.6 volts (the latter
> > assumes Q4 is a silicon transistor and turned fully on). The RF will
> > almost certainly get back into earlier stages of this modulator.
> > There needs to be something like a 0.005 uF capacitor from cathode
> > to ground to decouple RF at that point.
> >
> > The capacitor together with the 51K resistor forms a low pass filter
> > and the value of capacitance can be chosen to roll the audio off as
> > a bonus.
> >
> > 73 de Neil ZL1ANM
> >
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