[ARC5] SS Cathode modulator.

[AKLDGUY .]

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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