Using LM7805 regulators for AF / RF amps?

[ThaDood]

I've thought about using the LM**** series of voltage regulators as AM modulators, but never thought that they could be useful at RF FREQ's. FWR by Boomer:

https://www.antiqueradios.com/forums/viewtopic.php?f=12&t=425423

[Charlie_Dont_Surf]

Yes. In that circuit, its upper frequency limit is likely defined first by the internal parasitics of the silicon die, the package and the physical PCB layout than so much the internal circuit design.

Also, you folks seeking a simple AM modulator for your <15 Watt transmitter can do as shown in the applications section of the L78xx data sheet:
https://www.alldatasheet.com/datasheet-pdf/pdf/22615/STMICROELECTRONICS/L7805.html

Page 22, figure 34, “Power AM Modulator”.

The concepts of the two are similar. The regulator does exactly what it is supposed to do: regulate a voltage. In the case of these circuits, ground voltage is being brought up and down by the AC or RF input. Therefore, the user is impressing that AC or RF voltage on the output of the regulator, which is forced to follow.

[Stretchyman]

In the application you'd use it as a 'Series Modulator'. You're modulating the DC (at an audio frequency)not RF. Very linear and inefficient. Simple tho'.

Str.

[Charlie_Dont_Surf]

You're modulating the DC (at an audio frequency) not RF.

You're confused and simultaneously confusing, as usual. Let's clarify.

Forgetting about which circuit we're talking about for a second, if you modulate the DC that is fed to a final amplifier, then you will amplitude modulate the signal. That's pretty fundamental. You can do this with a Class-A modulator, such as shown here or you can do it other ways, like a Class-D modulator and LPF. Either way, you're making the DC fed to the final amplifier wiggle up and down and that produces amplitude modulation on the RF at the "wiggle frequency".

In the case of the circuit from the OP, it only produces RF (without AF modulation, unless the RF already has modulation on it) riding on the DC. In the case of the second circuit, the one that I posted, it probably won't work very well at RF because it needs an inductor that the first circuit has.