[Amps] Solid state amp, damage when powered while both input/output disconnected.

[Lukasz]

Hi, I've recently been testing cheap Chinese vhf amplifiers to boost few
tens of mW of power to a couple of watts. Each came with a warning (on a
sticker, or piece of paper attached) that says "do not power up without a
load".

These are all very wideband amplifiers. Some use chips like RF3809 (GaAs
predriver for cellular base stations). Others just transistors.

I wonder what is the mechanism of said damage. The only one I can think of
is self output to input feedback and oscillation, but if this is the case
it should be perfectly fine to run one with no load as well as the input is
shorted for example.

So what is the problem here? I'm trying to use these in a half duplex
scenario where the TX path is either connected to z dummy load or to the
antenna. But the relay is not instant. There will be time when it is
floating.

So my question is. Is it sufficient to ensure there is no drive signal at
such time, or does PA power have to be off as well? I'd prefer to not have
to power the whole thing up/dplown on each T/R cycle.

In "big solid state PAs" there is usually an enable line (that switches off
the bias when required). Do I need to modify these amps to support that? Or
are the warnings overblown and I'm fine as well as the input has no drive
signal ( or is shorted to ground).

73, Lukasz