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Respectfully, current limiting in the final amp is exactly how
high-level plate modulation in an old AM transmitter works. Your audio
amplifier, which is required to generate a significant amount of power,
is transformer coupled into the plate lead of your final amplifier.
This allows the audio to vary the voltage and current in the final
amplifier turning it into a mixer. That's how you get the audio
sidebands plus the carrier that is a high level generated AM signal.

Flat-topping in a linear amplifier is actually nothing more than a form
of current limiting and it too generates spurious responses by making
the amplifier non-linear.

Before you do anything to modulate the current and voltage being fed
into a linear amplifier you should have a two-tone audio generator and
a spectrum analyzer of some kind to monitor your signal.

tim

On Mon, 29 Jan 2018 09:49:00 -0700
"Arv Evans" <arvid.evans@...> wrote:

> Maybe part of the point has been missed at several levels.? The
> proposed current
> limiter is not much different from current limiters in many commercial
> power
> supplies.? It is not active until some current value has been
> reached.? Up to then
> there is no current limiting or "modulation" or "compression".? Yes,
> the in-series
> current sensing resistance does allow an output voltage drop until
> voltage across
> this resistance reaches about 0.65 volts to forward bias the sensing
> transistor.
> If your power supply output is not stiff enough to hold a steady
> voltage this could
> add to the sensing resistance and cause voltage droop and in really
> bad cases
> compression of RF output.
>
> Question...is RF compression all that bad?? It is what most transmit
> ALC circuits
> do.? It would also be possible to make the proposed circuit driven by
> reflected
> power instead of current.? Then we could call it a transmit ALC to
> protect the
> IRF510(s).
>
> It would be fairly simple to add a latching function to the limiter
> design so that it
> totally shuts down output when Imax has been reached.? This is
> possible but would
> then require a RESET button or power cycling to restore power to the
> RF PA stage.
>
> It is also possible to change the proposed design to put the current
> sensing ahead
> of the control MOSFET and make that MOSFET into a voltage control
> circuit. This
> was my initial approach but the component count became just too high
> to be an
> internal modification for transceivers.? It would be more suitable for
> converting
> an unregulated power supply to a regulated one.
>
> For those who are using modified PC power supplies it is possible to
> change the
> current sensing so that the switching regulator would shut down if
> Imax of 2 to 3
> amperes is exceeded.? This approach would require power cycling to
> restore operation.
>
> Problem with using fuses is that they have a short but measurable
> heating period
> before they go open.? When they do open they spark internally for
> another short
> period.? Both the delay and the spark are potential problems for
> circuit components.
> Poly-fuse? or resettable fuse devices are intended to address a
> different issue.
> Some are equivalent of FAST-BLOW fuses, and some are just conventional
> resettable circuit breakers.
>
> Suggestion here is that "
> *if you can come up with something else that is adequate, *
> *please submit a circuit drawing and explanation of how it works*"? We
> could all
> benefit from that.
>
> Arv? K7HKL
> _._
>
>
> On Fri, Jan 26, 2018 at 11:27 AM, Tim Gorman <tgorman2@...> wrote:
>
> > A current "limiter" which modulates the current, i.e. limiting the
> > current no matter the drive level, is like applying an ALC to the
> > exciter. Improper design can result in compression of the signal
> > which can then generate spurious responses in the output.
> >
> > If the current limiter is of a "drop-out" style which requires
> > resetting then this won't happen, it will just shut down the
> > exciter. In that case, however, it doesn't provide much efficiency
> > over a plain old fuse!
> >
> > tim ab0wr
> >
> > On Fri, 26 Jan 2018 05:32:08 -0800
> > "n5ib_2" <n5ib@...> wrote:
> >
> > > One other possibility to consider to protect against overcurrent
> > > situations is to use a current limiting power supply, or add a
> > > current limiter to an existing power supply or battery.
> > >
> > > The current limiter circuit whose link is at this site:
> > > <>
> > > will go up to 2.2 amps, and includes a low-dropout voltage
> > > regulator that also provides reverse polarity protection.
> > >
> > > There is a version, in a different package, of that same regulator
> > > chip that has an added "tracking" feature - being able to parallel
> > > multiple chips to increase the available current. Details are on
> > > the datasheet in the document.
> > >
> > > Jim, N5IB
> > >
> > >
> > >
> >
> >
> >
> >
> >