[Amps] BLF188xr amp

[Manfred Mornhinweg]

Hi Chris,

> They brag about 65:1 swr survivability, but this is pulsed. 

It's thermally limited. Basically this rating means that the FET can avalanche 
safely if a high SWR results in high voltage pulses, and that it can handle high 
current pulses too, without the bonding wires fusing. But certain high SWR 
conditions result in a very high dissipated power, and in this regard the 
survival of the FET depends on the total transient thermal resistance, and the 
time the high SWR condition lasts.

So, the 65:1 SWR survivability is very welcome, because it gives your protection 
circuit enough time to react. But it doesn't replace the protection circuit!

 > Clearly a post
> LPF reflectometer ref sample is necessary to shut it down with a poor match,
> say above 2:1. But I see another pitfall; the switched LPFs being set to the
> wrong band.
> 
> So, is the best solution adding another reflectometer after the output
> transformer and prior to the filter to protect against that?

That's exactly what some commercially made amplifiers do. For example, download 
and read the service manual for the Kenwood TL-120 amplifier. That's a 100 watt 
add-on amplifier for the TS-120V QRP rig, made 35 years ago. That amplifier has 
a forward/reflected power sensor after the filters, and an additional reflected 
power sensor (no forward) before the filters. The service manual is explicit 
about the protection circuit.

It's probably the simplest way to provide full protection.

 > With an
> expected third harmonic being 10-12 dB down at this point, the threshold
> might need to be high, like 5:1 ...

Yes, but the SWR into the wrong filter is sky-high! So you don't need much 
sensitivity there.

It's a good idea to couple a novel design feature to this protection circuit: A 
robotic arm that comes out of the amplifier and slaps the operator, whenever he 
selected the wrong band!

Manfred

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