[Amps] BLF188xr Amplifier

[Rick]

Good morning Chris,

... But if the wrong band LPF is selected, this does not protect the fets. ...

  
To "simplify" your protection you can

 1.   install reflected power detection before and after the low pass
    filter,
 2.   Install output  bandpass/terminated band stop filters with
    sufficient power handling to dissipate full power on the wrong band
 3. Install input AND output bandpass/terminated band stop filters.
 4. Install a microprocessor to read the input fundamental frequency,
    and lock out the amplifier if the input RF doesn't match the
    amplifier band settings. Lot of microprocessors are fast enough now
    to directly count above HF, and this solution can alert you (via LCD
    display, computer window, bright red "OUT OF BAND" light), and help
    you learn what not to do.

Option 1 has been previously discussed, along with monitoring the 
effects of high VSWR (over voltage, over current, over temperature).

Option 2 is going to be a big filter solution. While massive overkill, 
brute force does have it's simplistic aspects ... in that there is 
little to go wrong. Did I mention it will be big?

Option 3 will reduce your harmonic energy content into the amplifier, 
and will absorb any reflected harmonic energy from the input of your 
amplifier, presenting a better 50 ohm match to your transmitter. If you 
also insert an appropriate sized attenuator on the input of the 
bandpass/terminated band stop input filter, you will prevent overdriving 
your FETs, at the cost of dumping the extra power as heat. You will need 
to narrow your filters to a single band per filter set.

When you switch the input and output bandpass/terminated band stop 
filters together, the input bandpass/terminated band stop filter must 
handle full out-of-band power ... but with an attenuator in line, this 
means less than 100 watts to handle (depending on your drive level 
requirements).

Option 4 may not appeal to those who believe amplifiers should not have 
computer control. I offer it as an alternative to doubling the size of 
your amplifier to accommodate the additional filter elements.

While this adds some additional components, it certainly meets your 
requirements of protecting against selecting the wrong filter.

Good luck with your project, and enjoy the journey.

73,

Rick
NM3G