[Amps] BLF188xr Amplifier
nm3g at triad.rr.com
Sat Oct 17 09:33:20 EDT 2015
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
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
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.
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