Amplifier protection by nulling BIAS.

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Radio Garfield
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Amplifier protection by nulling BIAS.

Post by Radio Garfield » Sun Nov 25, 2018 10:03 am

Hi everyone.

I was wondering is it good and acceptable to protect FM amplifier, by pulling BIAS to low level, ( near zero V ) ?

Recently I was looking at the old FM amplifier boards from Elenos ( as I can remember) and noticed that they use protection method of pulling BIAS to 0V, when big SWR is present on the output. At the same time few Watts on RF input is still coming into MOSFET so I was wondering is that harmful to MOSFET, to have RF input going to device and no BIAS to gate, and therefore no output power at all.

By the way, I saw it on two boards. One is 300W ( 2x BLF 177 ) and the other one is 30W ( BLF 245 or so). Both have the same method used.

Thanks in advance for answers.

nrgkits.nz
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Re: Amplifier protection by nulling BIAS.

Post by nrgkits.nz » Sun Nov 25, 2018 9:27 pm

The driver should be dropping to zero output also, I have found on various PAs the VSWR presented to the driver goes up when the PA is not operating. There are several ways to disable the PA, one method I’m looking at is switching off the PA supply entirely, I’m using a Meanwell 600w 48v supply and it supports remote power down by applying a voltage to one of the pins, I’ll do this using the PIC that monitors VSWR, and at the same time it will disable the driver.

Albert H
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Re: Amplifier protection by nulling BIAS.

Post by Albert H » Mon Nov 26, 2018 10:13 am

I always leave the PLL and oscillator and buffer running, with the pre-drive, driver and output stage powered off. There's some small amount of RF (with the modulation on it) to confirm that the link and rig are basically working. Removing the power to my amplifier stages also removes the bias. When the VSWR issue has been fixed, the rig has to be reset (an internal push button) to re-start the amplifiers. On reset (or power-up) the bias comes up more slowly than the supply (deliberately), and the pre-drive output rises slowly, so the PA doesn't get hit with full power drive without the bias being set correctly - I destroyed a few FETs in the early days (really overrunning pairs of BLF177s) by applying drive too soon, before the bias was right.

If Elenos have found bias switching to be effective, then their design of PA is quite different to most. FETs I've worked with frequently provide lots of gain at zero bias (Class C operation), and one series of my SRF1616 PAs used RF-derived bias to just about switch them on - moving their output conduction angle somewhat, getting more power out.

Finally, it occurs to me that Elenos might have a stability problem with their PAs - switching the bias off would ensure that the PA didn't "hoot"!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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Radio Garfield
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Re: Amplifier protection by nulling BIAS.

Post by Radio Garfield » Mon Nov 26, 2018 4:02 pm

All right then, thank you very much for answers. So, conclusion is that it should not be done that way. I also have found few other articles on the internet about this and it seems that its wrong way.

Many thanks. :tup

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