Setting BIAS for PCS 300w amplifier

[Elopid]

Hi all, I am almost ready to test some new boards I recently got hold of from PCS electronics with 300w transistor device 'MRF151G'.
I want to make sure everything is set up perfectly before I run any RF into them. I would like like to clarify exactly where to place my positive test probe on my multimeter and exactly what voltage range of BIAS it requires. I will test the BIAS voltage before I add the transistor.
I'm also planning to initially switch it on with a lower voltage (approximately 44v) so will that mean the BIAS voltage needs completely resetting again when I up it to 48v? I'm glad they are solderless boards so the transistor can very easily be removed.

Many kind thanks in advance, EloPiD.

[s2000]

Hi there Elopid, I am by no means an expert on this but I believe you need to have the mosfet installed in the circuit to set up the amplifier properly. You would connect an amp meter in series with the power supply and slowly turn up the bias until it is drawing the correct amount of quiescent current (IDQ). From the mrf151g datasheet this appears to be 500mA. No rf drive is needed for this stage.

There's a good video on you tube...

https://www.youtube.com/watch?v=9ys8LsHGrUk

If any experts see anything wrong with this method please feel free to correct me as I am new to mosfets. I set up a blf244 a while ago and noticed it would only start drawing current once a certain voltage had been reached?

[Albert H]

S2000 - that's right. You have to get past a threshold voltage before the device will begin to conduct. It's sensible to turn the bias up very slowly, as the onset of conduction is very "all or nothing"!

[s2000]

Nice one Albert, good to have you back

[Elopid]

Thanks for your replies s2000 and Albert H,

Sorry for the late reply I've only just found the chance to log on. I appreciate all your help. Elo

[rigmo]

The great simple one...