how much bias current for the IRF520 mosfet?

[groundwire]

i did a full alignment on my galaxy dx86v using the dx979 service manual and it came out perfect. my only issue now is that i totally forgot that the driver and final bias current on the 979 radio (50ma/100ma) is not the same as the 86v, and of course i didnt note what the 86v bias setting was before i changed it . so now my 86v does 25w pep instead of the 45w its supposed to do. sooo, what is the bias current supposed to be for the IRF520 mosfet transistors? My google investigations led me to think 100ma for both driver and final will get me back to the 45w its supposed to do, or 100ma for driver & 200ma for the dual finals? or if you know of other radios that use the three IRF520 driver final setup that has an accessible service manual that would work as well. any help would be great. worse case scenario, my 86v output transistors should out live the radio at 25w pep lol.

 

[nomadradio]

When RCI was having warranty trouble with IRF520s blowing up in their 2995 radios, they recommended 10 mA for the driver and final both.

You would think that 50 or 60 mA would be plenty safe, but YMMV.

73

 

[loosecannon]

ranger and galaxy really did the CB world a disservice when they wrote in their service literature that the new MOSFET transistors were to have their bias set by adjusting gate voltages.

without a current meter on the input positive lead going in to the radio, you won't know when you have sent the quiescent current sky high by trying to get that last tenth of a volt on the gate.

It can happen on any radio with mosfet finals and drivers, and i found the DX86V to be quite sensitive to runaway.

I think more discussion should be had about what the quiescent current for each transistor in the chain should be, but a good starting point is 50mA per device.

set the radio to SSB, mic gain all the way down, current meter in line with the VCC+ coming in to the radio.

key the mic and adjust both the driver and final bias trimpots until they are drawing no current (minimum).

now adjust the driver bias control up until you see it pulling 50mA into the radio.
now adjust the final bias control until you see 100mA being pulled in to the radio.

if you had a second final, you would then adjust that trimpot until you saw 150mA.

I would love to hear what others have to say with regard to that 50mA number, as i will admit it's a bit of a guess.

just for fun, you can then hook your DC voltmeter up to the gate of the driver or final, and turn the pot up looking for that 3.5-4 volts. If you're watching your current meter while you do this you will watch it jump WAY up at some point.
LC

 

[CommandoTech]

I wonder why the manufacturers didn't incorporate a feedback circuit similar to the Cobra 148GTL driver stage? And I agree with loosecannon on watching your current draw on each device. If you go over 100 milliamps idle current on mosfets things can go badly quick.

 

[nomadradio]

I wonder why the manufacturers didn't incorporate a feedback circuit similar to the Cobra 148GTL driver stage?

The IRF parts don't have enough gain to justify reducing it with negative feedback. Even if it would serve to clean up the amplified signal, the penalty on power gain would not be popular.

73

 

[loosecannon]

so commando tech, you go with 100mA per device quiescent current?

what about you Nomad?
LC

 

[CommandoTech]

I would stay somewhere in the vicinity of 65-75 ma max 100 is the danger zone.

 

[Handy Andy]

When you hit 100mA - the Zeners (if they're still working) are going to be the next thing to screw up that stage...

Get's too close to that 5V "crowbar" moment.

IF you folks remember this thread - I was corrected to the fact that many a radio might not use Zeners...

https://www.worldwidedx.com/threads...virtually-no-output.262093/page-3#post-767029

Read that at your leisure...

 

[Handy Andy]

But you really should think thru what would be a good BIAS drive level for the MAJORITY of the MODE(s) you use...


Use AM, keep the mA kind of low, below 50mA for both - so the Envelope can SWING too...

Yes, Swing...

The Zener's, if used, at 50 mA and above - are only leaving a small window - which limits the RF pushing the Gate conduction upwards - like what others do with Volting - Gates can exhibit the same thing. They just need a presence so if you're an AM'er and don't really care for SSB - then LOWER the Gate mA drive - let the RF push it and switch them on in their linear region.

If you don't understand any of this - then use the "Good Book" you've got and run that with those values, you posted above and gripe later...

Why do I say what I do above?

Well, Connex showed you on a regular Galaxy EPT3600 board running a 300W on-board amp - what they did to the Driver. They used 82K for R219...

https://www.worldwidedx.com/threads...p-dual-mosfet-issue.262896/page-5#post-777894
​

Why?

Because of the reason why the thread blew up - Connex doesn't go with SSB all the time, so they don't need to use the "standard" default values and settings any Galaxy (or similar SEE:RCI) wired and sold with SSB. At least on Radios that are not set up for REQUIRING such a high drive level since they don't use the MODE for it.

AM you have the carrier - on SSB you have to talk or send some kind of audio signal talk, it becomes the carrier (or signal in the RF realm) as the Power envelope just "dies" when you DO NOT send any audio.

So, look above, the Driver? You'd be lucky to get 35mA MAX drive from that R219 value - so it tells you by reviewing the different resistor Bias values like R219 is, informs you of the TYPE of bias drive you'll need to think about resetting the new Driver and Final for later.

By why lower on the Driver? Well, it's [pretty] simple - you need envelope POWER (your PEP) to handle the audio information - so the Finals will take on most of that chore - the Driver will - by bias - send enough RF to offset that Audio drive present for the Drain to Source (Collector to Emitter) So again, like a Bipolar of old, The Base needs to have enough DRIVE present to handle the Collector to Emitter swing.

The Driver will easily form up to 5 watts of RF power at 35mA - above that you risk blowing up GATE's of the FINALS from the DRIVERS Own output to the Finals' INPUT DRIVE LEVEL being too high and exceeding 20V P-2-P