Setting bias

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
Post Reply
kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 28
Joined: Sun Apr 12, 2015 2:32 pm

Setting bias

Post by kevycorsa » Sun Feb 17, 2019 6:27 pm

Setting bias
Anyone want to write me a good idiots guide to setting up bias on amplifier's
What to use?
Where to measure from ?
What should it measure ect

s2000

Re: Setting bias

Post by s2000 » Sun Feb 17, 2019 8:18 pm

Watch this video....


User avatar
sinus trouble
proppa neck!
proppa neck!
Posts: 1410
Joined: Fri Aug 22, 2014 11:34 pm

Re: Setting bias

Post by sinus trouble » Mon Feb 18, 2019 1:51 pm

The bias setting will vary with different devices also the mode of operation you want to achieve!
The datasheet will usually give you a rough guide to biasing the device into its linear region

VGSth = (Voltage gate source threshold) is the min/max gate bias voltage range

Idq = (current drain quiescent) is the idle current that should be on the drain pin of the device (+volt pin)
usually in mA

Also I would highly recommend a bench PSU for setup! You can then limit the current to say? 100mA? It will avoid any costly mistakes whilst you determine the point at which the device starts conducting!
Then you can up the current a bit to reach your desired value!

I hope this helps
I am as stupid as I look! :|

nrgkits.nz
Neckmin
Neckmin
Posts: 337
Joined: Fri Oct 17, 2014 10:35 am

Re: Setting bias

Post by nrgkits.nz » Tue Feb 19, 2019 4:22 am

What I would recommend is firstly a bench PSU which can go from zero volts upto atleast 50v - if you intend on building PA's that run on 50v. The PSU should also have current limiting capabilities and be able to supply enough current to run the PA at full power. I have made a few costly mistakes because I didn't have a PSU that could limit the current, also had a load fail once which resulted in some smoke coming from the PA. It pays to check everything before powering it up, put a multimeter across the PA output and make sure it reads 50R if you're using a load (which you should be and not radiating from an antenna for testing)

Setting the bias: What I usually do when testing a new design is set the bias to a lower voltage below the current drain quiescent but not below the minimum recommended bias, limit the supply current and reduce the voltage right down to say 12v for a 50v PA, then apply drive. If all is good, then increase the bias (with no drive) so that the drains are drawing the Idq as per datasheet, apply drive again - slowly allow more current checking output power is increasing also (also monitor the SWR into the load). Once no more current is being limited at the PSU, take note of of the output power and with this and the current draw in amps you can calculate the efficiency. For modern LDMOS devices the efficiency should be around 75 to 80%, if you're getting less than this, you need to tweak the input and or output matching (or your drive might not be enough) - you can do this while testing it on a low supply voltage as the output impedance matching is going to be very much the same at 50v also.

Output impedance = Vcc*Vcc / 2*Pout. For a dual LDMOS device, the output impedance at each drain is added together because its being operated in a balanced configuration.

Post Reply