Bias

[kevycorsa]

Probably a silly question but as im still learning..
How do you read the bias voltage
Where do you put the multimeter
Think i have set bias to high and popped a 9180

[Analyser]

The bias of a FET is set by measuring the CURRENT that the amplifier draws with no RF input or output. Put your meter in series with the positive supply line of the amplifier and set it to amps (make sure the leads are the correct meter holes). Apply a supply voltage to the amp and slowly increase the bias, up to a maximum of around 500mA for this size of device (in my opinion).
The bias on the 9180 is very sensitive amd they are easy to pop, especially with the normal cheap bias pot supplied in the Chinese Ebay kits. I would fit a 10 turn preset to make this easier. The safest way to achieve a correct bias setting is to power the amp from a current limted power supply so you cannot overdo it. If you don't have this type power supply be very careful to make adjustments slowly and always make sure the bias pot is turned fully to zero before you power up.

Also note your bias will increase with temperture so as the amp gets hot it can also lead to a blown device.
Also, set the bias up at the full amplifier supply voltage because the bias will increase with increasing supply (ie. don't set the bias up with a supply of 24V and then try to use the amp at 28V). Of course there is nothig to stop you slowly increasing the supply voltage, but the final adjustment should be made at full supply voltage.

We've had quite a few stories now on the forum from people who have blown these devices from over bias. I also suspect they are quite sensitive to ESD on the input or that people are over driving them. It's much better to run the bias lower at say 50-100mA than try to get the last few watts out and run it ragged with it nearly popping, you won't notice the extra few watts on the and the amp will be more efficient, run cooler and last a lot longer.

Good luck!

[Maximus]

Thanks for the heads up Analyser. I found that the devices worked best at 24v-25v. The gain dropped off if I went any higher or lower, so possibly the bias was set wrong.


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[OgreVorbis]

As albert was saying, it is best to read the amperage, but if you are wondering about voltage, it should be about 3.5v for the device you are talking about (9180). For the 186, it's usually between 4.2 and 4.6v depending on the device. It varies a little between each device. You'll find it in the datasheet under gate threshold voltage if I remember correctly.

[OgreVorbis]

To measure the voltage, you can just put a volt meter on the gate of the fet (the input pads).

[Maximus]

Nice1 OgreVorbis. Will keep this in mind instead of guessing lol


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[Albert H]

To measure the voltage, you can just put a volt meter on the gate of the fet (the input pads).

You MUST use a high input impedance meter - the venerable AVO Meter will affect the voltage by loading it down.

The basic circuit I use to bias FETs is:

FET Bias.png

The circuit effectively isolates the meter from the RF part of the circuit since some multimeters (Fluke ones are susceptible) don't like RF getting into them!

My actual FET bias circuit is significantly more complex than shown - there's temperature compensation, VSWR shut down (which turns off the bias) and LED indication of function - but the circuit shown will work in most cases.

[kevycorsa]

Finally got round to doing this
Put a new chip in and set bias
170w no problem

[kevycorsa]

Screenshot_20170915-215902.png

[kevycorsa]

Screenshot_20170915-220513.png

[MC Spanner]

170w no problem

How much drive are you finding that you need to get your 170W out?

[kevycorsa]

Around 1.2w to 1.4w if i remember correctly
Nrg pll 1w setting running at 15v