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How to check bias of transistor?

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zhangm79

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Hello, i am designing LNA using FET transistor. But when connnect -Vgg with gate of FET, the voltage changed.the measured Vg is not -V as expected. source of FET connected to ground. Is it normal or something wrong?How to debug real bias? thanks.
 

Hi,
Post the biasing circuit topology. Did you put a resistor in it?
What is the supply voltage? And gate voltage?

Some RF power is maybe going through the bias network...
 

It is possible your LNA to oscillate.
 

Yes, I was thinking oscillation too. How are you measuring the gate voltage. You can not just attach a DVM probe to the gate!
 

thanks, guys. bias should be -0.5v on the gate, 3V on the drain. Biff44, how to measure gate voltage?

---------- Post added at 17:20 ---------- Previous post was at 17:20 ----------

50ohms resister in bias of gate and drain.
 

Bias resistor should more than 300R.
Bias voltage resistors is almost compared to resistor of the transistor.
 

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