Question about gate biasing of pHEMT amplifiers

[samchieh]

I have a simple question.

I measured a 2 different pHEMT amplifiers from 2 separate companies. I used GSG probes for the input and output, and DC probes for biasing the drain and gate.

When I do this, the gain is signifantly lower than from the spec sheet.

When I add a external 1k series resistor between the DC supply and the DC probe at the gate, I get the advertised gain.

Why? Was I seeing oscillation? Is it because the gate needs current limiting?

 

[precision]

Re: pHEMT Gate Biasing

Do you known the concept of Bias Tee?
You should insure that DC bias port is very-high impedance (open) for RF frequency.
The high resistance series resistor you used paved the way.

 

[samchieh]

Re: pHEMT Gate Biasing

The amplifier's I'm using are not just single FET's. They are MMIC amplifiers with internal biasing. Attached is a typical assembly for the Amplifier.

When the amplifier has internal biasing, unless I am mistaken, a bias-T is not necessary.

 

[BigBoss]

pHEMT Gate Biasing

Your circuit is not a simple GaAs pHEMT instead an amplifer with its own biasing.
Gate biasing is negative to Source in pHEMTs and it's provided by simple source resistance so it creates a negative GS voltage when G is grounded.
I think you have forgotten to connect RF choke btween Vdd and the circuit because some circuits don't have own on-chip RF chokes, this is connected externally.

 

[samchieh]

pHEMT Gate Biasing

That makes sense, but why does the 1K Gate resistor change the circuit so much.

Without the 1K series gate resistor, gain is around 0dB.

With the 1K series gate resistor, gain is around 20dB.