# How to bias a pHEMT (CS) for LNA application?

1. ## How to bias a pHEMT (CS) for LNA application?

If you go to the bottom of the datasheet for passive biasing with an ATF-55143 transistor, they use a bias point of 2.7V and 10 mA with Vdd at 3V. This doesn't make sense to me to have Vq at 2.7V since an LNA is suppose to be in class A operation, right?meaning that Vq should equal 1/2Vdd, correct? The datasheet is shown in the link here: https://www.google.com/url?sa=t&sour...E0RJO36Pyqf77l

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2. ## Re: How to bias a pHEMT (CS) for LNA application?

VDS (max) is 5V..
What else you would expect ?? The resistance has been used for-perhaps-stability or Current Limiting purposes..

3. ## Re: How to bias a pHEMT (CS) for LNA application?

Do you mean R1? And how does that resistor relate to class A biasing?

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4. ## Re: How to bias a pHEMT (CS) for LNA application?

Originally Posted by skatefast08
Do you mean R1? And how does that resistor relate to class A biasing?
R1,R2,R3 are biasing resistors.R3 limits the Drain Current with R1 and R2.
Didn't you read the Datasheet ??

5. ## Re: How to bias a pHEMT (CS) for LNA application?

Yea I understand that, but why do they pick Vdd=3V? Why not 5V, since then Vds=2.7V would be about half of 5V? Also they put bias point at Iq=10mA, which also isnt even half of the max current.

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6. ## Re: How to bias a pHEMT (CS) for LNA application?

Voltage: 5V is the absolute max rating. The device seems targeted at mobile devices, which typically have 3.7V LiIon batteries.
Current: Why would you bias to absolute max current rating?

7. ## Re: How to bias a pHEMT (CS) for LNA application?

Originally Posted by volker@muehlhaus
Voltage: 5V is the absolute max rating. The device seems targeted at mobile devices, which typically have 3.7V LiIon batteries.
Current: Why would you bias to absolute max current rating?
I said I would bias Id at 1/2 of max current and bias Vds at 1/2 of max voltage. This way my q point will be in the middle of the IV characteristic curve, which will achieve class A operation, right?

8. ## Re: How to bias a pHEMT (CS) for LNA application?

Originally Posted by skatefast08
I said I would bias Id at 1/2 of max current and bias Vds at 1/2 of max voltage. This way my q point will be in the middle of the IV characteristic curve, which will achieve class A operation, right?
Iq:Quiescent Current
So, Peak-to-Peak swing will be 20mA that is under the limit of the transistor.
This OP has been selected because it's a Low Noise Amplifier so the Best Noise Figure has been obtained on that Operating Point.
Read carefully the datasheet..You cannot select the Op as you wish, there are many constraint.

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