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

Status
Not open for further replies.
S

skatefast08

Full Member level 3
Joined
Jan 24, 2018
Messages
189
Helped
2
Reputation
4
Reaction score
2
Trophy points
18
Activity points
1,779
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&sou...FjAAegQIAxAB&usg=AOvVaw3c6xti4QE0RJO36Pyqf77l
 

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

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

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.
 

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?
 

volker@muehlhaus said:
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?
Click to expand...

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?
 

skatefast08 said:
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?
Click to expand...

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.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…