DC Biasing for Power Amplifier

[hari_preetha]

I want to design Ka- Band Power Amplifier using ADS software. I have taken 0.15 um GaN HEMT transistor for my design. According to datasheet, biasing conditions are VDS = 28 V and IDS = 100 mA. But when we checked DC IV Characteristics, I am getting operating point at VDS = 16 V, IDS =0.072 A, and VGS =-0.080 V in the active region. Once I select VDS =28 V, IDS =100 mA , am closer to Cutoff region.

I am getting confused by these two version, whether I have to follow datasheet biasing values or plotted DC IV characteristics values. Which I have to follow, clarify someone.

 

[FvM]

You should show a test circuit for clarification. How is the load line constructed?

 

[hari_preetha]

You should show a test circuit for clarification. How is the load line constructed?

I have attached the test circuit.

 

[BigBoss]

If your Vdd=28V, you should draw your Load Line from 2xVdd=56V (at least for Class-A)

 

[volker@muehlhaus]

According to datasheet, biasing conditions are VDS = 28 V and IDS = 100 mA.

I'm not sure what that means, but the datasheet says 100mA/mm not 100mA.
Current in the right plot (Id vs. power) is not 100mA either.

 

[hari_preetha]

If your Vdd=28V, you should draw your Load Line from 2xVdd=56V (at least for Class-A)

Yes, I have varied VDS from 0 to 60 V. Then I am getting VDS=31 V, IDS= 0.088 A, VGS = -0.055 V in the active region. Then can I use these biasing VDS and IDS values for getting optimum output power, power added efficiency and gain. Please clarify it.

I'm not sure what that means, but the datasheet says 100mA/mm not 100mA.
Current in the right plot (Id vs. power) is not 100mA either.

As you mentioned 100 mA/mm, it is not 100 mA. As you mentioned 100 mA/mm, it is right only.
Below I have attached simulated DC-IV Characteristics and relevant datasheet screen shots. I am beginner to this PA design, please clarify my doubts regarding biasing.

 

[BigBoss]

Yes, I have varied VDS from 0 to 60 V. Then I am getting VDS=31 V, IDS= 0.088 A, VGS = -0.055 V in the active region. Then can I use these biasing VDS and IDS values for getting optimum output power, power added efficiency and gain. Please clarify it.

As you mentioned 100 mA/mm, it is not 100 mA. As you mentioned 100 mA/mm, it is right only.
Below I have attached simulated DC-IV Characteristics and relevant datasheet screen shots. I am beginner to this PA design, please clarify my doubts regarding biasing.

You cannot use this Operating Point for ALL purpose. There is no transistor in this world that will give these simultaneously. Load Line is just a starting point for getting an idea about Load resistance. The exact Impedance for ONE of the specifications above will be obtained by Load Pull + Source Pull simulations.
Let me give you a Hint. I have worked on this PA and found a valuable starting point.( Idsq=20.3mA)
The rest is yours. Yo can improve this amplifier to get better specs.

 

[hari_preetha]

Let me give you a Hint. I have worked on this PA and found a valuable starting point.( Idsq=20.3mA)
The rest is yours. Yo can improve this amplifier to get better spec

Please let me know what about starting point of Vdsq. lt would help me lot.

 

[BigBoss]

Please let me know what about starting point of Vdsq. lt would help me lot.

28V