What is the criteria for choosing conduction angle?

[Osawa_Odessa]

Hi,
I am designing a RF power amplifier class AB. What is the criteria for choosing conduction angle?
Thanks.

 

[goldsmith]

Hi,
I am designing a RF power amplifier class AB. What is the criteria for choosing conduction angle?
Thanks.

Hi Osawa_Odessa

Are you sure you are not going through the class C amp ? because an amplifier which has been called class AB is only biased to prevent cross over distortion and it means a full mode class Ab amplifier conducts in whole cycle ( with two transistor of course )
So i think your question is so general . if i'm missing something please let me know ! complete your information about what you are exactly looking for then i'll guide you more .
Best Wishes
Goldsmith

 

[chuckey]

Class AB is really class B with a little forward bias, so if the conduction angle is 180 degrees - lowest power dissipation, highest distortion(worst I/Ps), increasing the angle increases power dissipation and decreases distortion and I//Ps.
Frank

 

[Osawa_Odessa]

Thank you, goldsmith and chuckey.
I am designing a RF power amplifier class AB. I know that in class AB the conduction angle can be any value from 180 to 360 degree.
I want to know what matter we need to consider when choosing conduction angle, for example Pout and PAE, power dissipation,...
I have read somewhere said that increasing conduction angle will increase Pout and lower PAE, it that right?

Class AB is really class B with a little forward bias, so if the conduction angle is 180 degrees - lowest power dissipation, highest distortion(worst I/Ps), increasing the angle increases power dissipation and decreases distortion and I//Ps.
Frank

Could you explain a little bit more?
Here is my understanding:
Conduction angle increases will increase bias voltage and the time that transistor conducts => Increase power dissipation.
However, I am not quite understand why increasing conduction angle will decrease distortion.
I see that the output is put through a filter therefore, with any conduction angle only the signal component that has the same frequency with resonant circuit get to output.

 

[chuckey]

This is only so at a first approximation, if you want I/Ps right down at -80 dBs then filters won't help as the products can be with in the pass band of the filter. Like wise any curvature of the transistors transfer charecteristic will be right down at the point where the transistor starts to conduct. it is best to use the manufacturers application notes because they would have done this work before.
Frank