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Design >> RF Design >> Class E power amplifier
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Message started by baab on Aug 27^th, 2013, 7:41pm

Title: Class E power amplifier
Post by baab on Aug 27^th, 2013, 7:41pm

Hi,
Please help me with these questions about class E power amplifier. Thanks.

http://imageshack.us/f/62/u9dr.jpg/
http://imageshack.us/f/21/umgr.jpg/
http://imageshack.us/f/14/jat9.jpg/

Title: Re: Class E power amplifier
Post by aaron_do on Aug 27^th, 2013, 8:20pm

Hi,

for the dV/dt part neat zero, I think the author is simply saying that if there is a time shift (such that there is overlap), you don't want a large change in voltage as this would result in power being burnt in the transistor. i.e. change in voltage / change in time should be zero.

The RFC has an average voltage drop across it of 0 V. When the transistor is ON, VX will be pulled down to ground making the voltage drop across it equal to VDD. In order for the average voltage drop across the RFC to be zero, when the transistor turns OFF, VX rises above VDD. In fact for the class-E PA I believe it is above 2*VDD, but I can't remember the theoretical number.

An ideal RFC provides a constant current (not voltage) because its AC impedance is infinite or dI/dV = 0.

Aaron

Title: Re: Class E power amplifier
Post by baab on Aug 28^th, 2013, 10:28pm

Thank you, Aaron and sorry for the inconvenience. I can't figure out how to attach images by using "[img][/img]".

Quote:

for the dV/dt part neat zero, I think the author is simply saying that if there is a time shift (such that there is overlap), you don't want a large change in voltage as this would result in power being burnt in the transistor. i.e. change in voltage / change in time should be zero.

When the transistor is OFF, Vx will increase but it can't increase suddenly because of the presence of the capacitor C1.
We need dVx/dt = 0 around the time transistor switches from ON to OFF. According to second condition the desired value of Vx is the smallest.
Then, apply both second and third conditions, we need Vx at the time when transistor switches from ON to OFF is very very small. Is this right that the voltage Vx is very small at the time transistor switches from ON to OFF?