Re: Excellent presentation on Class E Amplifiers and why the QCX finals sometimes can get fried

Glen Leinweber

A key difference from the classic Class C PA circuit is the lower value of inductance that feeds DC power to the MOSfet(s). In the classic circuit, this inductor is a choke, having high impedance at the operating frequency. For Class-E, it is a much lower impedance, so that it stores significant energy while the MOSfet(s) are ON (for half a cycle or less). When the MOSfets switch off, this energy is available for output power.

The word "flyback" comes to mind. During the half-cycle-or-less that the MOSfets are ON, current rises continuously in this inductor. After switching off, this inductor current dumps out in the form of voltage: MOSfet drain voltage swings far above the +12V DC supply voltage.

The LC low-pass filter feeding the antenna is quite conventional. Because the MOSfet drain voltage is somewhat pulse-like, harmonics are large - the filter must work hard to reject these harmonics effectively. This can be done various ways: higher Q, or an extra stage.

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