Brush up on the theory before designing a high power Class‐E amplifier: Page 2 of 15

June 29, 2016 //By Alfio Scuto, Roberto Cammarata, ST Microelectronics
The most promising switched-mode PA for RF applications, among the several types available, is the Class-E for several reasons: high efficiency, simplicity of the load network, and a satisfying performance even with a non-optimal drive signal.

3. Basic assumptions for a Class‐E amplifier

Generally the analysis of the circuit is done with the ideal simplified assumptions:

  1. The transistor is an ideal switch, i.e. a short circuit in the ON state and an open circuit in the OFF state, with an instant switching action.
  2. The switch is operated with a 50 % duty cycle, at the switching frequency.
  3. The switch can sustain the current running through it in the ON state and also must be able to stand the non-zero voltage that appears during the OFF state.
  4. The RF choke (DC-feeder) has a very large inductance and accordingly allows only DC current to flow through it.
  5. The loaded Q-factor (QL) of the series resonator L2-C2 is high enough so it can be considered that a purely sinusoidal current is running through the load R.

 

3.1. Design equations for infinite QL

Under the above ideal conditions, it’s possible to obtain design equations for each component of the circuits. It’s hypothesized that the circuit has reached a steady-state operation and the RF ON-OFF cycles are equally divided.

In Figure 3 are displayed the well-known ideal Class E waveforms in the switch, plus the shunt current through the capacitor C1.

From now on, the angular phase is defined as Ө = ωt with ω as the operation frequency.


Figure 3: Class-E waveforms.

 

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