3. Basic assumptions for a Class‐E amplifier
Generally the analysis of the circuit is done with the ideal simplified assumptions:
- 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.
- The switch is operated with a 50 % duty cycle, at the switching frequency.
- 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.
- The RF choke (DC-feeder) has a very large inductance and accordingly allows only DC current to flow through it.
- 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.