The idea behind class E is to reduce or eliminate the effects the
various capacitances within the MOSFET have on efficiency and operation
at high frequencies. The major operational condition is that the MOSFET
is only switched (turned on) when there is no voltage across the
device. This eliminates switching losses, a major loss component of
most RF amplifiers.
There are three capacitances at work within the MOSFET itself; the
input capacitance, the output capacitance and the so-called "transfer"
(drain to gate) capacitance. The effects of the capacitances within the
MOSFET can be reduced by making the capacitances part of resonant
circuits rather than "forcing" energy into and out of the capacitances,
and by controlling the timing of the switching of the MOSFET such that
the device is switched on only when the output capacitor is discharged.
Let's look at the various elements.
The element we must consider first, as far as class E operation is
concerned is the drain, or output capacitance. This capacitance exists
from drain to source. In normal switching arrangements, this
capacitance is simply charged and discharged by the MOSFET(s). However,
as the frequency is increased, more and more current is required to
quickly charge and discharge this MOSFET capacitance. If this current
flows through the MOSFET, the MOSFET's internal resistance will
dissipate power. The efficiency will drop dramatically as the frequency
is increased. In class E, the output network values are chosen such
that the output capacitance is part of a total resonant circuit. The
capacitor is "charged" by the flyback effect of the tuned circuit.
The diagram below shows a basic class E RF output stage, and the
drain and gate voltage waveforms when properly adjusted. The DC voltage
applied to the drain in this example is 50Vdc. Notice the peak RF drain
voltage rises to almost 200v.
The tuning and circuit values are chosen and adjusted such that the
drain capacitance (and shunt capacitor connected from drain to ground)
will fully discharge (drain voltage falls to zero) before the
MOSFET is turned on. In this way, the MOSFET is only switched on (by
the gate voltage) when there is already no voltage across the MOSFET,
drain to source. When the MOSFET is switched on, it isn't actually
"doing" anything at that moment, voltage-wise.
The gate, or "input" capacitance will prevent the MOSFET from being
driven easily at high frequencies. This capacitance is very high in
most MOSFETs - in some cases, in the order of thousands of picofarads
for a single MOSFET. Values which we would consider to be a "short
circuit" to RF in the vacuum tube world are commonplace operating
values in the MOSFET world. There are several ways to deal with the
input capacitance. One way is to make it part of a resonant circuit,
and drive it with a very low impedance driver. Another way is to charge
the gate capacitance by using the flyback effect of an inductor. Yet
another drive circuit simply involves a step-down transformer, with a
single turn secondary made of heavy wire, with the multi-turn primary
driven directly by an RF source.
All of the energy which is put into the gate is lost in the form of
heat, caused by the charging and discharging of the gate capacitance,
and by heating of the resistive component of the gate impedance. This
resistance is internal to the MOSFET, and varies from device type to
device type. MOSFETs with metal gate structures are much better in this
respect, and are capable of operating at higher frequencies, as opposed
to MOSFETs which use a silicon gate. However, the metal gate type
MOSFETs are considerably more expensive, for a given device
It is only necessary to drive the gate to about 12v
(positive); 24v peak to peak. The MOSFET will be fully saturated
at this point. It is possible to "drive" the MOSFET with a square wave,
however as the frequency is increased, the amount of power required to
force a square wave into the gate capacitance becomes excessive. A
trapezoidal waveform is generally the best compromise between good
switching times and driver power.
The reverse-transfer (drain to gate) capacitance effects the ability
of the MOSFET to be switched at high frequencies when high voltage is
present at the drain. Ideally, you want to choose a MOSFET which has a
low a reverse-transfer (also called the Miller capacitance). The
reverse-transfer capacitance causes the drain voltage to "work against"
the gate voltage. Improvements in technology and manufacturing
techniques have dramatically reduced reverse transfer capacitances over
the past few years. Be aware of this value, along with the related Gate
Charge value when choosing MOSFETs for RF applications. The lower
the gate charge, the better is the MOSFET for RF.