Variations of Class
H (Dual power supplies w/single ended output)
If
we add a capacitor to the variable voltage reference as shown in
last page of tracking amplifier, it will
form a peak detector to sample and hold voltage forming a
programmable power supply (relative to the peak input signal).
Voltage
on power devices will be always 10 V before clipping, but voltage
on collectors will be constant, not an envelope as in Class H.
At
low signal levels VCC stay fixed for 10V as example, determined
by the
sample and hold circuit (peak detector) MOSFET maintain PWM at 10V
regulated when there is no signal input. When
signal is received, C1 charges to the peak voltage,
and the DC-DC converter regulates the +VCC output Buss
accordingly reference level of peak detector.
Power
dissipation and efficiency then will be 75 %
even if the amplifier is running at 1/8, 1/3 or 3/4 of full power.
The efficiency will remain 75 % due
the fact that power supply rails are voltage variable
(pseudo-tracking) via the DC-DC converter.
Programmable
power supply amplifier is a variation of Class H, AB and Class D
topologies.
Advantages:
This
is high efficiency amplifier and it can deliver the specified
output power with a smaller power transformer, and a slightly
smaller heat-sink.
No
switching Noise due to the linear output.
RFI
(Radio Frequency Interference) is a lot less than Class D and Class H as the PWM is not
dynamically moving, but varies every time the output level is
adjusted .
Disadvantages:
Circuit
complexity increases, adding cost.
Average
transient response, EMI filter in power supply is required.
Conclusion:
PPS
amplifiers are used in high efficiency designs where cost is not
important factor, and efficiency is the most important issue.
