and IM, for any frequency selected on
the front panel. This output is then fed
into two, high-voltage operational ampli-
fiers, powered from a separate plus and
minus 25-volt supply, producing 10 volts
rms AC, in phase and out of phase.
Amplifier description.
There are two power amplifiers in the
Power Plant. Each produces 57.5 volts
rms AC at its output, for a total of 115
volts rms between the two balanced out-
puts. The two amplifiers form a bal-
anced bridge that is used to feed the AC
receptacles on the rear of the Power
Plant.
Input.
Bipolar differential pair, biased with an
active dual-device current source for
maximum linearity. The differential pair
has a dual-device current mirror feeding
it to increase gain, lower distortion, and
provide perfect balance.
Gain stage.
One leg of the current mirror is used to
feed the base of a pnp gain stage, which
is fed by an active current source. We
chose to incorporate this topology,
rather than any number of other possibil-
ities, because we felt this offered us the
highest linear amount of gain with mini-
mal componentry.
Current amplification.
The gain stage feeds a bipolar comple-
mentary driver stage, separated by a
thermal circuit which actively adjusts the
output stage current in a manner com-
mensurate with the transistor's safe
operating area vs. heat. The driver
stage feeds a complementary parallel
output stage, and the entire signal is
then fed back to the inverting input of
the differential pair in a loop feedback
scheme.
Catastrophic protection.
The output stage is ultimately protected
from an over-current situation by the cat-
astrophic sensing circuit. An opto-isola-
tor is placed across the emitter resistors
of the output stage. When a predeter-
mined voltage drop is developed across
the emitter resistor, the opto-isolator
fires and sends a signal to the Power
Plant's microprocessor, signaling for an
immediate shut down of the power acti-
vation relay. A front panel LED is also
activated by the microprocessor to indi-
cate to the customer that a fault condi-
tion has occurred.
Current clamp.
On the driver side of the current amplifi-
cation section, we have placed a unique
current clamp. Should the current draw
exceed the acceptable limits, based on
a predetermined time function, the
active clamps will conduct and limit the
current drive to the output stage. This
protection scheme is useful when
attempting to drive an extremely reactive
load, or a large turn-on inrush current. It
is convenient because it allows the
Power Plant to "soft start" a large trans-
former/capacitor combination without
shutting down, and without damage. In
essence, it is a limiter.
Soft sensing.
Using a small isolation transformer, con-
nected across one of the output stage
emitter resistors, we isolate and then
monitor the real time current draw pro-
duced by the Power Plant as it feeds a
load. The AC from the isolation trans-
former is rectified, then filtered, to pro-
duce an average level. That level is
then fed into an A to D converter, and
the digital information is fed into the on-
board microprocessor. The micro-
processor can then easily calculate the
average rms current, as delivered by the
Power Plant to its load, and then display
that information on the front panel of the
Power Plant when the Mode button has
been instructed to do so. Further, the
microprocessor has been programmed
to shut down the Power Plant when an
over-10% (of rated power) condition is
present.
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