Page 23
Reference Manual
Macro-Tech
®
24x6 & 36x12 Power Amplifiers
nal from the gain pot, and drives the voltage
translator
stage.
The voltage
translator stage channels the signal to the
Last Voltage Amplifiers (LVAs), depending on the sig-
nal polarity, from the error amp U104-A. The +LVA
(Q105,Q125) and the –LVA (Q110,Q126), with their
push-pull effect through the bias servo Q318, drive the
fully complementary output stage.
The bias servo Q318 is thermally coupled to the heat
sink, and sets the quiescent bias current in the output
stage to lower the distortion in the crossover region of
the output signal. D301, D302, D303, and D304 are
used to remove the charge on the unused portion of the
output stage, depending on the polarity of the output
signal.
With the voltage swing provided by the LVAs, the signal
then gains current amplification through the Darlington
emitter-follower output stage.
The bridge-balanced circuit (U104-B) receives a signal
from the output of the amplifier, and differences it with
the signal at the Vcc supply. The bridge-balanced cir-
cuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply
having exactly one
half of the output voltage added to
their quiescent voltage. D309, D310, D311 and a trim-
mer resistor set the quiescent current point for the
bridge-balanced output stage.
The protection mechanisms that affect the signal path
are implemented to protect the amplifier under real-
world conditions. These conditions are high instanta-
neous current, excessive temperature, and operation of
the output devices outside safe conditions.
Q107 and Q108 act as a conventional current limiter,
sensing current in the output stage. The allowable cur-
rent level is also adjusted as a function of voltage. When
current at any one instant exceeds the design criteria,
the limiters remove the drive from the LVAs, thus limiting
current in the output stage to a safe level.
To further protect the output stages, a specially devel-
oped ODEP (Output Device Emulation Protection) cir-
cuit is used. It produces an analog output proportional
to the always changing safe operating area of the out-
put transistors. This output controls the translator stage
by removing any drive that exceeds the safe operating
area of the output devices. Thermal sensor S100 gives
the ODEP circuits vital information on the operating tem-
perature of the heatsink on which the output devices
are mounted.
Should the amplifier fail in such a way that would cause
DC across the output lead, the DC protection circuit
senses this on the negative feedback loop and shuts
down the power supply until the DC is removed.
