Macro-Tech 24x6 Reference Manual - page 23
Page 23
Macro-Tech 24x6 & 36x12 Power Amplifiers
signal from the gain pot, and drives the voltage
trans-
lator 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 out-
put signal.
With the voltage swing provided by the LVAs, the sig-
nal then gains current amplification through the
Darlington emitter-follower output stage.
The bridge-balanced circuit (U104-B) receives a sig-
nal from the output of the amplifier, and differences it
with the signal at the Vcc supply. The bridge-balanced
circuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply hav-
ing exactly one
half of the output voltage added to their
quiescent voltage. D309, D310, D311 and a trimmer
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
temperature of the heatsink on which the output de-
vices 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.