Grounded Bridge Topology; High Side (Hs) - Crown Com-Tech CT-1610 Service Manual

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current amplification through the Darlington emitter-fol-
lower output stage.

5.4 Grounded Bridge Topology

Figure 5.2 is a simplified example of the grounded
bridge output topology. Grounded Bridge Topology con-
sists of four quadrants of Darlington emitter-follower
stages per channel: one NPN and one PNP on the High
Side of the bridge (driving the load), and one NPN and
one PNP on the Low Side of the bridge (controlling the
ground reference for the rails). The output stages are
biased to operate class AB+B, allowing for ultra low
distortion in the signal zero-crossing region, as well as
for high efficiency.

5.4.1 High Side (HS)

The High Side (HS) of the bridge operates much like a
conventional bipolar push-pull output configuration. As
the input drive voltage becomes more positive, the
HS NPN conducts and delivers positive voltage to the
load. Eventually the NPN devices reach full conduc-
tion and +Vcc is across the load. At this time the HS
PNP is biased off. When the drive signal is negative
going, the HS PNP conducts to deliver -Vcc to the load
and the HS NPN stage is off.
The output of the +LVA drives the base of predriver
device. Together, the predriver and driver form the first
two parts of the three-deep Darlington. They are bi-
ased class AB. They provide output drive through the
High Side
of Bridge
Input
Com-Tech 810 and 1610 Amplifier Service Manual
H.S.
NPN
Load
H.S.
PNP
Figure 5.2 Crown's Patented Grounded Bridge Topology
bias resistor, bypassing the output devices, at levels
below about 100mW. An RLC network between the
predriver and driver provide phase shift compensa-
tion and limit driver base current to safe levels. Output
devices are biased class B, just below cutoff. At about
100mW output they switch on to conduct high current
to the load. Together with predriver and driver, the out-
put device provides an overall class AB+B output.
The negative half of the HS is almost identical to the
positive half, except that the devices are PNP. One
difference is that the PNP bias resistor is slightly greater
in value so that the PNP output devices run closer to
the cutoff level under static (no signal) conditions. This
is because PNP devices require greater drive current.
HS bias is regulated by Q18, the Bias Servo. Q18 is a
Vbe multiplier which maintains approximately 3.2V Vce
under static conditions. The positive and negative
halves of the HS output are in parallel with this 3.2V.
With a full base-emitter on voltage drop across
predrivers and drivers, the balance of voltage results
in approximately .32V drop across the bias resistors in
the positive half, and about .46V across the bias resis-
tor in the negative half. Q18 conduction (and thus bias)
is adjustable.
A diode string prevents excessive charge build up
within the high conduction output devices when off.
Flyback diodes shunt back-EMF pulses from reactive
Low Side
of Bridge
L.S.
NPN
L.S.
PNP
NFb Loop
]
Inverter
5-3