MA-5002VZ Service Manual
4-2 Circuit Theory
130446-1 Rev. A
©2000 Crown International, Inc.
Figure 4.1 Simplified Grounded Bridge
by the output stages rather than the power supply.
Composite devices are constructed to function as gi-
gantic NPN and PNP devices since the available cur-
rents exceed the limits of existing individual devices.
The devices connected to the load are referred to as
“high-side NPN and PNP” and the devices connected
to ground are referred to as “low-side NPN and PNP.”
Positive voltage is delivered to the load by increasing
conductance simultaneously in the high-side NPN and
low-side PNP stage. At the same time, conductance of
the high-side PNP and low-side NPN is being de-
creased.
4.2.1 Grounded Bridge Operation
Figure 4.1 is a simplified example of Crown’s patented
Grounded Bridge output topology (ignoring the articu-
lating characteristics of the VZ supply). It consists of
four quadrants of three-deep Darlington (composite)
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 out-
put stages are biased to operate class AB+B for ultra
low distortion in the signal cross-over region.
The high side of the bridge operates similar to a con-
ventional bipolar push-pull output configuration. As the
input drive voltage becomes more positive, the high side
NPN conducts current and delivers positive voltage to
the speaker load. Eventually, full +Vcc is across the load.
At this time the high side PNP is biased off. When the
drive signal is negative going, the high side PNP con-
ducts to deliver –Vcc to the load and the high side NPN
stage is off.
The low side operates quite differently. The power sup-
ply bridge rectifier is not ground referenced. This al-
lows the power supply to deliver +Vcc and –Vcc from
the same bridge rectifier and filter as a total difference
in potential, regardless of their voltages with respect to
ground. The low side of bridge uses inverted feedback
from the high side output to control the ground refer-
ence for the rails.
As the output swings positive, the output signal is fed
back to the low side and is inverted to drive the low
side with a negative signal. The negative signal causes
the low side PNP to conduct (as the high side NPN con-
ducts) shifting the ground reference toward –Vcc until,
at the peak, –Vcc = 0V. At this time +Vcc equals the full
potential (from rail to rail, not rail to ground) of the power
supply with positive polarity. Since the high side is de-
livering +Vcc to the speaker load (which is ground ref-
erenced at all times), the speaker sees the full potential
developed by the power supply with a positive polarity.
When the input drive signal is negative and the high
side PNP conducts to deliver a negative voltage to the
load, that output is again fed to the low side and in-
verted to cause the low side NPN to conduct. As the
low side NPN conducts, +Vcc swings toward the 0V
ground potential. At the peak: +Vcc = 0V. At this time
–Vcc equals the full potential developed by the power
supply, but with negative polarity. Since the high side is
delivering –Vcc to the speaker load, the load sees the
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