Crown Audio 800CSL service manual Grounded Bridge Topology, Theory, High Side HS

Power Base-2 & 800CSL Amplifier Service Manual

Theory

allows the NFb loop to control Error Amp gain by	The output of the +LVA drives the base of predriver
feeding back to its non-inverting input (with its polarity	device. Together, the predriver and driver form the
opposite to the output of the VGS). With the added	first two parts of the three-deep Darlington and are
voltage swing provided by the LVAs, the signal then	biased class AB. They provide output drive through
gains current amplification through the Darlington	the bias resistor, bypassing the output devices, at
emitter-follower output stage.	levels below about 100mW. An RLC network between
GROUNDED BRIDGE TOPOLOGY	the predriver and driver provide phase shift compen-
	sation and limit driver base current to safe levels.
Figure 2 is a simplified example of the grounded	Output devices are biased class B, just below cutoff.
bridge output topology. It consists of four quadrants	At about 100mW output they switch on to conduct high
of three deep Darlington (composite) emitter-follower	current to the load. Together with predriver and driver,
stages per channel: one NPN and one PNP on the	the output device provide an overall class AB+B
High Side of the bridge (driving the load), and one	output.
NPN and one PNP on the Low Side of the bridge	The negative half of the HS is almost identical to the
(controlling the ground reference for the rails). The
output stages are biased to operate class AB+B for	positive half, except that the devices are PNP. One
ultra low distortion in the signal zero-crossing region	difference is that the PNP bias resistor is slighter
and high efficiency.	greater in value so that PNP output devices run closer
	to the cutoff level under static (no signal) conditions.
High Side (HS)	This is because PNP devices require greater drive
The High Side (HS) of the bridge operates much like	current.
a conventional bipolar push-pull output configuration.	HS bias is regulated by Q18, the Bias Servo. Q18 is a
As the input drive voltage becomes more positive, the
HS NPN conducts and delivers positive voltage to the	Vbe multiplier which maintains approximately 3.3V
load. Eventually the NPN devices reach full conduc-	Vce under static conditions. The positive and negative
tion and +Vcc is across the load. At this time the HS	halves of the HS output are in parallel with this 3.3V.
PNP is biased off. When the drive signal is negative	With a full base-emitter on voltage drop across
going, the HS PNP conducts to deliver -Vcc to the load	predrivers and drivers, the balance of voltage results
and the HS NPN stage is off.	in approximately .35V drop across the bias resistors in

+

	+Vcc (Positive Rail)
Input
signal	Load
	(speaker)
	Inverting Op-amp
HIGH SIDE	-Vcc (Negative Rail)	LOW SIDE

-

Figure 2. Crown Patented Grounded Bridge Topology

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