Crown Audio 1200 Voltage Amplification, Grounded Bridge Topology, Last Voltage Amplifiers LVAs

Micro-Tech 1200 Amplifier Service Manual

Theory

amplifies the difference between input and output	The ODEP transistors steal drive as dictated by the
signals, any difference in the two waveforms will	ODEP circuitry (discussed later). The control/protec-
produce a near open loop gain condition which in turn	tion transistors act as switches to totally shunt audio to
results in high peak output voltage. The output of the	ground during the turn-on delay, or during a DC/LF or
Error Amp, called the Error Signal (ES) drives the	Fault protective action.
Voltage Translators.
VOLTAGE AMPLIFICATION	Last Voltage Amplifiers (LVAs)
	The Voltage Translator stage channels the signal to
The Voltage Translator stage separates the output of	the Last Voltage Amplifiers (LVA's) in a balanced
the Error Amp into balanced positive and negative	configuration. The +LVA and -LVA, with their push-pull
drive voltages for the Last Voltage Amplifiers (LVAs),	effect through the Bias Servo, drive the fully comple-
translating the signal from ground referenced ±15V to	mentary output stage. The LVAs are configured as
±Vcc reference. LVAs provide the main voltage ampli-	common emitter amplifiers. This configuration pro-
fication and drive the High Side output stages. Gain	vides sufficient voltage gain and inverts the audio. The
from Voltage Translator input to amplifier output is a	polarity inversion is necessary to avoid an overall
factor of 25.2.	polarity inversion from input jack to output jack, and it
	allows the NFb loop to control Error Amp gain by
Voltage Translators	feeding back to its non-inverting input (with its polarity
A voltage divider network splits the Error Signal (ES)	opposite to the output of the VGS). With the added
into positive and negative drive signals for the bal-	voltage swing provided by the LVAs, the signal then
anced voltage translator stage. These offset reference	gains current amplification through the Darlington
voltages drive the input to the Voltage Translator	emitter-follower output stage.
transistors. A nested NFb loop from the output of the	GROUNDED BRIDGE TOPOLOGY
amplifier mixes with the inverted signal riding on the
offset references. This negative feedback fixes gain at	Figure 2 is a simplified example of the grounded
the offset reference points (and the output of the Error	bridge output topology. It consists of four quadrants
Amp) at a factor of -25.2 with respect to the amplifier	of three deep Darlington (composite) emitter-follower
output. The Voltage Translators are arranged in a	stages per channel: one NPN and one PNP on the
common base configuration for non-inverting voltage	High Side of the bridge (driving the load), and one
gain with equal gain. They shift the audio from the	NPN and one PNP on the Low Side of the bridge
±15V reference to VCC reference. Their outputs drive	(controlling the ground reference for the rails). The
their respective LVA.	output stages are biased to operate class AB+B for
	ultra low distortion in the signal zero-crossing region
Also tied into the Voltage Translator inputs are ODEP	and high efficiency.
limiting transistors and control/protection transistors.

	BGS	VGS	Error	+15V				Voltage	+VCC
								Translators
Audio	+	+	Amp	Divider					NPN Outputs (+HS)
			+
						Q100	Q121	Q101	Q105
Inputs				Voltage				Q102
	-	-	-			Q103			PNP Outputs (-HS)
							Q122		Q110
					- +	Mute			-VCC
				-15V	ODEP
						NFb Loop			LVA's

Figure 1. Typical Amplifier Front End and Voltage Amplification Stages.

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