Com-Tech 200 Amplifier Service Manual
8
Theory
Voltage Amplification
The Voltage Translator stage separates the output of
the Error Amp into balanced positive and negative
drive voltages for the Last Voltage Amplifiers (LVAs),
translating the signal from ground referenced ±15V to
±Vcc reference. LVAs provide the main voltage ampli-
fication and drive the High Side output stages. Gain
from Voltage Translator input to amplifier output is a
factor of 25.2.
Voltage Translators
A voltage divider network splits the Error Signal (ES)
into positive and negative drive signals for the bal-
anced voltage translator stage. These offset reference
voltages drive the input to the Voltage Translator tran-
sistors. A nested NFb loop from the output of the
amplifier mixes with the inverted signal riding on the
offset references. This negative feedback fixes gain at
the offset reference points (and the output of the Error
Amp) at a factor of -25.2 with respect to the amplifier
output. The Voltage Translators are arranged in a
common base configuration for non-inverting voltage
gain with equal gain. They shift the audio from the ±15V
reference to VCC reference. Their outputs drive their
respective LVA.
Also tied into the Voltage Translator inputs are ODEP
limiting transistors and control/protection transistors.
The ODEP transistors steal drive as dictated by the
ODEP circuitry (discussed later). The control/protec-
tion transistors act as switches to totally shunt audio to
ground during the turn-on delay, or during a DC/LF or
Fault protective action.
Last Voltage Amplifiers (LVAs)
The Voltage Translator stage channels the signal to the
Last Voltage Amplifiers (LVA's) in a balanced configu-
ration. The +LVA and -LVA, with their push-pull effect
through the Bias Servo, drive the fully complementary
output stage. The LVAs are configured as common
emitter amplifiers. This configuration provides suffi-
cient voltage gain and inverts the audio. The polarity
inversion is necessary to avoid an overall polarity
inversion from input jack to output jack, and it allows the
NFb loop to control Error Amp gain by feeding back to
its non-inverting input (with its polarity opposite to the
output of the VGS). With the added voltage swing
provided by the LVAs, the signal then gains current
amplification through the Darlington emitter-follower
output stage.
Grounded Bridge Topology
Figure 2 is a simplified example of the grounded bridge
output topology. 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 output stages are
biased to operate class AB+B for ultra low distortion in
the signal zero-crossing region and high efficiency.
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 conduction
and +Vcc is across the load. At this time the HS PNP is
+
-
+
-
+
-
BGS
VGS
Error
Amp
Audio
Inputs
Voltage Divider
NFb Loop
+
-
ODEP
Mute
+15V
-15V
+VCC
-VCC
NPN Outputs (+HS)
PNP Outputs (-HS)
Q100
Q103
Q121
Q122
Q101
Q102
Q105
Q110
Voltage
Translators
LVA's
Figure 1. Typical Amplifier Front End and Voltage Amplification Stages.
