Crown CE 2000, Service Manual

Page: 24 / 581

CE Series Power Amplifier Service Manual Rev. D
Circuit Theory 3-2
this case, the gain of the VGS is unity and the amplifier
will have an overall fixed gain of 20 volt/volt or 26- dB.
The VGS is an inverting stage.
3.4 Error Amp
The inverted output from the VGS is fed to the non-
inverting input of the Error Amp (U101-C) through an
AC coupling capacitor C103 and input resistor R106.
Diodes D103 and D104 prevent overdriving the Error
Amp. Amplifier output is fed back via the negative
feedback (NFb) loop through R112 and C106 (pre-ter-
minator network) and R322 (post-terminator network).
The overall closed-loop mid-band gain is set to be 20
or 26 dB by resistors R112 and R110.
The Error Amp's job is to keep both inputs at the same
potential. Since the signal fed to the inverting input is
1
/
20
of the amplifier output, the Error Amp output should
be the same as the non-inverting input, which should
be
1
/
20
of the output of the amplifier during linear op-
eration (i.e., what goes in, comes out with gain). Any
type of non-linearity in the output will cause the Error
Amp to compensate with the opposite of the non-lin-
earity. For example, if the amplifier clips, the error amp
will travel all the way to its opposite rail trying to com-
pensate. The output of the Error Amp, called the Error
Signal (ES) drives the Voltage Translator (Q103).
3.5 Voltage Amplification
The voltage amplification stage consists of the voltage
translator, last voltage amplifier and the bias servo.
Each of these items are discussed in this section.
3.5.1 Voltage Translator
The Error Amp output is only capable of swinging sev-
eral volts and therefore must be voltage amplified to
drive the output stage. The purpose of the voltage trans-
lator, Q103, is to level shift or translate the voltage
from a reference around ground to a reference just above
-Vcc. The result is higher voltage swing capabilities
from the LVA. This is required since the next stage is
referenced to -Vcc. The diode D105 protects the volt-
age translator from reverse biasing.
3.5.2 Last Voltage Amplifier (LVA)
The next stage is the Last Voltage Amplifier Q107. The
LVA provides voltage gain necessary to provide drive
to the output stage. R115 in the base of Q107 pro-
vides collector current for Q103, the voltage translator,
and it also allows the signal of the collector of Q107 to
be developed across it and thus amplified.
The series combination of D126 and D127, in parallel
with the base-emitter junction of Q107 and R136, form
a circuit that limits the current through Q107. One of
these diode drops equates to the base-emitter junc-
tion of Q107, and the other equates to the voltage on
R136. Therefore, the current through Q107 cannot rise
higher than that required to produce a diode drop across
R136.
3.5.3 Bias Servo
Q104, R132, R133 and R134 form the bias servo. This
circuit is also known as a V
BE
Multiplier or a level shifter.
Q104 is called the bias transistor. The purpose of this
circuit is to provide and control bias to the output stage.
By utilizing the constant current source Q105, the bias
servo effectively multiplies the voltage across the bias
transistors base-emitter junction and produces the out-
put voltage across the bias transistors collector-emit-
ter junction. The bias adjustment pot R134 is included
to allow adjustment of the bias voltage.
The bias transistor is mounted on the main module
and thermally connected to the heatsink. The purpose
of this is to allow the bias transistor to automatically
adjust the bias voltage as needed depending on the
temperature of the output devices. This is possible
because the forward voltage drop across a P-N junc-
tion decreases by approximately 2 mV for every 1° C
increase in temperature.
3.6 Output Topology
The output topology for the CE series amplifiers is a
type of quasi-complementary design using only N-P-N
output devices. They also have the classic CROWN
AB+B biasing configuration also known as MultiMode
®
or triple-deep Darlington. The pre-drivers and drivers
are biased at 0.6V and the output transistors have a
0.31VDC voltage from base-emitter and are therefore
at a sub-turn-on voltage. In this type of topology (AB+B),
the driver transistors carry the bias current, while the
output transistors sense when the drivers have devel-
oped significant current, and thus take over and de-
liver the needed current. This is a proven design that
provides maximum efficiency with minimum crossover
notch distortion and idling amplifier heat. Thus there is
no bias current adjustment, as the output circuit is not
temperature-tolerance critical.
This output topology has become quite common in
power amplifier design. Typical Darlington transistors,
connected in the Common Emitter configuration, are used