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Circuit Description
The
c200 is a full discrete, two voltage gain stage power amplifier
derived from the classic Lin topology. The power output stage
is configured as emitter followers and biased to class AB.
1st
Stage - the Input Differential
The
input signal is fed through a network comprising of C1,C2,R1
and R2. Input capacitor C1, prevents DC from the preceding
stage from being injected into the input differential. C1,
in combination with R2, form a high pass filter of 3.3Hz at
-3dB. R1 and C2 serve to prevent RF from entering the input
by setting the corner frequency at 408KHz (-3dB).The
signal meets Q1 of the input differential Q1 and Q2. Current
for the differential is set by a red led constant current
source Q3 at a total of 3.0mA.
2nd
Stage - Voltage Amplifier Stage (VAS)
The amplified inverted signal is direct coupled to
the 2nd stage through Q4, which is biased into class A by
another current source Q5. In this stage, current is set at
11.6mA by R15. Capacitor C7 (Miller compensation) sets the
dominant pole for stability. Q4, Q5 must be heatsinked.
Vbe
Multiplier
Q6 must be connected to the main heatsink where the
power transistors are. Apart from being a bias adjustment,
it also acts as a thermal tracking device. Prevents the output
transistors from thermal runaway.
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Output Stage
In
order to prevent the output stage from loading the VAS, drivers
Q9 and Q10 are inserted in between to act as buffers. Output
transistors Q11,Q12,Q13 and Q14 are configured as emitter
followers operating in class AB. Zobel network C10,R28 provides
final stability against RF oscillations. Q9-Q14 must be connected
to the main power heatsink.
Negative
Feedback
Negative feedback is global. A fraction of the output signal
is fed back into the inverted section of the input differential
via R7 and R8. Total Gain is set at 31x or 30dB. Capacitor
C3 is for high frequency compensation whilst C4 ensures that
overall dc gain is at unity.
VI
Limiting
Over current protection is by subcircuit Q7, Q8 and R16 through
R21, including D2 and D3. It is set to activate at load impedances
of 2 ohms and below, thus protecting the output transistors
from operating out of their SOA.
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| 5 | 6
| 7 | 8 >>Next
- powering up the c200
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