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.
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.
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
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.
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
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.
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.
| 2 | 3 | 4
| 5 | 6
| 7 | 8
Next - powering