Sony PS-2250 - Page 4

SERVO AMP
Fig. 1-2. Practical ac servo system
Stage/Control Function
Start Operation
When the power switch is turned on,
Q06 in the base circuit of QO7 is cutoff.
As a result, C8 in. the collector circuit
is charged through R27,R8, VR2, R10,
VRI and R9 when the 33 rpm button
is depressed. Note that VR2 and R10
is shorted during 45 rpm operation.
Q07 is forced into conduction when
C8 is charged up to some specified
voltage. As a result the following
conditions exist: :
Q09 QO10 QOlI Q3 Q4 Q5 Q6
OFF OFF ON ON ON ON ON
and a large enough ac voltage is applied
to the motor so the motor starts to
revolve rapidly.
Correct Speed Condition
Frequency
generator When the motor starts to revolve, the
frequency generator (F.G.) generates
ac voltage whose frequency is pro-
portional to the motor speed. As the
frequency generator is directly coupled
to the shaft of the drive motor, it
converts motor speed into frequency.
Differential Q1 and Q2 forma differential amplifier
amplifier which amplifies the input FG signal to
the level required for the following
Ql, Q2
limiter circuit.
Note that the output is extracted from
collector circuit of Ql] and Q2, and
then fed to the flip-flop stage through
diode limiter D1 and D2.
Stage/Control Function
Diode limiter Removes all amplitude variations from
D1, D2 the signal. Each diode conducts when
the signal across it exceeds the barrier
potential (0.6 V) in a forward biased
condition. Thus, the output signal is
limited to about 1.2 V peak-to-peak.
Flip-flop Q01 and QO02 form a flip-flop circuit
circuit which generates square output in ac-
Q01, Q02 cordance with the input trigger signal
(limiter’s output).
The flip-flop output is extracted at
collector circuit of QO2 and then fed to
the buffer amplifier stages.
Buffer/phase Q0O3, Q04 and QOS form a buffer
inverter amplifier stage. Note that Q03 and
Q03, Q04, Q05 are emitter followers but, Q04
Q05 acts as a phase inverter.
Differentiation Square wave output at the emitter
circuit circuit of QO5 is converted into spike
C7, RO12 pulses through the differentiator circuit
(formed by C7 and RO12) to trigger
the following saw-tooth wave generator
(Q06).
Saw-tooth Q06 and RC components (C8, R9,
wave VR1, R10, VR2) in the collector circuit
generator form a saw-tooth wave generator.
Q06, C8, R9, Note that the frequency of the saw-
VRI, R10 tooth wave is determined by the RC
VR2 time constants in the collector circuit.
Voltage The saw-tooth is fed to the voltage
comparator comparator formed by QO7 and QO8.
Q07, Q08 Q08 is forward biased through the
FINE control (VR3 paralleled by R12)
R11 and R28. The current flow in Q08
is controlled by the FINE control
(VR3), which. varies its base-emitter
voltage. Q07 conducts only when the
base voltage becomes higher than the
emitter voltage which is determined by
the current flow in QO8. Note that the
emitter voltage of QO8 serves as a
reference voltage.
Referring to Fig. 1-3, the comparator
operates as follows: At time T1 a saw-
tooth signal is applied to the base of
Q07.
At time T2 the voltage at the base of
Q07 is sufficient to turn on QO7
generating a negative pulse.
Note that the pulse width is determined
by saw-tooth signal waveform: