Harman Kardon FL8400 - Circuit Description; APC Circuit; Focus Servo

FL8400
CIRCUIT DESCRIPTION
1. APC CIRCUIT
— A semiconductor laser is used as the light source for the optical pickup. As the laser diode has large
negative temperature characteristics in its optical output when driven with a constant current, a circuit
must be provided to stabilize this output. For this purpose, a monitor diode which detects the optical
output of the laser diode is used in the semiconductor laser.
As the laser diode emits light from its bonded surface, light is emitted both in front and behind. The
light emitted behind is monitored with the monitor diode installed on its rear surface, and the optical
output is thus controlled. The light emitted in front becomes the light source for the pickup.
Fig. 1 Shows the APC circuit.
When the temperature rises and the optical output decreases, the monitor diode current (IS) decreases,
the electric potential of |C104 pin 5 rises, the base current of the driving transistor increases, and the
laser diode current increases. This causes the reduced optical output to return to its former level.
150K 43K 1.25V
Optical Pick-up
500 < |
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Fig. 1
2. FOCUS SERVO
2-1. Optical pickup
This set employs a three-beam optical pickup comprised of six
division photodiodes, A through F as shown in Fig. 2. The
four photodiodes (A through D) at the center provide focus
error detection by using their property to allow the beam to
focus into a round image only at a certain point.
The sums of outputs from diagonal two elements of four
division photodioddes (A+C and B+D) are compared by the
differential amplifier in 1C104 to detect the shape of the beam
image.
as ‘ “! : Three spotted (six-division)
The remaining two diodes (E and F) provide tracking error photo diodes
detection by means of sub-beam spots. Fig. 2
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