Hitachi, Ltd.

Tatsuo Baba

Harushige Nakagaki

Takao Arai

Nobutaka Amada

Masafumi Nakamura

Takashi Takeuchi

Yukio Fukui

Shigeki Inoue

Tohru Sampei

Hiroshi Fukushima

Yoshito Ohmura

Hideo Fukuma

A servo circuit for a signal reproducing apparatus for reproducing signals from a record medium such as a disc utilizing an optical pickup, wherein a variable gain amplifier is arranged in a servo loop between a pickup and an actuator so that the pickup is caused to track a signal track in accordance with a servo error signal produced by the variable gain amplifier. A playback condition of a disc is discriminated by detecting the number of errors in a signal reproduced from a recorded signal on the disc by the pickup, and a gain of the variable gain amplifier is controlled by the discrimination output so that the variation of the servo error signal caused by a difference in reflectance between individual discs is eliminated and a loop gain appropriate to an individual disc is automatically set.

Servo circuit for a signal reproducing apparatus

The output signal of an l-f (low-frequency) power amplifier is supplied to an electroacoustic transducer, through an inductive element for preventing parasitic oscillation, to cause the inductive element to generate a magnetic flux whose density depends on the output current signal of the power amplifier while a current according to the magnitude of the output voltage signal of the power amplifier is supplied to the current input terminals of a Hall-effect element disposed in the magnetic field induced by the inductive element, so that a voltage according to the output power of the l-f power amplifier is taken out of the output terminals of the Hall-effect element. When the voltage exceeds a predetermined level, the l-f signal supplied to the electroacoustic transducer is attenuated by a signal attenuating circuit so that the transducer is prevented from being damaged.

Protective circuit for electroacoustic transducer with hall element and

A low frequency power amplifier uses MOS FET's each having a semiconductor device unit including a source electrode, a drain electrode and an insulated gate electrode filled in a can type casing with the source electrode being electrically connected to the can type casing. When the MOS FET having its source electrode connected to the can type casing is mounted on a heat sink and operated in a source follower configuration, a stray capacity between the can-shaped casing and the heat sink is connected in parallel with a load so that the amplifier oscillates. The heat sink is grounded through an impedance element and the stray capacity is isolated from the load to prevent the oscillation.

Low frequency power amplifier using MOS FET's

An audio frequency power amplifier wherein an input signal is applied to respective input electrodes of a pair of transistors of the same conductivity type to produce an output signal at an output electrode of one of the pair of transistors. A common electrode of the one transistor is connected with an output electrode of the other transistor and a power from a first D.C. power supply is supplied to the junction thereof through a diode while a power is supplied to a common electrode of the other transistor from a second D.C. power supply having a higher voltage than that of the first D.C. power supply. The other transistor and the diode are alternately rendered conductive and non-conductive in response to the signal level of the input signal, a capacitor is connected to the input electrode of the other transistor, which capacitor is charged during the turn-off of the other transistor whereby the charge is supplied to the other transistor upon turning on of the other transistor to shorten the rise time.

Audio frequency power amplifier

Growing community of inventors

Toyokawa, Japan

Tatsuo Baba