Gain-variable amplifier circuit

A compensating circuit for a transmission signal transmitted through a transmission path having a predetermined transmission characteristic, comprising:

• first circuit means for amplifying the transmission signal with a frequency characteristic of compensating an amplitude attenuation of the transmission signal, and outputting a first current signal;
• second circuit means for amplifying the transmission signal with a predetermined frequency characteristic and outputting a second current signal;
• load means, connected to a power source circuit, for producing an output signal corresponding to the transmission signal;
• a first transistor having an emitter for receiving the first current signal, a base for receiving a first control signal, and a collector coupled to said power source circuit via said load means;
• a second transistor having an emitter for receiving the first current signal, a base for receiving a second control signal whose level change phase is opposite to a level change phase of the first control signal, and a collector connected to said power source circuit;
• a third transistor having an emitter for receiving the second current signal, a base for receiving the first control signal, and a collector connected to said power source circuit;
• and
• a fourth transistor having an emitter for receiving the second current signal, a base for receiving the second control signal, and a collector coupled to said power source circuit via said load means.

A compensating circuit for a balanced transmission signal, comprising:

• first balanced circuit means for amplifying the balanced transmission signal with a frequency characteristic of compensating an amplitude attenuation, and outputting a pair of first balanced current signals;
• second balanced circuit means for amplifying the balanced transmission signal with a predetermined frequency characteristic, and outputting a pair of second balanced current signals;
• first load means, connected to a power source circuit, for supplying a first output signal corresponding to the balanced transmission signal;
• second load means, connected to said power source circuit, for supplying a second output signal having a phase opposite to that of the first output signal;
• a first transistor having an emitter for receiving one of the first balanced current signals, a base for receiving a first control signal, and a collector coupled to said power source circuit via said first load means;
• a second transistor having an emitter for receiving one of the first balanced current signals, a base for receiving a second control signal whose level change phase is opposite to a level change phase of the first control signal, and a collector connected to said power source circuit;
• a third transistor having an emitter for receiving the other of the first balanced current signals, a base for receiving the second control signal, and a collector connected to said power source circuit;
• a fourth transistor having an emitter for receiving the other of the first balanced current signals, a base for receiving the first control signal, and a collector connected to said power source circuit via said second load means;
• a fifth transistor having an emitter for receiving one of the second balanced current signals, a base for receiving the first control signal, and a collector connected to said power source circuit;
• a sixth transistor having an emitter for receiving one of the second balanced current signals, a base for receiving the second control signal, and a collector coupled to said power source circuit via said first load means;
• a seventh transistor having an emitter for receiving the other of the second balanced current signals, a base for receiving the second control signal, and a collector coupled to said power source circuit via said second load means;
• and
• an eighth transistor having an emitter for receiving the other of the second balanced current signals, a base for receiving the first control signal, and a collector connected to said power source circuit.

A variable gain amplifier for compensating for an amplitude frequency response characteristic of a transmission signal transmitted through a transmission path having a predetermined attenuation characteristic, comprising:

• first amplifying means, having a first predetermined frequency characteristic, for amplifying the transmission signal and outputting a first amplified signal;
• second amplifying means, having a second predetermined frequency characteristic, for amplifying the transmission signal and outputting a second amplified signal, wherein the second predetermined frequency characteristic has a high-frequency range increasing characteristic of compensating for the attenuation characteristic which is changed by a length of said transmission path;
• first amplitude changing means, connected to said first amplifying means, for outputting a first change signal obtained by changing an amplitude of the first amplified signal with a first coefficient in accordance with a predetermined control signal;
• second amplitude changing means, connected to said second amplifying means, for outputting a second change signal obtained by changing an amplitude of the second amplified signal with a second coefficient in accordance with the predetermined control signal, wherein a magnitude of the second coefficient is reduced when a magnitude of the first coefficient is increased and increased when the magnitude of the first coefficient is reduced;
• and
• means, connected to said first and second amplitude changing means, for providing an output signal obtained by combining the first change signal with the second change signal.

A compensating apparatus for automatically compensating for a frequency characteristic of a trans mission signal transmitted through a transmission path having a predetermined transmission characteristic, comprising:

• (a) a first compensating circuit including:
• first means, having a first predetermined frequency characteristic and a first predetermined amplification factor, for outputting a first signal obtained by changing an amplitude of the transmission signal with a first coefficient in accordance with a predetermined control signal;
• second means, having a second predetermined frequency characteristic and a second predetermined amplification factor, for outputting a second signal obtained by changing the amplitude of the transmission signal with a second coefficient in accordance with the predetermined control signal, wherein the second predetermined frequency characteristic has a characteristic of compensating for a characteristic depending on said transmission path, and a magnitude of the second coefficient is reduced when a magnitude of the first coefficient is increased and increased when the magnitude of the first coefficient is reduced;
• and
• first synthesizing means, connected to said first and second means, for synthesizing the first and second signals and supplying a first output signal;
• (b) a second compensating circuit connected to said first compensating circuit, including:
• third means, having a third predetermined frequency characteristic and a third predetermined amplification factor, for outputting a third signal obtained by changing an amplitude of the first output signal with the first coefficient in accordance with the predetermined control signal;
• fourth means, having a fourth predetermined frequency characteristic and a fourth predetermined amplification factor, for outputting a fourth signal obtained by changing the amplitude of the first output signal with the second coefficient in accordance with the predetermined control signal, wherein the fourth predetermined frequency characteristic has a characteristic of compensating for the characteristic depending on said transmission path;
• and
• second synthesizing means, connected to said third and fourth means, for synthesizing the third and fourth signals and supplying a second output signal;
• and
• (c) a coefficient controller, connected to said second compensating circuit, for supplying a compensation output corresponding to the second output signal, and generating, as the predetermined control signal, a signal corresponding to a difference between a signal level of the compensation output and a predetermined reference level.

A variable gain amplifier for compensating for an amplitude frequency response characteristic of a signal transmitted through a transmission path having a predetermined attenuation characteristic, comprising:

• a first amplifier for amplifying a signal transmitted through said transmission path and having a substantially flat amplitude frequency response characteristic, and providing a first output signal;
• a second amplifier for amplifying a signal transmitted through said transmission path and having a predetermined amplitude frequency response characteristic corresponding to an attenuation characteristic of said transmission path, and providing a second output signal;
• an adder circuit for reciprocally level-controlling the first and second output signals from said first and second amplifiers, so that a level of one of the first and second output signals is increased when a level of the other thereof is reduced, and adding the level controlled first and second output signals to each other;
• and
• an addition controller for variably setting an amount of the level control performed by said adder circuit in accordance with a line length of said transmission path.