A gain controlled amplifier comprising a first means for providing a first current and a second current corresponding to an input signal, a second means for providing a first attenuated current corresponding to the first current, a third means for providing a second attenuated current corresponding to the second current, a fourth means for providing a control signal for changing magnitude of the first and second attenuated currents, and fifth means for providing an output signal corresponding to amplitudes of the first and second attenuator currents. The first means comprises a split current generator for separately providing the first current corresponding to the positive component of the input signal and the second current corresponding to the negative component of the input signal.

A constant bandwidth automatic gain control (AGC) system is presented. The system is especially suitable for use in the positioning servo system of a magnetic disk storage system. The system includes a double balanced modulator circuit and a fixed gain video amplifier circuit. The double balanced modulator circuit functions as a variable gain element, and the gain thereof is set by a control voltage derived from an otherwise conventional AGC loop. The gain of the current steering stage of the double balanced modulator is limited to values no greater than 0.5.

A gain control circuit particularly suitable for compressing or expanding the dynamic range of an audio signal, and thereby reducing noise produced during recording and playback comprises an input circuit receiving an input signal, first and second differential amplifiers each supplied from the input circuit with a signal derived from the input signal, with the amplifying elements of each being complementary to the amplifying elements of the other, a first pair of transistors having their emitters coupled to the output of the first differential amplifier and a second pair of transistors having their emitters coupled to the output of the second differential amplifier. The transistors of each pair are of the same conductivity type as the amplifying elements of the associated differential amplifier. The collectors of the transistors of each pair are jointed respectively to the collectors of the corresponding transistors of the other pair. Control voltage input terminals are respectively connected to one transistor of each pair and an opposite transistor of the other pair. A feedback signal is applied from the joined collectors of the one transistors to the input circuit, and an output current is applied from the joined collectors of the other transistors to an output stage which can include a load resistor or a current-to-voltage converter circuit. This arrangement prevents variation in total static current when gain is varied, thereby achieving a superior signal-to-noise characteristic.

A voltage controlled attenuator having extremely wide dynamic range and having low distortion is achieved by limiting the current density in each unit area of the transistors to minimize the current noise and by utilizing high current sources to minimize the noise voltage. A pair of compound gain cells having low current densities are each directly fed by a linearized current source. The gain cells are controlled by a single gain control voltage and feed a pair of linearized buffer amplifiers for minimizing loading on the gain cells. The output of the buffer amplifiers feed a differential output amplifier for generating an output signal that eliminates the effect of the control voltage in the output signal and any DC shift caused by changes in control voltage and nulls remaining distortion products.

A low power, differential ECL line driver has a switching network that applies a constant current source as the emitter load to only one of the output transistors at a time while applying a constant current source to the base of the other output transistor.