Importance

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Abstract

A gain-controlled amplifier amplifies with reduced distortion an input signal at a gain which varies exponentially in accordance with a control voltage, and has a wide dynamic range irrespectively of the value of the control voltage. The gain-controlled amplifier comprises first and second operational amplifiers, first and second transistor pairs, a bias current control circuit and a subtracting circuit. The first transistor pair comprises emitter-coupled first and second transistors of the same conduction type (PNP or NPN), and the second transistor pair comprises emitter-coupled third and fourth transistors of the same conduction type as the first and second transistors. The common emitters of the first and second transistor pairs are connected to output terminals of the first and second operational amplifiers, respectively. Non-inverting input terminals of the first and second operational amplifiers are grounded. Collectors of the first and third transistors are connected to the inverting input terminals of the first and second operational amplifiers, respectively. The input signal and an inversion thereof are supplied to the inverting input terminals of the first and second operational amplifiers, respectively. The control voltage is supplied between bases of each of the first and second transistor pairs. The bias current control circuit generates currents which vary exponentially in accordance with the control voltage, and supplies the currents to the inverting input terminals of the first and second operational amplifiers. The subtracting circuit subtracts a signal at the collector of the second transistor from a signal at the collector of the fourth transistor and outputs the subtraction result as an output signal of the gain-controlled amplifier.

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First Claim