Gain control circuit

DETAILED DESCRIPTION The principal object of this invention is to realize a gain control circuit which varies the gain of an amplifier circuit approximately at geometric intervals, in other words, the gain in logarithmic representation linearly (linearly in decibel) by the use of digital codes.
That is, the object is to realize a gain control circuit with which, when a controlling digital code signal varying linearly is applied, a switching circuit (in general, switches corresponding to the number of bits of the codes) is driven directly without decoding the code signal, and besides, the transfer function (this may be considered the gain) of a controlled circuit at the time of the drive varies linearly in decibel.
Another object of this invention is to realize a circuit arrangement which lessens the number of constituents of the gain control circuit and which is readily put into the form of an integrated circuit.
In order to accomplish the objects, this invention exploits the theory of a variable equalizer and is based on the use of an amplifier which has a fixed gain between an input and output thereof and a resistor network which has a variable resistance circuit in an input or output circuit or feedback circuit thereof connected so that the transfer function between the input and output may become: ##EQU2## (where A and B are constants), and wherein the circuit for linearly controlling the value G is constructed of switches which are operated by bit signals constituting digital codes for control, and resistance elements.
According to such gain control circuit of this invention, a control portion of the circuit is realized in such a way that the plurality of resistance elements connected in parallel or in series and the plurality of switches connected in series or in parallel with the resistance elements are turned "on" or "off" directly by the plurality of bit signals constituting the controlling digital codes, and hence, many stages of gain can be obtained with a very small number of resistance elements and switches