Class A gain control systems having extremely good linearity and wide dynamic range, and suitable for monolithic integration, are described in embodiments realizing several classical multiplier functions such as voltage controlled attenuators, exponentially controlled multipliers and four quadrant multipliers. Two identical current steering circuits, each comprising a matched, emitter coupled pair of transistors, or effective transistors, are fed from an actively linearized input circuit which derives the required well balanced signals from a single ended or balanced input source, and current mirror circuits are used to subtract unwanted common mode products from the output signals.
An integrated circuit voltage controlled attenuator which reduces control voltage feedthrough by employing NPN transistors for both sets of current steering elements is disclosed. A pair of emitter coupled NPN transistors are provided, connected to a common current source. The collector currents of these transistors form input currents of two precision PNP current mirrors. Output currents from the mirrors are coupled to a second pair of emitter coupled NPN transistors, having a current source connected to their emitters, whose value is adjusted by feedback to exactly match the sum of output currents of the precision mirrors. A single set of control voltages is supplied to both sets of emitter coupled NPN transistors. Signal input and output current is coupled from the precision current source output legs and made available in voltage form through operational amplifiers which perform input voltage to current, and output current to voltage conversions, respectively. An integrated control voltage generation technique is also disclosed, which provides accurate gain settings for a voltage controlled attenuator without the need for absolute value external components or signal feedback between the voltage controlled attenuator and control circuit.
A gain cell circuit includes a logarithmic transformation circuit. The logarithmic transformation circuit includes a pair of first and second transistors, each of which has first and second current carrying electrodes and a control electrode. The control electrodes of the first and second transistors are coupled to input terminals of the logarithmic transformation circuit. The logarithmic transformation circuit further includes third and fourth transistors coupled to the first and second transistors. The third and fourth transistors have control electrodes serving as output terminals of the logarithmic transformation circuit, first current carrying electrodes connected at first and second circuit nodes to the second current carrying electrodes of the first and second transistors, and second current carrying electrodes coupled to a power supply voltage. The logarithmic transformation circuit further includes an impedance element connected between the first and second nodes, and level-shift circuits connected to the second current carrying electrodes of the first and second transistors and to the control electrodes of the third and fourth transistors.
The differential amplifier circuit of the invention comprises operational amplifiers 3 and 4 having the negative inputs connected to input terminals 1 and 2 respectively, transistors 21 and 22 having the bases connected to each of the outputs of the operational amplifiers 3 and 4 respectively, constant current sources 41 and 42 connected between each emitter of the transistors 21 and 22 and a ground terminal 8 respectively, constant current sources 43 and 44 connected between each collector of the transistors 21 and 22 and a power supply terminal 7 respectively, a resistor 31 connected between the collectors of the transistors 21 and 22, transistors 23 and 24 having the emitters connected to each emitter of the transistors 21 and 22 respectively and the bases commonly connected to each other, and load resistors 32 and 33 connected between each collector of the transistors 23 and 24 and the power supply terminal 7 respectively.
A multi-stage low power, high dynamic range variable gain amplifier comprises an input stage cascaded with one or more current amplifier stages, whereby the gain of each stage may be independently controlled. The input stage may be comprised of a variable transconductance amplifier using variable emitter degeneration. The current amplifier may be comprised of a differential Darlington amplifier coupled to a differential cascode amplifier. The transconductance amplifier converts an input voltage signal to a current signal. The variable gain amplifier is designed for efficient low power operation.
The output terminal of a first voltage-current conversion circuit, which includes operational amplifiers and a resistor, is connected to the output terminal of a second voltage-current conversion circuit which includes operational amplifiers and a resistor, and also to the negative input terminal of the second voltage-current conversion circuit via a source follower as an impedance conversion circuit which is formed by an NMOS transistor and constant current source. Furthermore, the output terminal of the first voltage-current conversion circuit serves as the output terminal of a differential amplifier circuit. The positive-phase input terminal of the second voltage-current conversion circuit is connected to a reference voltage. Since the node of the current outputs of the first and second voltage-current conversion circuits has a high impedance, these circuits operate to equalize their output currents.
