The Gilbert cell analog multiplier for a current. It was developed in 1968 by Barrie Gilbert, who worked at Tektronix in Beaverton at the time. The output current of the cell is an exact multiplication of the differential input currents. The Gilbert cell is used for example when mixing signals or voltage-controlled amplifiers.
The circuit configuration is similar to the differential amplifier, wherein the current of the constant current source can be changed via an input. The current of the constant current source, the gain of the differential amplifier depends. Since the latter is controlled, resulting in a multiplication by the signal at the differential input. The differential stage is largely carried out twice, only the load resistors (in the picture not drawn), pairs exchanged, shared. At both levels is the same signal. In this way, the currents that result from the gain, otherwise appeared at the output except the product and the amplified input signals compensate.
The Gilbert cell is a component of or as a basis for many mixer serving or those containing integrated circuits, since it has a low distortion and carrier suppression by integration and can achieve the required symmetry of the components particularly.
The activation of the circuit can also be greatly controlled, the behavior now corresponds to the XOR gate in digital technology. This property presented in subsequent patent litigation is a problem, as the Gilbert cell was registered as an XOR gate and not as an analog mixer for a patent.