This invention relates to a transistor circuit comprising an emitter-coupled transistor pair comprising a first and a second transistor, and a third transistor to compensate for the base-current of the first transistor, which third transistor has its collector-emitter path arranged in series with the collector-emitter path of one of the transistors of the transistor pair.
Such a transistor circuit is disclosed in U.S. Pat. No. 3,714,600. This prior-art transistor circuit comprises an emitter-coupled transistor pair, the base-current of the first transistor being compesated for by the base-current of a third transistor having its collector-emitter path arranged in the collector line of the first transistor. The base-current compensation causes the impedance at the base of the compensated first transistor to increase so that the load on the signal source to which said base in coupled is reduced substantially. The base-current of the second transistor can be compensated for identically.
In gyrator-capacitor filters, which are described , inter alia, in "Integration of Analog Filters in a Bipolar Process", IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 4, Aug. 1982, pp. 713-722, the emitter-coupled transistor pair is an important element, which is often referred to as a transconductance circuit or transconductor, the voltage difference between the bases of the transistors of one pair being converted into two balanced output currents. By coupling two emittercoupled transistor pairs a gyrator can be formed. The quality and precision of the gyrator-capacitor filters depends inter alia on the manner in which a gyrator loads the other filter components, such as capacitors, resistors and other gyrators. In the ideal case a gyrator comprises two transconductors each having an infinitely high input impedance such that the transconductors neither load one another nor the filter components coupled to the gyrator. A high input impedance might be obtained by base-current compensation as used in the prior art transistor circuit. However, the prior-art transistor circuit requires a current mirror for the purpose of base-current compensation. The transistors of this current mirror are of a conductivity type opposite to that of the transistor in the imetter-coupled transistor pair. In integrated circuits such transistor pairs are formed by NPN transistors on account of thier superior signal-amplifying properties. This means that PNP transistors will be needed for the current mirror. The currents in the PNP current mirror should be most faithful replicas of the base current of the transistor to be compensated. However, PNP transistors have a lower current gain and an inferior high-frequency performance in comparison with NPN transistors. Therefore, a current mirror comprising PNP transistors is less accurate and less suitable for use at high frequencies.