Abstract: The invention provides an improved variable capacitance circuit which is substantially linearly controlled using voltage or current control. The circuit comprises two emitter connector transistors and a constant current source or drain forming a current steering circuit. A fixed capacitor is connected to the emitters of the transistors forming a port into the circuit. AC current into the port flows through the fixed capacitor and is fed back through the transistors in a proportion depending on the voltage at their bases. The effective capacitance of the circuit is varied using this variable AC feedback arrangement. Other embodiments use current control to vary the circuit capacitance by varying the effective constant current source level.

Abstract: A transformer-coupled mixer circuit including dc voltage supply; and four transistors, arranged as a first transistor pair and a second transistor pair. Each transistor pair has a first transistor and a second transistor and each transistor has a base, a collector and an emitter. The emitter of the first transistor in each transistor pair is connected to the collector of the second transistor in that transistor pair, and the emitter of the second transistor in each transistor pair is connected to a current sink. The circuit also comprises a transformer, having a set of primary windings and first and second sets of secondary windings, the first set of secondary windings being connected in series with the emitter of the first transistor in the first transistor pair and the second set of secondary windings being connected in series with the emitter of the first transistor in the second transistor pair.