Nortel Networks Corporation

Northern Telecom Limited

Gregory Weng Yuen

Peter John Mole

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.

Variable capacitance circuit

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. The windings are arranged such that outputs taken from the first and second sets of secondary windings are 180 degrees out of phase, and the primary windings of the transformer are arranged to receive an input signal. The bases of all of the transistors are arranged to receive local oscillator signals, the dc voltage applied to the second transistor in each transistor pair being less than the dc voltage applied to the first transistor in each transistor pair. Finally, the signals applied to the first transistor pair are the inverse of the signals applied to the second transistor pair, such that the local oscillator signals activate only one transistor pair at a time.

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Harlow, United Kingdom

Gregory Weng Yuen