Integratable tunable resonant circuit for use in filters and oscillators

Abstract

A circuitry arrangement for a resonant circuit that may be completely monolithically integrated and electrically tuned has a differential amplifier stage with two transistors T.sub.1,T.sub.1 ' fed by a constant current source I.sub.0 that are differentially loaded at their collectors by a voltage-dependent capacity and inductively loaded in relation to the circuit ground by a pair of emitter followers T.sub.2,T.sub.2 ' with electrically adjustable impedance that act upon the base. To implement the electrically adjustable impedances, transistors with associated resistances are provided (for example a pair of emitter followers T.sub.3,T.sub.3 ' with base pre-resistances R.sub.1,R.sub.1 ') which may be adjusted by control currents (for example I.sub.1 I.sub.1 ') . The voltage-dependant capacity is implemented by transistors T.sub.4,T.sub.4 ' whose short-circuited emitters and collectors are connected so that the circuit node thus obtained is applied to the circuit ground through a voltage source U.sub.1. This circuit arrangement may also be used in a controllable oscillator by interconnecting two resonant circuits in a loop. The output signal of the first resonant circuit is used to control the second resonant circuit and the output signal of the second resonant circuit is inverted and used to control the first resonant circuit. Thus, two signals may be tapped at the outputs of both resonant circuits. The frequency of said signals may be varied by a control voltage and their frequency-independent phase difference equals .pi./2. An advantageous implementation of such an oscillator is obtained by decoupling the output signals of both resonant circuits through two identical output drive stages.

Dependencies are inferred from the claim text.