An integrated circuit (shown below dashed line A--A) for a voltage controlled oscillator comprises a first transistor (T.sub.1) having its collector coupled to a first port (Port 1) via a filter comprising a capacitor (C.sub.f) and an inductor (L.sub.f) and its emitter coupled to the emitter of a second transistor (T.sub.2) whose collector is coupled to a second port (Port 2). The collector of the first transistor is also coupled via a capacitive divider (C.sub.1, C.sub.2) to the base of the second transistor. The base of the first transistor is AC decoupled (C.sub.3). The emitters of the first and second transistors are fed by a current source (I.sub.1). A capacitor (C.sub.s) connects the base of the second transistor to the first port. The first and second ports are for connection to an external resonator (L.sub.1, D.sub.1, C.sub.p, L.sub.p) (shown above dashed line A--A) where C.sub.p and L.sub.p are parasitic capacitances and inductances respectively, the latter being typically bond wires connecting the ports to the rest of the external resonator. The filter (C.sub.f, L.sub.f) causes the circuit to exhibit negative impedance across the first and second ports. A VCO embodying the invention is tunable across the intended frequency range without exhibiting frequency hopping or mode switching.