An oscillator circuit topology using a one-pin external resonator suitable for integrated-circuit low-voltage, low-power applications that require a fast-starting accurate clock is disclosed. The circuit incorporates a novel arrangement of a plurality of active transconductance cells that respond to a digital control and provide adjustable loop gain for the oscillator. A programmable number of start-up transconductance cells are engaged in the initial phase of the oscillation for temporarily increasing the loop gain and energizing the resonator, and are disengaged from the oscillator core once the oscillation level is sufficiently large. The start-up transconductance cells may be identical to the always-on transconductance cells in the oscillator core, or they may be scaled versions of those cells. In addition, a programmable number of identical or scaled transconductance cells may be provided in the oscillator core itself, for accommodating different resonators. Internal circuit implementations of the transconductance cells that enable their efficient combination for increasing the oscillator loop gain are also disclosed.