Op-amp LC oscillator uses tank losses for amplitude stabilization

This proposed design eliminates traditional additional requirements for AGC, AC coupling, and post amplification circuitry.

This Design Idea presents a simple op-amp-based sine wave oscillator that directly generates a low-impedance bipolar output of approximately 20 Vpp at frequencies above 100 kHz. The circuit was developed to directly drive an AD633 in an on–off keying (OOK) digital transmission system.

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Many sinewave oscillators reported in the literature provide a unipolar output and require additional circuitry for automatic gain control (AGC), AC coupling, and post amplification to achieve the desired amplitude. The proposed circuit eliminates these requirements by directly generating a high-amplitude bipolar sinewave.

The circuit (Figure 1) consists of an LC tank connected in a feedback loop with an op-amp configured as an inverting amplifier with a gain set by the R3/R2 ratio. At the resonance frequency, fosc = 1/(2π√(L·Cs)), where Cs = C1 || C2, the LC network introduces 180° phase shift, while the inverting amplifier provides an additional 180°, resulting in a total loop phase shift of 360°, thus satisfying the Barkhausen phase condition.

Figure 1 This simple LC oscillator uses an op-amp and the intrinsic losses of the resonant tank to generate a stable low-distortion 20 Vpp sine wave without AGC.

The topology can be viewed as a simplified Colpitts oscillator in which the op-amp both sustains oscillation and compensates for the losses of the resonant network. A key practical advantage is that the oscillation amplitude is set by the op-amp closed-loop gain, primarily through the feedback resistor R3, allowing straightforward amplitude control without additional circuitry.

The prototype was implemented using the LT1357, a high-speed op-amp featuring high slew rate and wide gain-bandwidth product. In general, a wideband op-amp with sufficient slew rate and gain-bandwidth product is required, particularly as the oscillation frequency increases.

Resistor R1 is not critical in value and primarily serves to isolate the op-amp output from the LC tank, preventing degradation of the phase margin due to the reactive load. At resonance, assuming an ideal inductor, R1 is effectively in series with R2 and forms a voltage divider. Its value should therefore be kept small relative to R2 to limit attenuation of the LC network, but not so small as to excessively load the op-amp, resulting in a practical design trade-off.

At startup, the loop gain is greater than unity, allowing oscillation to build up from noise. As the amplitude increases, current in the LC tank also increases, leading to higher losses due to winding resistance and ferrite core dissipation. These losses introduce additional attenuation in the resonant network, progressively reducing the loop gain until it reaches unity.

At equilibrium, the energy provided by the op-amp exactly compensates for the tank losses, and the oscillation amplitude stabilizes. The op-amp remains in its linear region, and the resulting waveform is very close to an ideal sine wave, as confirmed by oscilloscope capture (Figure 2).

Figure 2 The waveform generated by the circuit is very close to an ideal sine wave.

The oscillation frequency remains essentially constant as the amplitude varies, indicating that inductance variation due to core nonlinearity is negligible. The circuit was built and tested experimentally (Figure 3).

Figure 3 The circuit was breadboarded, versus simply simulated, to more definitively validate its functionality.

The measured frequency is approximately 120 kHz, compared to a nominal value of about 124 kHz, with the difference mainly attributable to component tolerances, particularly in the ceramic capacitors and the inductor .

The value of R3 depends on the characteristics and quality factor of the inductor. In the prototype, the inductor was hand-wound on a ferrite toroid to obtain approximately 100 µH. When reproducing the circuit, R3 may require empirical adjustment depending on the specific inductor used.

This oscillator provides a simple and effective solution for generating low-distortion sine waves at medium–high frequencies, offering easy amplitude control via op-amp gain while eliminating the need for dedicated amplitude control circuitry and directly delivering a low-impedance bipolar output.

—Luca Bruno has a Master’s Degree in Electronic Engineering from Politecnico of Milano. He taught electronics and telecommunications for many years at ITI Hensemberger and has published numerous Design Ideas in EDN on analog and electronic circuit design.

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