The latest wave of high-performance op amps allows you to incorporate "negatrons" (synthesized negative resistors) into your oscillators and filters. Fig 1 shows the universal filter/oscillator circuit based on a simulated lossy inductor and a negative resistance, which compensates the lossy inductor. Note that the circuit has no physical inductors. The performance of this circuit is very predictable and repeatable. Op-amp IC₂ and its associated components form a gyrator that simulates the inductance L_EQ=C₂R₄R₅. Paralleling L_EQ with C₁ forms a parallel-resonant LC tank circuit. Op amp IC₁ and its associated components form the negatron that simulates a negative resistance, -(R₁R₃)/R₂.

Fig 2 shows the simplified equivalent circuit of Fig 1's circuit. Obviously, to obtain continuous oscillation from IC₂'s simulated tank circuit, you must trim its shunt resistance to zero or a negative value. Otherwise, oscillations would simply die out. IC₁'s negatron cancels the tank circuit's shunt resistance.

One further detail: To stabilize the amplitude of oscillation, you must implement a nonlinear circuit in IC₁'s negatron. The simplest solution is the diode clamp, comprising D₁, D₂, and R_C, in parallel with Fig 1's R₂.

Configured as an oscillator, the circuit produces a 1V-rms, 1-kHz sinewave having <1% THD at OUT₁. OUT₂ produces a 0.7V-rms sinewave, 90° phase-shifted from OUT₁'s signal. The inexpensive LM358s in Fig 1 work fine up to 100 kHz. For higher frequencies, you have to use faster and more expensive op amps.

To configure the circuit as a filter, first remove the clamping diodes and R_C and then disconnect the input IN from ground. Next, trim R₂ to more than 7.5 kOhms. You can use OUT₁, OUT₂, or both, simultaneously. OUT₁ is a bandpass output, and OUT₂ is a lowpass output; OUT₂ is still 90° phase-shifted from OUT₁.

With R₂ set to 10 kOhms, Fig 3 shows the circuit's response to a 0.5V-rms input. The filter's maximum resonant amplification is approximately 18 dB at the 1-kHz center frequency, f_C. You can adjust f_C by varying R₄, R₅, C₁,
or C₂. Varying R₂ adjusts the filter's Q factor separately from its f_C. (DI #1560)