I am designing a voltage-controlled state variable filter for use in a modular synthesizer. Its function is to produce low-pass, band-pass, and high-pass responses for a given audio input signal. Corner frequency and resonance (Q) are determined by LM13700 amplifier bias currents, which are controlled by external voltages.
Here is the circuit I'm simulating and breadboarding. Resonance control is simplified in this schematic, and signal ac-coupling and output buffering is not shown.
Behavior of the simulation and breadboard prototype is as-expected for normal control voltage and signal input frequencies in the range of <1Hz to 20KHz.
Out of curiosity I simulated the circuit to 100MHz and found that, in the 50KHz-200KHz range, gain starts increasing. This is most visible in the low-pass output. The frequency at which gain increases is independent of Q and corner frequency, and seems to be determined entirely by the op amps used for U6 and U7. Responses for TL072 and LT1022 show a 180-degree phase shift at different frequencies, leading me to believe this is non-ideal op amp behavior.
I am not too concerned since the circuit functions properly in its intended application, but would like to understand why this is happening.
Is non-ideal op amp behavior the cause of this high frequency behavior? If so, what parameters are relevant?