What causes intermodulation in an electronic circuit?

Intermodulation occurs when two or more signals mix together to produce unwanted, phantom signals at new frequencies. For this mixing to happen, the signals must pass through a nonlinear circuit or device, such as a diode, a transistor, or an amplifier driven into saturation. In a perfectly linear circuit, multiple signals simply pass through independently without interacting. However, no practical circuit is perfectly linear, and strong signals often push receiver front-ends into their nonlinear operating regions, causing them to act like accidental mixers. Distractors like "lack of neutralization" or "positive feedback" refer to conditions that cause unwanted self-oscillation, not the mixing of external signals. Too little gain simply results in a weak signal, not the generation of intermodulation products.