Multiplier circuits encompass various types, including voltage, frequency, photo, and digital multipliers, with a focus on analog multipliers in this discussion. The Gilbert cell, invented by Barrie Gilbert, is highlighted for its application in variable gain control and RF mixing, with two main designs noted for their differing capabilities. Resources such as Wikipedia and Analog Devices provide additional information, while specific circuit schematics are referenced for deeper understanding. Techniques for analog multiplication include PWM methods, though digital approaches are often preferred for cost-effectiveness and stability. The conversation emphasizes the evolution and complexity of multiplier circuits in both analog and digital domains.