A crude example is a typical inverting op-amp configuration with a light-dependent resistor (LDR) in the feedback loop.
The VCR is one of the numerous interesting circuit elements that can be produced by using a JFET (junction field-effect transistor) with simple biasing.
[1] Analog multipliers are a type of VCA designed to have accurate linear characteristics, the two inputs are identical and often work in all four voltage quadrants, unlike most other VCAs.
[4] Another early VCA capability on a sound mixer was the series of MCI JH500 studio recording desks introduced in 1975.
The digitally controlled amplifier uses a stepped approach giving the circuit graduated increments of gain selection.
At its most basic form, a toggle switch strapped across the feedback resistor can provide two discrete gain settings.
Other devices such as the CD4053 bi-directional CMOS analog multiplexer integrated circuit and digital potentiometers (combined resistor string and MUXes) can serve well as the switching function.