What is Analog multiplier using emitter coupled transistor pair?

Answer 1

The standard analog multiplier circuit (i.e. multiplying two analog signals to generate a product analog signal) sums the logs of the inputs then takes the antilog to form the output. The log and antilog functions are approximated using the base-emitter curve of transistors. The rest of the circuit is opamps, which have emitter coupled differential transistor amplifiers at their core. This however is limited to positive values only, and is called a one quadrant multiplier.

Perhaps you are asking instead about a type of analog multiplier using two cross-connected emitter coupled differential transistor amplifiers and several constant current sources for bias. The main advantages of these is they can correctly handle negative numbers, and are thus called four quadrant multipliers, and they are almost trivial to integrate compared to the opamp version.