A transistor circuit for a signal multiplier used in, for example, a demodulator by means of the Costas loop method is disclosed. The transistor circuit comprises first to third circuit stages each including first and second transistors coupled in a differential form. The first to third circuit stages are connected in tandem with one another such that the output signal current of each circuit stage is supplied to the succeeding circuit stage without a substantial change. Further, each of the first to third circuit stages is supplied with one of or both of two input signals P and Q. As a result, the transistor circuit produces an output signal representing a signal multiplication: P.times.Q.times.(P+Q).times.(P-Q).