In transistor differential amplifiers, Fig. 1, the common-mode rejection suffers badly if resistor R is made small enough to give good amplifier sta¬bilization. A technique permitting reasonably high-operating current and high-rejection ratio, but low supply voltage was required.

Substituting a third transistor for the emitter resistor of the conventional circuit, as shown in Fig. 2, permits operation at fairly high currents. Also, it has the effect of a very large emitter resistor, since the collector characteristics of a tran¬sistor present a very high impedance to any col¬lector load. For example, if a stage current of 5 ma is required and the emitter resistance is de¬sired around 100 K, the conventional circuit would require a 500-v supply. Using the third transistor, it is quite easy to get the same results with a 10-v, or less, supply.