less. Figure 9-10 shows the ac equivalent circuit for common-mode gain. here, the 47-£ emit- ter resistances of the transistors are elintinattetl. They are so small compared with 7.8 kl’! that they can be ignored. RE is physically a 3.9-kt! resistor. However, it appears to be twice that value because it supports both transistor cur— rents. As discussed in the first section of this chapter, the ideal situation is a constant iota] emitter current. However, this is not the case with Fig. 9-10. A common-niode signal will change the total emitter current because the impedance is not infinite. In the case where at common-mode signal is driving both hascs in a positive direction, both transistors are turneti on harder. Rt; must support twice the increase in current that it would if it were serving just a single transistor. The output signal is taken from Q2 in Fig. 9-10. As far as Q2 is concerned, its emitter is loaded by 7.8 kfl. This large re- sistance makes the common-mode gain less than l: