As we know, the function of the emitter-bypass capacitor in the CE confi guration is to establish an ac ground at the emitter. The fact that the resistance seen looking into the emitter is usually low...

As we know, the function of the emitter-bypass capacitor in the CE confi guration is to establish an ac ground at the emitter. The fact that the resistance seen looking into the emitter is usually low requires a fairly large capacitance, which is undesirable. The circuit of Fig. P2.67 eliminates altogether the need for such a capacitance by using the diode-connected BJT Q2 instead. The ac resistance re2 presented by Q2, though not zero, is small (26 V at IC 5 1 mA), so the small amount of emitter degeneration that it introduces for Q1 is a price well worth the elimination of the bulky bypass capacitor. This technique is widely used in IC implementations, where Q1 and Q2 are matched devices. With a signal source having a dc component of 0 V, the two BJTs experience the same VBE drop and thus carry the same current IC. Consequently, RE has to be specifi ed so as to carry twice as much current. (a) Regarding Q1 as a CE-ED amplifi er with a total emitter-degeneration resistance of RE//re2, fi nd expressions for the small-signal parameters Ri , Rc, Ro, and voyvsig. (b) Assuming matched BJTs with F 5 200, VBE(on) 5 0.7 V, and VA 5 100 V, and assuming the signal source has a dc component of 0 V, fi nd the above parameters numerically. Hint: thanks to BJT matching, we have I E1 5 I E2 5 I R E y2.