Emitter output circuit analysis (a)

The collector bootstrap circuit was introduced in the OTL power amplifier circuit in Chapter 1. It
increases the collector voltage through the charging of the bootstrap capacitor, so that the output voltage of the output tube in the positive half cycle of the signal is symmetrical with the output voltage in the negative half cycle. In addition
, collector bootstrapping can also increase the input impedance of the circuit and increase the circuit gain. Of course, due to the influence of the bootstrap capacitor,
the low-frequency response of the circuit will be subject to certain limitations. Its application should be appropriately restricted in high-fidelity audio circuits.
    In addition to collector bootstrap, there is also a base bootstrap circuit. The use of base bootstrap can increase the input impedance of the amplifier.
In the emitter output circuit of Figure 8-9, if the shunting effect of the bias resistors Rbl and ‰ is not considered, the input impedance can be
expressed by R = rbe + (1 + Lu) RA.
    However, the shunting effect of Rbl on the base actually exists and cannot be ignored. Therefore, the input resistance of the circuit
must change. For AC signals, the base bias resistors Rb1 and Rk are connected in parallel, and their parallel resistance is R b=
Rtl∥R b2. The actual input impedance of the circuit is Ri=R:∥R b, as shown in Figure 8-9 (b)o In Figure (b), if the transistor
R = 100, Re.RL is the value given in Figure (a), according to the formula R and rk + (Wang + Lu). R: Calculation, Re= Re∥RL=
1kCI', be - 11dl, then: R:=102kflo If calculated according to 旯i=R j∥R b, since Rb=Rbl∥R b2=
20kCI, then Ri= 17kflo It can be seen that the actual input impedance R is greater than that of the group.