![]() ![]() With both CE and CB circuits, the ac input (v i) is developed across the base-emitter junction, and the ac output (v o) is produced at the transistor collector terminal, (see Fig 6-42). So, a CC circuit (an emitter follower) has a voltage gain of 1. In the case of a circuit with an unbypassed emitter resistor, the ac voltage at the emitter follows the ac input at the transistor base. 6-41, the circuit output impedance at the collector is essentially, This is normally a very large quantity at low and medium signal frequencies. ![]() So, the impedance ‘looking into’ the collector is the device output impedance. The output for CE and CB circuits is produced at the transistor collector terminal. A CB circuit normally has its base bypassed as shown, so that the impedance at the emitter is,įor all circuit arrangements, the impedance at the transistor emitter terminal is, 6-40, R B includes the signal source resistance (r s) for a CC circuit. Where R B is the impedance ‘looking back’ from the transistor base.Īs shown in Fig. For a CC circuit or a CB circuit with an unbypassed base (using CE parameters), Although the Z e equations for the two circuits look different, they can be shown to give exactly the same result in similar situations. So, the device impedance in each case is the impedance ‘looking into’ the transistor emitter terminal (Z e). The transistor emitter is the output terminal for a CC circuit and the input terminal for a CB circuit. The circuit input impedance for both CE and CC configurations is Z b in parallel with the bias resistors, A rough approximation for the base input impedance of any transistor circuit with an unbypassed emitter resistor is, The CC equations for Z b are almost identical to the CE equations, except that they use h ic and h fc, which are essentially equal to h ie and h fe, (see Table 6-3). When R E is bypassed, the R E(1 + h fe) portion can be treated as zero, so that, Figure 6-39 shows that, for a CE circuit with an unbypassed emitter resistor, So, Z i is the impedance ‘looking into’ the base. In both the CE and CC circuits, the input signal is applied to the transistor base terminal. In the case of input and output impedances, it is helpful to think in terms of the terminal being looked into.Ĭonsideration of each type of circuit shows that the input impedance (Z i) depends upon which transistor terminal is involved. Table 6-3 gives the circuit impedance equations in terms of CE h-parameters. Although these can be converted to CC and CB parameters, it is convenient to use CE parameters for all three types of circuits. Device manufacturers normally only list the CE h-parameters on a transistor data sheet. ![]()
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