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The calculation of hi2b is facilitated if one assumes that V2 =. V'c; then

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The approximate form h22b=juCc is useful if one is interested in the high-frequency performance where the capacitive reactance is much less than r'c. Also, the load impedance appears directly in parallel with h22b, and this must be much smaller than r'c if the transistor collector circuit is not to be the limitation on the high-frequency response. With low load impedances the effect of r'c is negligible at all frequencies.

The approximate form for hi2b shows that it is constant at low frequencies and then increases linearly with frequency. Again with small load impedances the effect of h\2b can be ignored at low frequencies; hence only the term jwr'bCc need be retained in most cases. With these approximations,

Fig. 2-22 The circuit representation of the short-circuit input impedance hm-

Fig. 2-22 The circuit representation of the short-circuit input impedance hm-

the equivalent circuit may be modified to the form shown in Fig. 2-23, where the components ¬°xec and r'c are omitted.

2-7 The Hybrid-pi High-frequency Equivalent Circuit. Although the equivalent circuit of Fig. 2-23 may be used for the transistor connected in any manner, it is frequently more convenient for the common-emitter

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