Rl jooCc j

Consequently the effect of Cc is about the same as connecting a "Miller-effect" capacitance of value Cc( 1 + a0RL/r'e) from V' to ground and omitting the capacitor Cc entirely. The resulting equivalent circuit is shown in Fig. 3-38c. Since usually 1/cotr'e Cc and a0 = 1, the value for Ceq is conveniently approximated by

1 CcRl 1 Ceq = —+ --- = —7 (1 + co tCcR£) (3-104)

oitre re wtre

With the aid of this simplified circuit, the gain and bandwidth are very easily found. The voltage gain is

The final approximate equation is usually valid in a video-type amplifier because Rg is of necessity small. For the common-emitter amplifier the bandwidth

(Rg + rOC,*, (Rg + 4)(1 + o,,CcRl) ^ j is increased by decreasing the source impedance Rg. The maximum obtainable bandwidth, which is obtained by using a voltage source (Rg = 0), is 1 /r'bCeq- From this it is seen that a transistor suitable for wide bandwidths has a high oit, a low collector capacitance Cc, and low base resistance r'b.

The bandwidth with very large RK may be obtained from Eq. (3-105) by finding the limit of Br as Re approaches infinity.

This limiting bandwidth (if Rl is small) is the so-called beta-cutoff frequency, Ujs 4 (1 — a0)ui, diminished by the effect of Cc. This bandwidth is approximately that of a resistance-coupled stage, designed to give high gain as in an audio amplifier.

Since a single stage rarely produces enough gain, the multistage situation depicted in Fig. 3-39a is of particular interest. Because the output circuit

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