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Whites, EE 320 Lecture 23 Page 1 of 17

Lecture 23: Common Emitter AmplifierFrequency Response. Millers Theorem.

Well use the high frequency model for the BJT we developed inthe previous lecture and compute the frequency response of acommon emitter amplifier, as shown below in Fig. 5.71a.

(Fig. 5.71)

As we discussed in the previous lecture, there are three distinctregion of frequency operation for this and most transistor amplifier circuits. Well examine the operation of this CE

2009 Keith W. Whites

7/27/2019 320Lecture23 (1)

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Whites, EE 320 Lecture 23 Page 2 of 17

amplifier more closely when operated in three frequencyregimes.

Mid-band Frequency Response of the CE Amplifier

At the mid-band frequencies, the DC blocking capacitors areassumed to have very small impedances so they can be replaced

by short circuits, while the impedances of C and C are very

large so they can be replaced by open circuits. The equivalentsmall-signal model for the mid-band frequency response is then

Well define

so that at the output

R L = r

o || R C || R L (1)

V o = g m R L V (2)

Using Thvenins theorem followed by voltage division at theinput we find