r bb and r o are omitted (insignificant) R B represents parallel combination of R B1 and R B2 At high frequencies C 1, C 2 and C 3 approximate short circuits. Problem : C BC influences the input and output halves of the circuit
Extending the Upper Cut-Off Use a different transistor – lower C BC. Reduce the gain; C IN is proportional to gain. Reduce the source resistance. Eliminate the Miller effect – use a different amplifier configuration.
High Frequency Effects Neither C BC or C BE connects v in to v out. There is, therefore, no Miller effect. C in = C BE C out = C BC
C-B vs. C-E Comparison Identical quiescent conditions Identical voltage gain (except C-E inverts) Identical output resistance Common-Base input impedance is very low Common-Emitter suffers Miller effect
Summary Common-emitter upper cut-off frequency is disappointingly low due, mainly, to the Miller effect. Common-base configuration does not suffer Miller effect but has impractically low input impedance. Solution : combine the two ?