<Tr> <Th> Input impedance </Th> <Td> r in ≜ v in i in (\ displaystyle r_ (\ text (in)) \ triangleq (\ frac (v_ (\ text (in))) (i_ (\ text (in)))) \,) </Td> <Td> r π + (β + 1) R E (\ displaystyle r_ (\ pi) + (\ beta + 1) R_ (\ text (E)) \,) </Td> <Td> r π (\ displaystyle r_ (\ pi)) </Td> </Tr> <Tr> <Th> Output impedance </Th> <Td> r out ≜ v out i out (\ displaystyle r_ (\ text (out)) \ triangleq (\ frac (v_ (\ text (out))) (i_ (\ text (out)))) \,) </Td> <Td> R C (\ displaystyle R_ (\ text (C)) \,) </Td> <Td> R C (\ displaystyle R_ (\ text (C))) </Td> </Tr> <P> If the emitter degeneration resistor is not present, then R E = 0 Ω (\ displaystyle R_ (\ text (E)) = 0 \, \ Omega), and the expressions effectively simplify to the ones given by the rightmost column (note that the voltage gain is an ideal value; the actual gain is somewhat unpredictable). As expected, when R E (\ displaystyle R_ (\ text (E)) \,) is increased, the input impedance is increased and the voltage gain A v (\ displaystyle A_ (\ text (v)) \,) is reduced . </P> <P> The bandwidth of the common - emitter amplifier tends to be low due to high capacitance resulting from the Miller effect . The parasitic base - collector capacitance C CB (\ displaystyle C_ (\ text (CB)) \,) appears like a larger parasitic capacitor C CB (1 − A v) (\ displaystyle C_ (\ text (CB)) (1 - A_ (\ text (v))) \,) (where A v (\ displaystyle A_ (\ text (v)) \,) is negative) from the base to ground . This large capacitor greatly decreases the bandwidth of the amplifier as it makes the time constant of the parasitic input RC filter r s (1 − A V) C CB (\ displaystyle r_ (\ text (s)) (1 - A_ (\ text (V))) C_ (\ text (CB)) \,) where r s (\ displaystyle r_ (\ text (s)) \,) is the output impedance of the signal source connected to the ideal base . </P>

In ce configuration of transistor the input impedance is