<P> (While this is often described as "negative feedback", as it reduces gain, raises input impedance, and reduces distortion, it predates the invention of negative feedback and does not reduce output impedance or increase bandwidth, as true negative feedback would do .) </P> <P> At low frequencies and using a simplified hybrid - pi model, the following small - signal characteristics can be derived . </P> <Table> <Tr> <Th> </Th> <Th> Definition </Th> <Th> Expression (with emitter degeneration) </Th> <Th> Expression (without emitter degeneration, i.e., R = 0) </Th> </Tr> <Tr> <Th> Current gain </Th> <Td> A i ≜ i out i in (\ displaystyle A_ (\ text (i)) \ triangleq (\ frac (i_ (\ text (out))) (i_ (\ text (in)))) \,) </Td> <Td> β (\ displaystyle \ beta \,) </Td> <Td> β (\ displaystyle \ beta) </Td> </Tr> <Tr> <Th> Voltage gain </Th> <Td> A v ≜ v out v in (\ displaystyle A_ (\ text (v)) \ triangleq (\ frac (v_ (\ text (out))) (v_ (\ text (in)))) \,) </Td> <Td> − β R C r π + (β + 1) R E (\ displaystyle (\ begin (matrix) - (\ frac (\ beta R_ (\ text (C))) (r_ (\ pi) + (\ beta + 1) R_ (\ text (E)))) \ end (matrix)) \,) </Td> <Td> − g m R C (\ displaystyle - g_ (m) R_ (\ text (C))) </Td> </Tr> <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> </Table> <Tr> <Th> </Th> <Th> Definition </Th> <Th> Expression (with emitter degeneration) </Th> <Th> Expression (without emitter degeneration, i.e., R = 0) </Th> </Tr>

Define current gain of the transistor in ce configuration