SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

7.5. HIGH-FREQUENCY BEHAVIOR OF A BJT 343

Zo CBC

1

ωTCBC

Figure 7.19: Equivalent circuit representation of the output impedanceZo.

output impedance

Zo=

Vo

Io

(7.5.84)

From nodal analysis, we get the following expression forIo:

Io=

−jωT

ω

·iB+iB=

[

−jωT

ω

+1

]

iB (7.5.85)

We now assumeCBC<< Cin, and since

iB=jωCBC(Vo−vBE)

and
iB=jωCinvBE

we can combine these expressions to show that

vBE=

(

CBC

Cin+CBC

)

Vo

This shows thatvBE<< Vo,andsoiB jωCBCVo.
We can now writeIoas a function ofVo.

Io =

[

−jωT

ω

+1

]

·jωCBCVo

=[ωTCBC+jωCBC]Vo (7.5.86)

=[Gs+jX]Vo

This shows us that the output impedance can be expressed as a resistor 1 /(ωTCBC)in parallel
with a capacitorCBC, as shown in figure 7.19. The dc output conductance

Ro−^1 =

IC

VA