BJT ac Equivalent Circuit
In order to better visualize the operation
of a BJT as an amplifier, it is useful to
represent it by an equivalent circuit
Two types of equivalent circuits
representations are used based on h-
parameters (Hybrid) and r-parameters
For this course we shall limit our
discussion to r-parameters
Department of EE 1
,BJT ac Equivalent Circuit…
Recall that we used αdc and βdc in the
previous discussion
These are dc parameters
The corresponding ac parameters
are: ic i
ac ac c
ib i e
ac current
gain
Department of EE 2
, Simplified BJT r-parameter
Equivalent Circuit
C C The base-emitter
ic junction is forward
ib biased
B B ac signal applied to
re the base will see a
resistance of a forward
ie
E biased junction, re
E
ie = ic + ib 26mV
re
ic = βacib IE
Department of EE 3
, Simplified BJT r-parameter
Equivalent Circuit
C
C Current Source
ic
ib
B ic = βacib
B
ie ib
E re
ie = ic + ib 26mV
E re
ic = βacib IE
Department of EE 4
In order to better visualize the operation
of a BJT as an amplifier, it is useful to
represent it by an equivalent circuit
Two types of equivalent circuits
representations are used based on h-
parameters (Hybrid) and r-parameters
For this course we shall limit our
discussion to r-parameters
Department of EE 1
,BJT ac Equivalent Circuit…
Recall that we used αdc and βdc in the
previous discussion
These are dc parameters
The corresponding ac parameters
are: ic i
ac ac c
ib i e
ac current
gain
Department of EE 2
, Simplified BJT r-parameter
Equivalent Circuit
C C The base-emitter
ic junction is forward
ib biased
B B ac signal applied to
re the base will see a
resistance of a forward
ie
E biased junction, re
E
ie = ic + ib 26mV
re
ic = βacib IE
Department of EE 3
, Simplified BJT r-parameter
Equivalent Circuit
C
C Current Source
ic
ib
B ic = βacib
B
ie ib
E re
ie = ic + ib 26mV
E re
ic = βacib IE
Department of EE 4
