Programming instructions

Darlington Transistor (NPN & PNP)
 National Instruments Corporation 4-9 Multisim Component Reference Guide
4.3.1 DC Bias Model
If a Darlington transistor with a very high current gain, β
D
, is used, the base current may be
calculated from
This equation is the same for a regular transistor, however, the value of
β
D
is much greater,
and the value of
V
BE
is larger.
The emitter current is then
DC voltages are:
4.3.2 AC Model
The AC input signal is applied to the base of the Darlington transistor through capacitor C
1
,
with the ac output,
V
o
, obtained from the emitter through capacitor C
2
. The Darlington
transistor is replaced by an ac equivalent circuit made up of an input resistance,
r
i
, and an
output current source,
β
D
I
b
.
4.3.2.1 AC Input Impedance
The AC input impedance looking into the transistor base is then
The AC input impedance looking into the circuit is
I
B
V
CC
V
BE
–
R
B
β
D
R
E
+
---------------------------=
I
E
β
D
1+()I
B
β
D
I
B
≈=
V
E
I
E
R
E
=
V
B
V
E
V
BE
+=
V
i
I
b
----- r
i
β
D
R
E
+=
Z
i
R
B
r
i
β
D
R
E
+()
||
=
ComponentRef.book Page 9 Thursday, December 7, 2006 10:12 AM