1
Chapter 4
BIPOLAR JUNCTION
TRANSISTORs
(BJTs)
DC Biasing
Chapter 4 BJTs DC Biasing
1
So far, we have learned the following important basic relationships for a transistor.
The term Biasing is a term for the application of dc voltages to establish a fixed level
of current and voltage.
For transistor amplifiers the resulting dc current and voltage establish an operating point
on the characteristics that define the region that will be employed for amplification of
the applied signal.
Chapter 4 BJTs DC Biasing
2
Since the operating point is a fixed point on the characteristics, it is called the
Quiescent point (Q-point). Quiescent means quiet, still, inactive.
2
Saturation region:
Cutoff region:
Chapter 4 BJTs DC Biasing
3
General output characteristics
Q-point at A – No bias were used, the device would initially be completely off.
Namely, zero current through the device.
Q-point at B – The best operating point in terms of linear gain and largest possible voltage
and current swing.
Q-point at C –
Q-point at D –
Chapter 4 BJTs DC Biasing
4
3
For BJTs to be biased in its linear or active operating region the following must be true.
1.
The base-emitter junction must be forward-biased (p-region voltage more positive),
with a resulting forward-bias voltage of about 0.6 to 0.7V.
2.
The base-collector junction must be reverse-biased (n-region more positive), with
th
bi
lt
b i
l
ithi
th
i
li
it
f th d
i
e reverse- as vo age e ng any va ue w
n e max mum m s o e ev ce.
Operation in the cutoff, saturation and linear (active) regions of the BJT characteristics
are provided as follows:
1.
Linear-region operation
Base-emitter junction forward biased
Base-collector junction reverse biased
2
Cutoff region operation
Chapter 4 BJTs DC Biasing
5
.
-
Base-emitter junction reverse biased
Base-collector junction reverse biased
3.
Saturation-region operation
Base-emitter junction forward biased
Base-collector junction forward biased
BJT DC BIASING ANALYSIS
(A) FIXED-BIAS CIRCUIT
Here, even though the network
employs an npn transisto