Lecture 24: BJT Operation Regions (Active, Saturation and Cut-Off)
Biasing
Figure 1 shows a bias arrangement for both npn and pnp BJTs for operation as an amplifier.
Notice that in both cases the base-emitter (BE) junction is forward-biased and the base-collector
(BC) junction is reverse-biased. This condition is called forward-reverse bias.
Fig. 1: Biasing of an npn and pnp BJTs.
Collector Characteristic Curves
Consider the circuit diagram shown in Fig. 2a and notice in the circuit diagram that both VBB
and VCC are variable sources of voltage. Assume that VBB is set to produce a certain value of IB
and VCC is zero. For this condition, both the base-emitter junction and the base-collector junction
are forward-biased because the base is at approximately 0.7 V while the emitter and the collector
are at 0 V. The base current is through the base-emitter junction because of the low impedance
path to ground and, therefore, IC is zero. When both junctions are forward-biased, the transistor
is in the saturation region of its operation. Saturation is the state of a BJT in which the collector
current is independent of the base current. As VCC is increased, VCE increases as the collector
current increases. This is indicated by the portion of the characteristic curve between points A
and B in Fig. 2b. IC increases as VCC is increased because VCE remains less than 0.7 V due to the
forward-biased base-collector junction.
Ideally, when VCE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the
transistor goes into the active, or linear, region of its operation. Once the base collector junction is
reverse-biased, IC levels off and remains essentially constant for a given value of IB as VCE
continues to increase. This is shown by the portion of the characteristic curve between points B
and C in Fig. 2b. For this portion of the characteristic curve, the value of IC is determined only by
the relationship expressed as IC=βIB.
When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into
breakdown; and the collector current increases rapidly as indicated by the part of the curve to the
right of point C in Fig. 2b. A transistor should never be operated in this breakdown region.
Fig. 2: (a) Test circuit of npn BJT (b) collector current characteristic curve at one IB value (c)
collector current characteristic curve at multiple IB values
A family of collector characteristic curves is produced when IC versus VCE is plotted for several
values of IB, as illustrated in Fig. 2c. When IB 0, the transistor is in the cutoff region although there
is a very small collector leakage current as indicated. Cutoff is the non-conducting state of a
transistor. The amount of collector leakage current for IB = 0 is exaggerated on the graph for
illustration.
Saturation:
When the base-emitter junction becomes forward-biased and the base current is increased, the
collector current also increases (IC=βIB) and VCE decreases as a result of more drop across the
collector resistor. This is illustrated in Fig. 3. When VCE reaches its saturation value, VCE(sat),
the base-collector junction becomes forward-biased and IC can increase no further even with a
continued increase in IB. At the point of saturation, the relation IC=βIB is no longer valid. VCE(sat)
for a transistor occurs somewhere below the knee of the collector curves, and it is usually only a
few tenths of a volt.
Fig. 3: Circuit for saturated configuration of npn transistor.
DC Load Line:
Cutoff and saturation can be illustrated in relation to the collector characteristic curves by the use
of a load line. Fig. 4 shows a dc load line drawn on a family of curves connecting the cutoff point
and the saturation point. The bottom of the load line is at ideal cutoff where IC=0 and VCE=VCC.
The top of the load line is at saturation where IC=IC(sat) and VCE=VCE(sat). In between cutoff
and saturation along the load line is the active region of the transistor’s operation.
Fig. 4: DC load line on collector current characteristic curve.
