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Transistor circuit (commonemitter configuration) is commonly used
for voltage amplification, as shown in the example below.
Here we assume and
, and .
Given the input voltage , the base current can be found
by the input characteristic plot as shown below:
From the plot above we see that the voltage between B and E is a superposition
of DC component and an AC component
:
The resistance, the reciprocal of the slope of the curve around ,
is approximately
. Correspondingly the
base current is also a superposition of a DC component and an
AC component with an amplitude
:
(Why can't we get
?)
The output characteristic plot is shown below:
Note that the value is
.
The load line is the straight line that goes through the two points
The collector current and the output voltage can be
found either algebraically or graphically:
 Find output current:
 Find output voltage:
Note:
Switch Circuit
From the currentvoltage plot of the output characteristics, we see that
the operation of a transistor can be in one of the three possible regions:
 Linear region: When the input voltage is about ,
the transistor works in the linear range where the collector current
is proportional to base current . Amplification
takes place in the linear region due to this relationship.
 Cutoff region: When the input voltage (possibly
negative), and
is close to zero. The transistor is said to be cut off.
 Saturation region: When the input voltage is higher
than , will significantly increase (due to the exponential
relationship between and ). But as the maximum value of
is restricted by the voltage supply and the collector and emitter
resistors (
), the linear relationship
no longer holds. In this case, the transistor is said to be saturated
and
.
Severe distortion in output will be caused if a transistor
amplification circuit is working near either the cutoff or the
saturation region, as can be seen in the following sections.
Example
Assume ,
, . Given the input voltage
or , find the output voltage .

, the forward bias of BE PNjunction is insufficient
for it to conduct current, we have ,
,
. The transistor is cutoff (the switch is
open or opencircuit).
 , the BE PNjunction is forward biased, from the input
characteristics, we can find , here assumed to be , then
,
.
The transistor is in linear region.

, the BE junction is forward biased, from the input
characteristics, we can find , here assumed to be . If the
linear relationship could hold, then we have
,
. But this result
is obviously wrong, we realize the transistor is actually in the saturation
region (the switch is closed or shortcircut), in which the linear
relation no longer holds. In fact, it is impossible for the
transistor to draw from the voltage source, as the maximum current
is only
. In this case,
can be determined on the output characteristics to be about
(intersection of load line and the curve corresponding to ),
and
.
Conclusion: a change in input from 0.2V to 0.8V switches the output
current from 0 to about 10 mA, and the output voltage from 15V to 0.2V,
and the transistor is in cutoff, linear, and saturation region, respectively.
is only valid when the transistor is in the linear region.
Next: DC Biasing
Up: ch4
Previous: Bipolar Junction Transistor (BJT)
Ruye Wang
20140415