Page 770 - Mechatronics with Experiments

756 MECHATRONICS
the transistor saturates (fully ON) and collector current and V CE output voltage does not
change, V CE = V CE,min if input voltage is increased beyond that value (V in,sat ).
The V in,min is the voltage value which is necessary to provide the forward bias
voltage plus just a bit more in order to make the transistor start conducting. So, this value
is expected to be close to the forward-bias voltage of the transistor. This can be determined
experimentally by slowly increasing V and monitoring the change in V CE .
in
The V in,sat value is the value which provides a base current (i ), and after being
B
amplified results in i = i such that V CE = V CE,min . Voltages more than that will not
C
B
result in any change in the output since the V CE can be no less that V CE,min ≈ 0.2 V and
the rest of the available supply voltage is used to generate the current i . From the voltage
C
relations of the output and input circuits, we can calculate the input voltage saturation value
and experimentally measure to confirm it approximately,
V − V CE
s
i = (11.32)
C
R 2
V − 0.2
s
= (11.33)
R 2
1
i = ⋅ i (11.34)
B C

1 V − 0.2
s
= ⋅ (11.35)
R
2
V = R ⋅ i + V (11.36)
in,sat 1 B BE
1 V − V CE,min
s
= R ⋅ ⋅ + V FB (11.37)
1
R 2
1 V s
V in,sat ≈ R ⋅ ⋅ + V FB (11.38)
1
R 2
where V ≈ V = 0.6V to 0.8 V range, V ≈ 0.2 V. Exact values of these voltages
BE FB CE,min
may vary a little from transistor to transistor. However, we can accurately measure them
experimentally by carefully recording when the transistor makes the transition from cutoff
state to conducting state in the linear region and from linear region to fully ON saturation
region.
Procedure
1. Design and build the circuit shown in Figures 11.5 and 11.6.
2. Choose R = 10 kΩ, R = 1.0kΩ, V = 9 V. Other values for R and R can be
2
1
s
1
2
chosen. And other available DC voltages can be used for V .
s
3. Provide a means of adjusting the voltage input at the base of the amplifier at point A.
This can be done by using either a DC power supply or a battery (i.e., 9 V plus an
adjustable series resistor (R ) that would act as a voltage divider when used in series
s
with the R resistor) or through the DAC output channel of the PIC microcontroller.
1
4. Measure the voltage at the following points, V , V , V , V , V , and make a
in AB BE CE DC
table of them, each voltage representing a column on Table 11.1.
5. Set V to different values in increments, that is, 0.0V, 0.1V, … ,2.0 V, and measure
in
the other four voltages and record them using your digital multimeter (DMM) or
oscilloscope.
6. Plot the input voltage (V ) and output voltage relationships (V CE , V DC ) and conclude
in
in what range of input voltage the transistor is fully OFF, fully ON, and in proportional
(linear) amplifier mode.