Page 769 - Mechatronics with Experiments

LABORATORY EXPERIMENTS 755
V
s

R D
2
C i
c
A R 1 B
V
in
i
B
E
FIGURE 11.5: Circuit of a voltage
amplifier using a transistor in a
common emitter configuration.

The relationships between electrical variables (voltages and currents) and electri-
cal parameters (resistance, transistor parameters) can be shown as follows (Figure 11.5).
Kirchoff’s voltage law between base (B) and emitter (E) indicates
V = V + V (11.22)
in AB BE
= R ⋅ i + V (11.23)
1 B BE
where V = V = 0.6V to 0.8 V range. V is the forward bias voltage between base
BE FB FB
and emitter and it can be up to 0.6V to 0.8 V range typically. This value is a property of a
transistor and is given in the datasheet of the transistor. If V is so small that that V < V ,
in BE FB
then no current flows at the base i = 0 and the transistor is in the cutoff (OFF state) mode.
B
Similarly, applying Kirchoff’s voltage law on the output side of the transistor, and noting
that the transistor acts as a current amplifier between its fully OFF (cutoff) and fully ON
states (saturation). The current gain ( ) is a property of the transistor and provided in its
datasheet. There can be variations in that gain up to 100% due to manufacturing variations.
Robust circuit designs should not rely on the exact value of the current gain of the transistor.
i = ⋅ i B (11.24)
C
V = V DC + V CE (11.25)
s
= R ⋅ i + V CE (11.26)
2
C
= R ⋅ ( ⋅ i ) + V CE (11.27)
B
2
( )
V − V FB
in
V = R ⋅ + V ; for V ≥ V (11.28)
s 2 CE in FB
R
1
( )
R ⋅
2
V CE = V − ⋅ (V − V ) (11.29)
FB
in
s
R 1
V = V − V (11.30)
DC s CE
( )
R ⋅
2
= ⋅ (V − V ) (11.31)
FB
in
R
1
Notice that V CE can be between maximum value of V (when i = 0for i = 0 when V <
B
C
in
s
V ), transistor is in cutoff (OFF) mode), and minimum value of about V CE,min = 0.2V
FB
(when the transistor is saturated, fully ON) which is a property of the particular transistor.
We can measure the two input voltage values of interest: first, the minimum input
voltage necessary to start making the transistor conduct, V in,min . For input voltages below
that the transistor will not conduct (OFF). Second, the value of the input voltage for which