24.2 Bipolar Transistors
701
For small collector–emitter voltage, the current quickly drops to zero. V CE is typically split in such
a way that the emitter–base diode is well biased forward and the CB diode has a high reverse voltage.
If V CE drops below a certain value (≈ 1 V for silicon transistors), there is no longer any bias at the CB
diode. A further reduction of V CE biases the CB diode in the forward direction and quickly brings the
collector current down to zero.
24.2.5 Basic Circuits
24.2.5.1 Common Base Circuit
In the common base configuration, there is no current amplification since the currents flowing through
emitter and base are almost the same. However, there is voltage gain since the collector current causes
a large voltage drop across the load resistor.
24.2.5.2 Common Emitter Circuit
The input resistance of the common emitter circuit (Fig. 24.10b) depends on the emitter–base diode
and varies between a value of the order of 100 k at small current and a few at larger current and
high V EB . The voltage gain is
r V =
V CE
V EB
=
I C
V EB
R L ,
(24.31)
where R L is the load resistance in the output circuit (see Fig. 24.4b). The ratio g m = I C /V EB is called
the forward transconductance. Also, the differential transconductance g
m = ∂ I C /∂V EB is used. The
voltage gain of the common emitter circuit is typically 10
2 –10
3 . Since current and voltage are amplified,
this circuit has the highest power gain.
If the input voltage V EB is increased, the collector current rises. This increase causes an increase of
the voltage drop across the load resistance R L and a decrease of the output voltage V CE . Therefore, the
phase of the input signal is reversed and the amplifier is inverting.
(a)
(b)
(c)
Fig. 24.10 a Common base, b common emitter and c common collector circuits with external loads
701
For small collector–emitter voltage, the current quickly drops to zero. V CE is typically split in such
a way that the emitter–base diode is well biased forward and the CB diode has a high reverse voltage.
If V CE drops below a certain value (≈ 1 V for silicon transistors), there is no longer any bias at the CB
diode. A further reduction of V CE biases the CB diode in the forward direction and quickly brings the
collector current down to zero.
24.2.5 Basic Circuits
24.2.5.1 Common Base Circuit
In the common base configuration, there is no current amplification since the currents flowing through
emitter and base are almost the same. However, there is voltage gain since the collector current causes
a large voltage drop across the load resistor.
24.2.5.2 Common Emitter Circuit
The input resistance of the common emitter circuit (Fig. 24.10b) depends on the emitter–base diode
and varies between a value of the order of 100 k at small current and a few at larger current and
high V EB . The voltage gain is
r V =
V CE
V EB
=
I C
V EB
R L ,
(24.31)
where R L is the load resistance in the output circuit (see Fig. 24.4b). The ratio g m = I C /V EB is called
the forward transconductance. Also, the differential transconductance g
m = ∂ I C /∂V EB is used. The
voltage gain of the common emitter circuit is typically 10
2 –10
3 . Since current and voltage are amplified,
this circuit has the highest power gain.
If the input voltage V EB is increased, the collector current rises. This increase causes an increase of
the voltage drop across the load resistance R L and a decrease of the output voltage V CE . Therefore, the
phase of the input signal is reversed and the amplifier is inverting.
(a)
(b)
(c)
Fig. 24.10 a Common base, b common emitter and c common collector circuits with external loads