How do you analyze a common-emitter transistor circuit?
1 Answer
Analyzing a common-emitter transistor circuit involves understanding its operating principles, characteristics, and calculations related to voltage, current, and gain. The common-emitter configuration is one of the three basic transistor amplifier configurations, along with common-base and common-collector. In this configuration, the emitter of the transistor is the common terminal for both input and output.
Here's a step-by-step guide on how to analyze a common-emitter transistor circuit:
Identify the Components:
Identify the components in the circuit, including the NPN transistor, resistors, power supplies, and any signal sources.
Draw the Circuit Diagram:
Draw the circuit diagram with labeled component values and connections.
Operating Point Analysis (DC Analysis):
Analyze the circuit's DC operating point to determine the transistor's operating conditions when no AC signals are applied. This involves calculating base current (
I
B
), collector current (
I
C
), and collector-emitter voltage (
V
CE
).
a. Use Ohm's law to calculate the base current:
=
−
I
B
=
R
B
V
IN
−V
BE
Where
V
IN
is the input voltage,
V
BE
is the base-emitter voltage (typically around 0.7V for silicon transistors), and
R
B
is the base resistor.
b. Calculate the collector current using the transistor's current gain (
ℎ
h
FE
or
β):
=
×
I
C
=β×I
B
c. Calculate the collector-emitter voltage:
=
−
×
V
CE
=V
CC
−I
C
×R
C
Where
V
CC
is the supply voltage and
R
C
is the collector resistor.
AC Analysis (Small-Signal Analysis):
The AC analysis focuses on the small variations around the DC operating point caused by the input AC signal. This involves determining the voltage gain and input/output impedance.
a. Substitute the transistor with its small-signal model, which includes an AC current source (representing
I
C
variation) and a transconductance (
g
m
) representing the change in collector current with respect to base-emitter voltage.
b. Calculate the transistor's transconductance (
g
m
):
=
g
m
=
V
T
I
C
Where
V
T
is the thermal voltage (
×
/
k×T/q, where
k is Boltzmann's constant,
T is temperature in Kelvin, and
q is the electron charge).
c. Determine the AC voltage gain (
A
v
):
=
−
×
A
v
=−g
m
×R
C
This is the voltage gain without considering the effect of load and bypass capacitors.
Calculate Input and Output Impedances:
Calculate the input impedance (
Z
in
) and output impedance (
Z
out
) of the circuit.
a. Input Impedance:
The input impedance for common-emitter configuration is primarily determined by the base biasing resistors. Depending on the configuration, it could be
R
B
in parallel with the input resistance of the transistor.
b. Output Impedance:
The output impedance depends on the collector resistor (
R
C
) and the early voltage (
V
A
) of the transistor. It can be approximated as
R
C
in parallel with
r
o
, where
=
r
o
=
I
C
V
A
.
Remember that these calculations provide approximate results and are based on simplifications and assumptions. Depending on the complexity of the circuit and the accuracy required, more detailed analysis may be necessary, such as accounting for early effect, temperature effects, and higher-order transistor models.
Here's a step-by-step guide on how to analyze a common-emitter transistor circuit:
Identify the Components:
Identify the components in the circuit, including the NPN transistor, resistors, power supplies, and any signal sources.
Draw the Circuit Diagram:
Draw the circuit diagram with labeled component values and connections.
Operating Point Analysis (DC Analysis):
Analyze the circuit's DC operating point to determine the transistor's operating conditions when no AC signals are applied. This involves calculating base current (
I
B
), collector current (
I
C
), and collector-emitter voltage (
V
CE
).
a. Use Ohm's law to calculate the base current:
=
−
I
B
=
R
B
V
IN
−V
BE
Where
V
IN
is the input voltage,
V
BE
is the base-emitter voltage (typically around 0.7V for silicon transistors), and
R
B
is the base resistor.
b. Calculate the collector current using the transistor's current gain (
ℎ
h
FE
or
β):
=
×
I
C
=β×I
B
c. Calculate the collector-emitter voltage:
=
−
×
V
CE
=V
CC
−I
C
×R
C
Where
V
CC
is the supply voltage and
R
C
is the collector resistor.
AC Analysis (Small-Signal Analysis):
The AC analysis focuses on the small variations around the DC operating point caused by the input AC signal. This involves determining the voltage gain and input/output impedance.
a. Substitute the transistor with its small-signal model, which includes an AC current source (representing
I
C
variation) and a transconductance (
g
m
) representing the change in collector current with respect to base-emitter voltage.
b. Calculate the transistor's transconductance (
g
m
):
=
g
m
=
V
T
I
C
Where
V
T
is the thermal voltage (
×
/
k×T/q, where
k is Boltzmann's constant,
T is temperature in Kelvin, and
q is the electron charge).
c. Determine the AC voltage gain (
A
v
):
=
−
×
A
v
=−g
m
×R
C
This is the voltage gain without considering the effect of load and bypass capacitors.
Calculate Input and Output Impedances:
Calculate the input impedance (
Z
in
) and output impedance (
Z
out
) of the circuit.
a. Input Impedance:
The input impedance for common-emitter configuration is primarily determined by the base biasing resistors. Depending on the configuration, it could be
R
B
in parallel with the input resistance of the transistor.
b. Output Impedance:
The output impedance depends on the collector resistor (
R
C
) and the early voltage (
V
A
) of the transistor. It can be approximated as
R
C
in parallel with
r
o
, where
=
r
o
=
I
C
V
A
.
Remember that these calculations provide approximate results and are based on simplifications and assumptions. Depending on the complexity of the circuit and the accuracy required, more detailed analysis may be necessary, such as accounting for early effect, temperature effects, and higher-order transistor models.