Electronic Circuits Laboratory
EE462G
Lab #8
BJT Common Emitter Amplifier
npn Bipolar Junction Transistor
BJT in a common-emitter configuration
C
B
C
n
+
VCE
_
B
p
+
VBE
_
n
B
E
B – Base
C – Collector
E
E – Emitter
For most applications the BJT is
operated in the active region where:
VBE ≅ 0.6V and VCE > VBE
npn BJT Operation
BJT in a common-emitter configuration in active
region (VCE > VBE ~ .6V):
The pn junction for VBE is forward
biased and current iBE flows
according to the Shockley
equation:
C
-
B
+
+
+
+
+
n
+
VBE iE
_
n (-)
E
+
VCE
_
  VBE  
 − 1
iE = I ES exp
  VT  
where VT ≅ .26 mV and IES ranges
from 10-12 to 10-17.
Electrons from the emitter flow
into the base and are pulled into the
depletion region of the reversed
biased collector-base junction.
npn BJT Characteristics
BJT Transfer Characteristics in active region with
β = IC / IB = 100:
-5
5
Amps (Collector Current)
Amps (Base Current)
1
x 10
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
Volts (VBE)
0.8
1
x 10
-4
IB = 4 microamps
4
IB = 3 microamps
3
IB = 2 microamps
2
IB = 1 microamp1
1
0
0
1
2
3
4
Volts (VCE)
5
6
7
DC (Biasing) Model
Equations for the DC operating point,
R2
assume IB <<<< I2:
VBB = VCC
R1 + R2
VCC
I2
R1
VBB
IC
RC
C
IB
B
R2
VCC = I C RC + VCE + I E RE
IE
E
RE
I C = βI B
IC + I B = I E
Key Result (Load-line Equation):
IC =
RC +
VCC
β +1
β
−
RE
RC +
VCE
β +1
β
RE
How sensitive is Ic to changes in β?
DC (Biasing) Equivalent Model
Apply Thévenin model to base terminal:
R1
VCC
R2
VB = VCC
RC
VCC
R2
R1 + R2
RE
R2 R1
RB =
R1 + R2
RC
RB
IB
Load-Line Equation
VB
VBE
IB =
−
RB + (1 + β ) RE RB + (1 + β ) RE
VB
RE
VCC
Load Line Analysis:
If β changes, IC changes
VCC
Amps (Base Current)
1
x 10
RC +
β
β +1
5
RE
Amps (Collector Current)
VB
RB + (1 + β ) RE
-5
0.8
0.6
0.4
-4
IB = 4 microamps
4
IB = 3 microamps
3
IB = 2 microamps
2
IB = 1 microamp1
1
0
0
0.2
0
0
x 10
1
2
3
4
Volts (VCE)
5
6
VCC
0.2
0.4
0.6
Volts (VBE)
0.8
1
VB
Qualitatively describe what
happens in both curves when β
increases(or decreases).
7
Large-Signal (DC) Model
VCC
I2
R1
VBB
IC
RC
C
IC
β IB
C
IB
B
R2
RC
IE
RB
E
RE
B
VBE
VCC
E
IB
VB
RE
IE
BJT Amplifier
Once the DC operating point is set, the AC input and output are
coupled to the amplifier with capacitors so as not to perturb the
operating point.
VCC
R1
Rs
vs
Cout
RC
Cin
R2
RE
CE
RL
BJT Amp Small-Signal Model
Consider the capacitors as short circuits for the small signal AC and open circuit for
DC to obtain the model below. The resistor ro accounts for the small slope of the I-V
characteristics in the forward-active region (often assumed to be infinite). The
resistance rπ is found from linearizing the nonlinear base-emitter characteristic,
which is an exponential diode curve.
Rs
vs
+
vin
-
B
C
iB
RB
R2 R1
RB =
R1 + R2
β iB
rπ
ro
E
RC
+
vout
-
RL
BJT Amp Small-Signal Model
Determine the voltage gain. How would the emitter
resistor affect the gain if it was not bypassed?
Rs
B
C
vs
+
vin
-
iB
RB
β iB
rπ
ro
E
RC
+
vout
-
RE replaces the short here!
RL
Taylor Series
Recall that a function can be expressed as a
polynomial through a Taylor Series expansion:
( x − a ) df ( x)
( x − a) 2 d 2 f ( x)
+
+
f ( x) = f (a) +
2
1!
2!
dx x = a
dx
x=a
where a is a point about which the function is
expanded. Note that if a represents a quiescent
point for a voltage, then the reciprocal of the
coefficient first linear represents the small
signal impedance.
SPICE Example
The amplifier circuit can be constructed in B2SPICE using the
BJT npn (Q) part from the menu. The “edit simulation model”
option can then be used to set the “ideal forward beta”
The input can be set to a sinusoid at desired frequency and
amplitude for a transient analysis.
SPICE can also do a Fourier analysis to observed effects of
clipping and distortion. There should be no harmonic energy for
perfect amplification.
SPICE Example
Example circuit with meters to monitor input and
output:
R3
1K
R1
10K
C3
1u
V1
12
Q1
1u
beta= 100
R6
1K
C2
1u
600
V2
R4
0
R2
R5
5k
50
IVm1
C1
IVm2
SPICE Example
Graphic output for .03 V sine wave input (IVM1) at 10 kHz.
Output is shown for meter IVM2.
What would the gain of this amplifier be at 10 kHz?
SPICE Example
Fourier analysis of output. Frequency magnitude plot for
output at meter IVM2.
bjtexamAC-Fourier-0-Graph
0.0
20.000k
40.000k
60.000k
Frequency (Hz)
80.000k
500.000m
0.0
freq
-1.000
D(norm_mag_v8) -1.000
norm_mag_v8
-1.000
D(freq)
-7.623
SPICE Example
Graphic output for .08 V sine wave input (IVM1) at 10 kHz.
Output is show for meter IVM2.
What would the gain of this amplifier be at 10 kHz?
SPICE Example
Fourier analysis of output. Frequency magnitude plot for
output at meter IVM2.
bjtexamAC-Fourier-2-Graph
0.0
20.000k
40.000k
60.000k
Frequency (Hz)
80.000k
500.000m
0.0
freq
-1.000
D(norm_mag_v8) -2.653Meg
norm_mag_v8
-1.000
D(freq)
-5.780
SPICE Example
Graphic output for 1.2 V sine wave input (IVM1) at 10 kHz.
Output is show for meter IVM2.
What would the gain of this amplifier be at 10 kHz?
SPICE Example
Fourier analysis of output. Frequency magnitude plot for
output at meter IVM2.
bjtexamAC-Fourier-3-Graph
0.0
20.000k
40.000k
60.000k
Frequency (Hz)
80.000k
500.000m
0.0
freq
-1.000
D(norm_mag_v8) -1.661
norm_mag_v8
-1.000
D(freq)
-1.661
BJT Circuit Parameters
How can β be found experimentally using the
curve tracer?
How can the input and output resistances be
determined experimentally?
How can voltage gain be determined
experimentally?