Electronic Circuits Laboratory EE462G Lab #8 BJT Common Emitter Amplifier
Feb 04, 2016
Electronic Circuits LaboratoryEE462GLab #8
BJT Common Emitter Amplifier
npn Bipolar Junction Transistor
BJT in a common-emitter configuration
+VBE_
+VCE_
C
B
EB
B – Base
C – Collector
E – Emitter
For most applications the BJT is operated in the active region where:
p
n
n
B
C
E
BECEBE VVV and V60.
npn BJT Operation
BJT in a common-emitter configuration in active region (VCE > VBE ~ .6V):
+VBE_
+VCE_
C
B
E
The pn junction for VBE is forward biased and current iBE flows according to the Shockley equation:
n
n
1
T
BEESE V
VIi exp
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.
iE
- - - - -
+ + + + +
(-)
npn BJT Characteristics
BJT Transfer Characteristics in active region with = IC / IB = 100:
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1x 10
-5
Volts (VBE)
Am
ps
(Ba
se C
urr
en
t)
0 1 2 3 4 5 6 70
1
2
3
4
5x 10
-4
Volts (VCE)
Am
ps
(Co
llect
or
Cu
rre
nt) IB = 4 microamps
IB = 3 microamps
IB = 2 microamps
IB = 1 microamp1
DC (Biasing) Model
Equations for the DC operating point,
assume IB <<<< I2:
21
2
RR
RVV CCBB
R1
R2
RC
RE
VCC
VBBIB
BE
C
IC
IE
I2 EECECCCC RIVRIV
BC II EBC III
Key Result (Load-line Equation):
EC
CE
EC
CCC
RR
V
RR
VI
11
How sensitive is Ic to changes in ?
R1
R2
RC
RE
VCC VCC
DC (Biasing) Equivalent Model
Apply Thévenin model to base terminal:
21
2
RR
RVV CCB
21
12
RR
RRRB
Load-Line Equation
EB
BE
EB
BB RR
V
RR
VI
)()(
11
VB
RC
RE
RB
IB
VCC
Load Line Analysis:
0 0.2 0.4 0.6 0.8 10
0.2
0.4
0.6
0.8
1x 10
-5
Volts (VBE)
Am
ps
(Ba
se C
urr
en
t)
0 1 2 3 4 5 6 70
1
2
3
4
5x 10
-4
Volts (VCE)A
mp
s (C
olle
cto
r C
urr
en
t) IB = 4 microamps
IB = 3 microamps
IB = 2 microamps
IB = 1 microamp1
EB
B
RR
V
)( 1
BV
EC
CC
RR
V
1
CCV
Qualitatively describe what happens in both curves when increases(or decreases).
Large-Signal (DC) Model
VB
VBE VCCRB
RC
RE
IB
IB
IC
IE
BE
C
R1
R2
RC
RE
VCC
VBBIB
BE
C
IC
IE
I2
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.
vs
Rs Cin
R1
R2
RC
RE
Cout
CE
RL
VCC
BJT Amp Small-Signal ModelConsider 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 r is found from linearizing the nonlinear base-emitter characteristic,
which is an exponential diode curve.
21
12
RR
RRRB
Rs
vs r
iB
BC
Ero RL
+vin
-
iB +vout
-
RCRB
BJT Amp Small-Signal Model
Determine the voltage gain. How would the emitter resistor affect the gain if it was not bypassed?
Rs
vs r
iB
BC
Ero RL
+vin
-
iB +vout
-
RCRB
RE replaces the short here!
BJT Circuit Parameters
How can be found experimentally using the curve tracer?
How can the input and output resistances be determined experimentally?
How can voltage gain be determined experimentally?
Taylor Series
Recall that a function can be expressed as a polynomial through a Taylor Series expansion:
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.
axax dx
xfdax
dx
xdfaxafxf
2
22 )(
!2
)()(
!1
)()()(
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:
Q1beta= 100
R1
10
KR
25
k
R3
1K
V1
12
R5
50
R4
60
0
C11u
V2
0
C2
1u
C31u
R6
1KIVm1
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?
bjtexamAC-Transient-0-Graph Time (s)
0.0 200.000u 400.000u 600.000u 800.000u 1.000m
(V)
-200.000m
0.0
200.000m
max:355.280u, 0.286
min:405.280u, -0.291
max:625.280u, 0.00981
min:675.280u, -0.00980
TIME 586.280u v(IVm1) -7.532m v(IVm2) -107.794m
D(TIME) 0.0 D(v(IVm2)) 0.0
SPICE Example
Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.
Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-0-Graph Frequency (Hz)
0.0 20.000k 40.000k 60.000k 80.000k
0.0
500.000m
freq -1.000 norm_mag_v8 -1.000 D(freq) -7.623
D(norm_mag_v8) -1.000
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?
bjtexamAC-Transient-1-Graph Time (s)
0.0 200.000u 400.000u 600.000u 800.000u 1.000m
(V)
-2.000
-1.000
0.0
1.000
max:560.280u, 1.971
min:603.280u, -2.396
max:725.280u, 0.0785
min:775.280u, -0.0782
TIME 709.280u v(IVm1) 42.303m v(IVm2) -2.217
D(TIME) 0.0 D(v(IVm2)) 0.0
SPICE Example
Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.
Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-2-Graph Frequency (Hz)
0.0 20.000k 40.000k 60.000k 80.000k
0.0
500.000m
freq -1.000 norm_mag_v8 -1.000 D(freq) -5.780
D(norm_mag_v8) -2.653Meg
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?
bjtexamAC-Transient-3-Graph Time (s)
0.0 200.000u 400.000u 600.000u 800.000u 1.000m
(V)
-4.000
-2.000
0.0
2.000max:539.280u, 2.434
min:610.280u, -4.323
max:833.280u, 1.040
min:875.280u, -1.166
TIME 850.280u v(IVm1) -4.199m v(IVm2) 2.322
D(TIME) 0.0 D(v(IVm2)) 0.0
SPICE Example
Fourier analysis of output. Frequency magnitude plot for output at meter IVM2.
Where should the energy be in the frequency domain for this output? bjtexamAC-Fourier-3-Graph Frequency (Hz)
0.0 20.000k 40.000k 60.000k 80.000k
0.0
500.000m
freq -1.000 norm_mag_v8 -1.000 D(freq) -1.661
D(norm_mag_v8) -1.661