LABORATORY 5
FREQUENCY RESPONSE OF A BJT AMPLIFIER
OBJECTIVES
1. To determine quiescent point of a common emitter and common collector
amplifiers and calculate the values of the resistors Rc, Re, R1 and R2. .
2. To measure upper and lower cutoff frequencies of a common-emitter amplifier.
3. To simulate amplifier frequency response measurements using MicroCap software.
4. To study the frequency response of the common emitter (CE) and common
collector (CC) BJT transistor amplifiers in the low to medium frequency range (50
Hz to 1 MHZ).
INFORMATION
In this lab, two BJT amplifier configurations will be investigated: the common-emitter, and
the common collector amplifier. Both amplifiers typically use a self-biasing scheme and
have a relatively linear output.
1. Common-Emitter Amplifier
The common emitter amplifier in Figure 5.1 is characterized by high voltage (Av) and
current gain (Ai). The amplifier typically has a relatively high input resistance (1 - 10 KΩ)
and a fairly high output resistance. Therefore it is generally used to drive medium to high
resistance loads. The circuit for the common-emitter amplifier can be seen in Figure 5.1. It
is typically used in applications where a small voltage signal needs to be amplified to a
large voltage signal. Since the amplifier cannot drive low resistance loads, it is usually
cascaded with a buffer that can act as a driver.
Vcc
IR
R1
Ic
Rc
C2
Ib
C1
1uF
Q1
1uF
Vin
R2
Ie
Re
RL
C3
47uF
.
3.9k
Vout
.
Figure 5.1. The common emitter amplifier
5-1
2. Common-Collector Amplifier
The common-collector (CC) amplifier, or the emitter-follower as it is sometimes called, is
a unity voltage gain, high current gain amplifier. The input resistance for this type of amp
is usually 1KΩ to 100KΩ. A typical CC amp can be seen in Figure 5.2. Because the
amplifier has a voltage gain of one, it is useful as a buffer amplifier, providing isolation
between two circuits while providing driving capability for low resistance loads.
Vcc
IR
R1
Ic
Q1
C1
C2
1uF
47uF
Ib
Vin
R2
RL
Ie
Re
.
Vout
3.9k
.
Figure 5.2. The common collector amplifier
EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
8.
Digital multimeter (Fluke 8010A, BK PRECISION 2831B)
Digital oscilloscope Tektronix TDS 210
Function Generator Wavetek FG3B.
PROTO-BOARD PB-503 (breadboard)
BJT 2N3904
Resistor RL=3.9 kΩ
Resistors Rc, Re , R1 and R2 according to prelab calculations
Capacitors 2 x 1µF, 1 x 47 µF.
PRE-LABORATORY PREPARATION
The lab preparation must be completed before coming to the lab. Show it to your TA for
checking and grading (out of 15) at the beginning of the lab and get his/her signature.
1. Common-Emitter amplifier
1.1. Use the BJT curve tracer in lab 3108 and follow the procedure explained in details in
the Lab #8 of ECE240a course to obtain the input and output characteristics of the BJT
2N3904 from your lab parts kit.
1.2. Follow the same explanations to calculate the β of the measured transistor. You will
use this value for determining the unknown resistor values.
1.3. For the circuit in Figure 5.1 calculate the values of Rc, Re, R1 and R2. DC Bias this
circuit for Vcc=10V, Vce=5V and Ic=5mA.
5-2
The resistors R1 and R2 form a potential divider, which will fix the base potential of the
transistor. The current IR through the R1 is usually set at least to 10 times the base current Ib
required by the transistor. The base emitter voltage drop of the transistor is approximated
as 0.7 volts. There will also be a voltage drop across the emitter resistor Re. The inclusion
of this resistor also helps to stabilize the bias: If the temperature increases, then extra
collector current will flow. If Ic increases, then so will Ie as Ie= Ib + Ic. The extra current
flow through Re increases the voltage drop across this resistor reducing the effective base
emitter voltage and therefore stabilizing the collector current.
Assume that Ve = Vcc/10 and IR = 10Ib and use Equations (5.1) to (5.5) to obtain Rc, Re,
R1 and R2 values. For your Lab setup choose the closest standard resistor values.
Vcc = I c Rc + Vce + I e Re
Equation (5.1)
I e = I b + I c as Ic >> Ib, then Ic ~ Ie
Equation (5.2)
Vb = Ve + 0.7
Equation (5.3)
Vb
V
= b
I R 10 I b
Equation (5.4)
Vcc − Vb Vcc − Vb
=
IR
10 I b
Equation (5.5)
R2 =
R1 =
1.4. Calculate the voltage gain, the current gain, the input resistance for this amplifier. Use
the β value that you measured in the pre-lab. (If you do not have this value, then use β
=200). All the calculations must be shown.
1.5. Simulate the above circuit in MicroCap using standard resistors values and attach the
bias point results. For this you must show the values of the all the bias currents and
voltages on your schematic.*
1.6. Using the MicroCap AC analysis function obtain the gain and phase frequency
response for this amplifier from 10Hz to 1MHz and find the 3dB point. Print the results
and bring these plots with you to the lab session for comparing with the experimental
data.** Bring the print-out of the DC quiescent point values and the Bode plots of the
magnitude (in dB) and phase angle (in degrees) of the gain ratio to the lab. You should use
the Bode plot printouts as the graphs on which to plot your experimental data.
Attention: You must plot the Bode plots, i.e. the ratio of the output voltage over the input
voltage!
MicroCap simulations tips:
• * To provide a power supply to the circuit use the “Battery” source from the
MicroCap library and set it to a 10V value .
• *To obtain the values of all the bias currents and voltages on your schematic
choose the Probe AC mode from Analysis menu and click on Node Voltages and
Currents icons on the toolbar.
5-3
•
**For a sine wave signal source (used for simulating the Vin), use a 1MHz Sinusoidal
Source from the Micro–Cap library. Set the AC Amplitude to A= 0.02(V) in the
model description area of the signal source. Note that A=0.02V corresponds to a
magnitude of Vp-p=0.04V.
• **To obtain the gain and phase frequency response plots for this circuit you must
run “AC ANALYSIS”. To get best results for your plots set the AC Analysis
Limits as follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
P
X-Expression
Y-Expression
X-Range
Y-Range**
Voltage Gain
Phase
1
2
F
F
dB(V(3)/V(1))* 1e6,10,1k
ph(V(3)/V(1))* 1e6,10,1k
50,-10,10
10,-190,20
Note: * V(1) and V(3) are the AC voltages at corresponding nodes(1) at the input
and (3) at the output of the simulation circuit set up. In your particular case they could
have different numeration.
2. Common-Collector amplifier
2.1. For the circuit shown in Figure 5.2 calculate the value of Re, R1 and R2. Bias this circuit
for Vcc=10V, Vce=5V and Ic=5mA.
The procedure for calculation of Re, R1 and R2 values is very similar to that used for
common-emitter amplifier. The current IR through the R1 is usually set at 10 times the base
current Ib required by the transistor. The base emitter voltage drop of the transistor is
approximated as 0.7 volts. There will also be a voltage drop across the emitter resistor Re.
Assume that IR = 10Ib and use Equations (5.6) to (5.10) to obtain Re, R1 and R2 values. For
your Lab setup choose the closest standard resistor values.
Vcc = Vce + I e Re
I e = I b + I c as Ic>>Ib, then Ic~Ie
Vb = Ve + 0.7
R2 =
R1 =
Vb
V
= b
I R 10 I b
Vcc − Vb Vcc − Vb
=
IR
10 I b
Equation (5.6)
Equation (5.7)
Equation (5.8)
Equation (5.9)
Equation (5.10)
2.2. Calculate the voltage gain, the current gain, the input resistance, and the output
resistance for this amplifier. Use the β value that you measured in the pre-lab. (If you do
not have this value, then use β =200). All the calculations must be shown.
5-4
2.3. Simulate the above circuit in MicroCap using standard resistors values and attach the
bias point results. For this you must show the values of the all the bias currents and
voltages on your schematic.*
4.4. Using the MicroCap AC analysis function obtain the gain and phase frequency
response for this amplifier from 10Hz to 1MHz and find the 3dB point. Print the results
and bring these plots with you to the lab session for comparing with the experimental
results. Bring the print-out of the DC quiescent point values and the Bode plots of the
magnitude (in dB) and phase angle (in degrees) gain ratio to the lab. You should use the
Bode plot printouts as the graphs on which to plot your experimental data.
Attention: You must plot the Bode plots, i.e. the ratio of the output voltage over the input
voltage!
MicroCap simulations tips:
• For a sine wave signal source use a 1MHz Sinusoidal Source. Set the AC Amplitude
to A= 1(V) in the model description area of the signal source. Note that A=1V
corresponds to a magnitude of Vp-p=2V.
• To get better results for your plots set the AC Analysis Limits as follows:
Frequency Range: 1E6,10 which corresponds to a frequency range from 10 Hz to 1MHz
Plot parameters:
P
X-Expression
Y-Expression
X-Range
Y-Range**
1
2
Voltage Gain
Phase
F
F
dB(V(3)/V(1))* 1e6,10,1k
ph(V(3)/V(1))* 1e6,10,1k
0,-10,1
90,00
Note: *V(1) and V(2) are the AC voltages at corresponding nodes (1)- input and (2)output of the simulation circuit set up. In your particular case they could have different
numeration.
PROCEDURE
1. Common-Emitter amplifier frequency response.
1.1. Build the circuit in Figure 5.3 using the components values calculated in the pre-lab.
Use standard parts when building the amplifier. Do not use multiple resistors to match
the specified values. Also measure the actual value of the resistors using the multimeter.
CH1
CH2
PHASE METER
Vcc
R1
Rc
RED
CH1
FG
C1
Q1
1uF
Rp
10k
CH1
GND
GND
Vin
R2
CH2
CH2
C2 1uF
Ib
Re
RL
Vout
C3 3.9k
47uF
GND
.
GND
OSCILLOSCOPE
BLACK
Figure 5.3. Common –emitter amplifier circuit measurements
5-5
CH1
CH2
When the layout has been completed, have your TA check your breadboard for errors
and get his/her signature on the Marking Sheet.
1.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using
the Digital multimeter. Measure the DC quiescent conditions. Make sure your circuit is
biased correctly, your measurements should deviate no more than 10% from the chosen
values for Ic and Vce. If your values deviate more than that, adjust the resistance
values, and provide an explanation in your report.
Attention: Remember that inserting any meter, such as an ammeter, will add additional
resistances in your circuit. Try to measure all currents as voltage drops across a resistor. It
becomes critical not to damage the BJT, otherwise you may have to start over again!
1.3. Connect the Function Generator (FG) to supply the input ac signal to the CE amplifier
circuit. To obtain the necessary value of Vin use the 10 kΩ potentiometer built in the
Proto Board as it shown in Figure 5.3.
1.4. Connect CH1 of the Oscilloscope and CH1 of the Phase meter in parallel with the
input of the CE amplifier to measure the parameters of the input signal Vin. Connect
CH2 of the Oscilloscope and CH2 of the Phase meter in parallel with the load resistor
RL to measure the parameters of the output signal Vout.
1.5. Set the input voltage level to Vin=20mV, as measured by the CH1 of the oscilloscope.
Starting from 50 Hz , sweep the input frequency up to 1 MHz.
1.6. For each of the selected frequencies read the RMS voltage of the Vin ( CH1) and Vout
(CH2) from the oscilloscope display and record the data in Table 5.1. For the same
frequencies record the Phase Meter readings (display will show the phase angle
between these two signals in [deg]).
1.7. For each of the measurements, calculate the voltage gain in Av(dB).
AV (dB) = 20 log
f [Hz]
50
75
100
200
500
1k
2k
5k
10k
20k
50k
100k
200k
500k
1M
Vin [V]
Vout
Vin
Vout [V]
Equation (5.11).
Θ[deg]
Av[dB]
Table 5.1. CE and CC amplifier frequency response
5-6
1.8. Plot the obtained data on top of your MicroCap simulations. Do they agree? If there is
a difference, explain what could be the reason for it.
1.9. Increase the input signal level until output voltage clipping occurs. Record the
maximum input and output levels of undistorted sine wave signal.
1.10. Note the phase shift between output and input. Is your amplifier inverting or noninverting?
2. Common-Collector amplifier frequency response.
2.1. Build the circuit in Figure 5.4 using the components given in the pre-lab. Use standard
parts when building the amplifier. Do not use multiple resistors to match the specified
values. Also measure the actual value of the resistors using the multi-meter.
CH1
CH2
PHASE METER
R1
RED
CH1
CH1
C1
Vcc
Q1
FG
Vin
GND
GND
CH2
C2 47uF
1uF
R2
RL
Re
CH2
Vout
3.9k
GND
GND
OSCILLOSCOPE
CH1
CH2
.
BLACK
Figure 5.4. Common –collector amplifier circuit measurements
When the layout has been completed, have your TA check your breadboard for errors
and get his/her signature on the Marking Sheet.
2.2. Initially apply only DC power to the circuit and measure the amplifier's Q point using
the Digital multimeter. Measure the DC quiescent conditions. Make sure your circuit is biased
correctly, your measurements should deviate no more than 10% from the chosen values for
Ic and Vce. If your values deviate more than 10%, adjust the resistance values, and provide
an explanation in your report.
Attention: Remember that inserting any meter, such as an ammeter, will add additional
resistances in your circuit. Try to measure all currents as voltage drops across a resistor. It
becomes critical not to damage the BJT, otherwise you may have to start over again!
2.3. Connect the Function Generator (FG) to supply the input ac signal to the CE amplifier
circuit.
2.3. Connect CH1 of the Oscilloscope and CH1 of the Phase meter in parallel with the
input of the CE amplifier to measure the parameters of the input signal Vin. Connect CH2
5-7
of the Oscilloscope and CH2 of the Phase meter in parallel with the load resistor RL to
measure the parameters of the output signal Vout.
2.4. Set the input voltage level to Vin=1V, as measured by the CH1 of the oscilloscope.
Starting from 50 Hz , sweep the input frequency up to 1 MHz.
For each of the selected frequencies read the RMS voltage of the Vin (CH1) and Vout
(CH2) from the oscilloscope display and record the data in separate table similar to Table
5.1. For the same frequencies record the Phase Meter readings (display will show the phase
angle between these two signals in [deg]).
For each of the measurements, calculate the voltage gain in (dB), using the Equation
(5.11).
2.5. Plot the obtained data on top of your MicroCap simulations. Do they agree? If there is
a difference, explain what could be the reason for it.
2.6. Increase the input signal level until output voltage clipping occurs. Record the
maximum input and output levels of undistorted sine wave signal.
2.7. Note the phase shift between output and input. Is your amplifier inverting or non-inverting?
2.8. Answer following questions:
• Do your experimental results agree with the MicroCap simulation? Comment on
both the DC and the AC values. Explain any discrepancies.
• What will the voltage gain, of your CE amplifier, be if RL=8Ω (input resistance of a
speaker)?
• Is your CE amplifier suitable for an audio amplifier?
• What is the main purpose of using the emitter follower?
REPORT
Your Lab report is due one week later. Please submit it to your TA in the beginning of
your next lab session.
Note: You must copy/print the Signature and Marking Sheet from your manual
before coming to the lab session.
5-8
SIGNATURE AND MARKING SHEET – LAB 5
To be completed by TA during your lab session
Student Name:____________________
TA Name:___________________
Student # : _____________________
Check
boxes
Task
Max.
Marks
Pre-lab completed
15
CE Amplifier completed
10
CC Amplifier completed
10
Overall Report Preparation
65
TOTAL MARKS
100
5-9
Granted
TA
Marks Signature