lab 4: bjt amplifier - college of engineering - university of … · · 2016-10-19lab 4: bjt...

EE221-2016 Laboratory #4 2016-10-16

Rev A Copyright 2016 © University of Saskatchewan of 12

Lab 4: BJT Amplifier

NOTE: The Prelab must be completed and handed in separately at the BEGINNING OF YOUR LAB SESSION. It is worth 2/10, while the remainder of your lab write up (due one week later), will be worth 8/10. Note also that you are required to bring the 2N2222A NPN Transistor you used in Lab #3 as you want to use the same transistor (have actual measured parameters for the transistor).

1. Learning Outcomes In this lab, students design and implement a Common Emitter (CE) amplifier and observe amplitude and frequency responses. A Breadboard and the Analog Discovery will be used in this lab. The students will use various tools and functions from the Waveform software to perform measurement and determine the amplifier response.

2. Health and Safety Any laboratory environment may contain conditions that are potentially hazardous to a person’s health if not handled appropriately. The Electrical Engineering laboratories obviously have electrical potentials that may be lethal and must be treated with respect. In addition, there are also mechanical hazards, particularly when dealing with rotating machines, and chemical hazards because of the materials used in various components. Our LEARNING OUTCOME is to educate all laboratory users to be able to handle laboratory materials and situations safely and thereby ensure a safe and healthy experience for all. Watch for posted information in and around the laboratories, and on the class web site.

3. Lab Report Students work in groups of 2 with laboratories being on alternative week (in 2C80/82). Each student must have a lab book for the labs. The lab book is used for lab preparation, notes, record, and lab reports. The lab books must be handed before 5:00 pm on the due date (same day of the following week) into the box labeled for your section across from 2C94.

4. Material and Equipment

Supplied by Department Supplied by student

Resistors: 1 kΩ, other values TBD 2N2222A (NPN Transistor) – from Lab #3

Capacitors: 1 µF x 2, 4.7 µF Analog Discovery

Digital Multi-Meter (DMM) Waveforms 2015 software

Breadboard and wiring kit

EE221-2016 Laboratory #4 2016-10-16


5. Prelab 1. Design a Common-Emitter (CE) amplifier (i.e. determine the required resistor values) using a 2N2222A

NPN transistor shown in Figure 5-1 with the following characteristics:

1.1. Voltage gain = -4

1.2. Collector current = 5 mA

1.3. Collector voltage = 2.5 V (provides approximately maximum voltage swing)

1.4. The bias resistances R1 and R2 should be in the range of 2 kΩ to 20 kΩ

1.5. Use the value of 𝛽𝛽𝐷𝐷𝐷𝐷 for the 2N2222A determined from Lab #3 (normally between 100 – 200)

1.6. Assume VBE = 0.7 V

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

Figure 5-1: CE Amplifier Circuit

2. Given the resistance values determined above and VIN = 0 V, calculate the DC values for VC, IC, VB, IB, VE and IE.

3. Answer the following questions:

3.1. What is the required ratio between R1 and R2?

3.2. Why is it important to limit the range of values for R1 and R2?

3.3. Why would you not want to use smaller capacitors for CB and CC?

EE221-2016 Laboratory #4 2016-10-16


4. Consider adding a 1 kΩ load resistor RL as shown in Figure 5-2. What would be the gain of the amplifier at 1 kHz? How would the gain of the amplifier be effected by increasing the frequency?

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

RL 1 kΩ

Figure 5-2: CE Amplifier with Load Resistance

5. Consider instead adding a 1 µF capacitor CE as shown in Figure 5-3. What would be the gain of the amplifier at 1 kHz? How would the gain of the amplifier be effected by increasing the frequency?

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

CE 1 µF

Figure 5-3: CE Amplifier with Bypass Capacitor

EE221-2016 Laboratory #4 2016-10-16


6. A template SPICE circuit file for the circuit shown in Figure 5-1 is given in Figure 5-4:

6.1. Set the appropriate values for the four resistors (i.e. replace the "XX" for the four resistor values). For example, "100" for a resistor value of 100 Ω and "1k" for a resistor value of 1 kΩ.

6.2. Use LTspice (a guide to using LTspice can be found in Laboratory #1 – Appendix A) to run the SPICE simulation.

6.3. Add the waveforms "V(vi)" and "V(vo)" (using "Plot Settings | Add Trace").

6.4. Include a screen capture of the plot in your lab submission. The plot should look similar to Figure 5-5.

6.5. Are the results what you were expecting?

BJT CE Amplifier (NPN) ** Circuit Description ** Vcc vcc 0 DC 5V ; 5 V voltage source Vin vi 0 SIN(0 100mV 1kHz) ; 100 mV, 1 kHz AC input CB vi vb 1uF ; DC blocking capacitor R1 vcc vb XX ; Bias resistor R2 vb 0 XX ; Bias resistor Q1 vc vb ve 2N2222A ; NPN transistor Rc vcc vc XX ; Collector resistor Re ve 0 XX ; Emitter resistor Cc vc vo 4.7uF ; DC blocking capacitor ** Rl vo 0 1k ; Optional load resistor ** Ce ve 0 1uF ; Optional bypass capacitor across Re .TRAN 0.01m 4m ; Simulate in 0.01 ms steps until 4 ms ****** *SRC=2n2222a;2n2222a;BJTs NPN; Si; 75.0V 0.800A 300MHz Central Semi Central Semi .MODEL 2N2222A NPN (IS=2.20f NF=1.00 BF=240 VAF=114 + IKF=0.293 ISE=2.73p NE=2.00 BR=4.00 NR=1.00 + VAR=24.0 IKR=0.600 RE=0.194 RB=0.777 RC=77.7m + XTB=1.5 CJE=24.9p VJE=1.10 MJE=0.500 CJC=12.4p VJC=0.300 + MJC=0.300 TF=371p TR=64.0n EG=1.12 ) ****** .end

Figure 5-4: Template SPICE Circuit File

7. Enable the load resistor "Rl" in the SPICE file (i.e. remove the "** " at the beginning of the line). Simulate and include a screen capture of the plot in your lab submission. Are the results what you were expecting?

EE221-2016 Laboratory #4 2016-10-16


8. Disable the load resistor "Rl" in the SPICE file (i.e. add back in the "** " at the beginning of the line) and enable bypass capacitor "Ce". Simulate and include a screen capture of the plot in your lab submission. Are the results what you were expecting?

Figure 5-5: Example SPICE Plot

EE221-2016 Laboratory #4 2016-10-16


6. Lab Procedures

Debugging (or What To Try When Things Aren't Working)

There are a number of things/procedures you should use to debugging circuits when things are not working correctly. These include (but are not limited to):

• Check that all component pins are correctly inserted in the breadboard (sometimes they get bent underneath a component).

• Make sure that components are not "misaligned" in the breadboard (e.g. off by one row).

• Double check component values (you can measure resistors, capacitors, and inductors).

• Try a different section in the breadboard (in case there is a bad internal connection).

• Measure the source voltages to verify power input.

• Measure key points in the circuit for proper voltage/waveform (i.e. divide-and-conquer).

Amplifier DC Characteristics

1. Construct the circuit shown in Figure 6-1 on your breadboard:

1.1. Use the Analog Discovery +5 V supply for VCC.

1.2. For this step, do NOT connect anything to VIN.

1.3. Record the actual resistor values used (the ones that were available and most closely match your determined values in the Prelab). Measure and record the actual values of the resistors using a Digital Multi-Meter (DMM). Don't forget you may be able to get closer values by combining two resistors in series or parallel.

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

Figure 6-1: CE Amplifier Circuit

EE221-2016 Laboratory #4 2016-10-16


2. Fill in Table 6-1. Use the Voltmeter Tool on your Analog Discovery to measure the DC voltages. Currents can be "measured" by measuring the voltage across a suitable resistor(s) and applying Ohm's law and/or applying KCL using some measured currents.

Parameter Measured Value Expected Value (from Prelab)

VC

IC

VB

IB

VE

IE

Table 6-1: Measure DC Values

Amplifier Gain

1. For the circuit in Figure 6-1, use WaveGen Channel 1 to generate VIN. Set to a 1 kHz, 100 mV peak sine wave.

2. Use Scope Channel 1 on the Analog Discovery to measure the input voltage (i.e. VIN) and Scope Channel 2 to measure the output voltage (i.e. VOUT).

3. Use the Scope tool on the Analog Discovery to display the input and output voltages and use the Measure Tool in the Scope to display the "Amplitude" of the input and output voltages and the "Frequency" of the signals. What is the gain of the amplifier? Why is it considered to be negative? How does it compare to the expected value?

4. If the measured gain is not between -3.9 and -4.1, adjust the value of RE to achieve a gain in this range. How does this value of RE compared to calculated required value of RE?

5. Include a screen capture of the Scope screen in your lab submission (which should look similar to Figure 6-2). REQUIRED: Demonstrate to a lab instructor and make sure your demonstration is recorded by the lab instructor.

6. Move Channel 2 so that you can observe both VIN and VB. Note how the DC voltage at the base is preserved by using coupling capacitor CB. Measure and record both the AC and DC components of VB (Hint: You can get both from the Scope measurement tool).

7. Now observe VC and VOUT and note how the coupling capacitor CC blocks all of the DC component in VC so that VOUT is a pure AC signal. Measure and record both the AC and DC components of VC.

8. Move Channel 1 back to VIN and Channel 2 to VOUT. On the Scope window, click on "View | Add XY" (use Channel 1 for "X" and Channel 2 for "Y"). Adjust the size of the plot (including using the C1 and C2 controls) so that is gives a good view of waveform (should look similar to Figure 6-3). Normally you would expect this to be a straight line. What does the slight oval shape represent?

EE221-2016 Laboratory #4 2016-10-16


Figure 6-2: Example Scope Screen

Figure 6-3: VO versus VI Plot

EE221-2016 Laboratory #4 2016-10-16


9. Next we want to determine the "bandwidth" of the filter (i.e. the low and high frequencies were the voltage gain of the amplifier is at 0.707 (half-power point)):

9.1. Reduce the frequency of VIN below 1 kHz until the gain of the amplifier reaches 70.7% of its "normal" gain. Document this frequency and include a screen shot of the Scope window (adjust the Time Base so that you can see a few cycles of the waveforms). Comment on the relative phase of VOUT versus VIN.

9.2. Increase the frequency past 1 kHz until the gain of the amplifier reaches 70.7% of its "normal" gain. At high frequencies, you may have to adjust the Amplitude of WaveGen Channel 1 to get a VIN of 100 mV. Document this frequency and include a screen shot of the Scope window (adjust the Time Base so that you can see a few cycles of the waveforms). Comment on the relative phase of VOUT versus VIN.

9.3. What is the bandwidth of your amplifier?

9.4. At what frequency do VOUT and VIN appear to be actually in phase?

Load Resistor

1. Add a 1 kΩ load resistor RL to the circuit from section 6.3, resulting in the circuit shown in Figure 6-4.

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

RL 1 kΩ

Figure 6-4: Amplifier with Load Resistor

2. Set VIN to a 1 kHz, 100 mV peak sine wave.

3. Determine the amplifier gain. How does this gain compared to the calculated gain from the Prelab? How is the phase affected between VIN and VOUT (relative to the amplifier without the resistor RL)?

EE221-2016 Laboratory #4 2016-10-16


Emitter Bypass Capacitor

1. Remove the 1 kΩ load resistor RL from the circuit and add a 1 µF capacitor CE across RE, resulting in the circuit shown in Figure 6-5.

VIN

R1

R2

VC

VE

VB2N2222A

Q1

RE

RC

CB

1 µF

CC

4.7 µF

VOUT

+5 V

CE 1 µF

Figure 6-5: Amplifier with Emitter Bypass Capacitor

2. Set VIN to a 1 kHz, 100 mV peak sine wave.

3. Determine the amplifier gain. How does this gain compared to the calculated gain from the Prelab? How is the phase affected between VIN and VOUT (relative to the amplifier without the capacitor CE)?

4. Set VIN to a 20 kHz, 100 mV peak sine wave. Use Channel 1 to measure VIN and Channel 2 to measure VOUT. Capture a "Reference" trace (Add Channel | Add Reference Channel) of Channel 2 on the Scope and move Channel 2 to measure VC. Include a screen capture of the waveforms. Explain why the waveforms are distorted.

Sweep Generator Function

We want to further investigate the amplifier gain as a function of frequency for the amplifier with the emitter bypass capacitor shown in Figure 6-5.

1. Select the "Sweep" tab in the WaveGen window and set the following (example given in Figure 6-6):

1.1. Type = Sine

1.2. Frequency = 1 Hz to 10 kHz in 10 ms

1.3. Amplitude = 100 mV

1.4. Auto synchronization

EE221-2016 Laboratory #4 2016-10-16


Figure 6-6: Sweep Generator

2. Use Channel 1 to measure VIN and Channel 2 to measure VOUT.

3. Set the Trigger for the Scope to "Source: Wavegen 1". Set Time Base to "1 ms/div" and Time Pos to "5 ms" and adjust the parameters for C1 and C2 to provide a reasonable view of the waveforms. Include a screen capture of the Scope screen in your lab submission (which should look similar to Figure 6-7). REQUIRED: Demonstrate to a lab instructor and make sure your demonstration is recorded by the lab instructor.

4. Use the "Y Cursors" to measure the peak-to-peak value of VOUT at various frequencies and fill in Table 6-2.

Frequency (kHz) VOUT (Peak-to-Peak) Gain

1

2

3

4

5

6

7

8

9

10

Table 6-2: Gain versus Frequency

EE221-2016 Laboratory #4 2016-10-16


Figure 6-7: VOUT and VIN with Frequency Swept from 1 Hz to 10 kHz

lab 4: bjt amplifier - college of engineering - university of … · · 2016-10-19lab 4: bjt...

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