BJT class-A amplifier Solution



Design a twostage BJT classA amplifier to provide the following characteristics.

This project is an individual effort.


Transistors: type 2N2222, nominal β = 200 with βmin = 100 and βmax =300

(For OrCAD, select Q2N2222 in Eval” library and edit thePSpice Model” to change the

value for βf right mouse click on transistor)

Power Supply: VCC from Project #1

Source frequency = 1kHz

Load Impedance: 64Ω (impedance of speaker)

Performance Requirements

1. Primary Goal: Output load current peak swing, 𝑖��𝐿��,𝑃𝑃𝑃𝑃𝑃��𝑃𝑃 ≥ 70𝑚𝑚𝑚𝑚 before clipping (140mA

𝑖𝑖 𝑖𝑖

peaktopeak) over the range 100 𝛽𝛽 300. Extra points for 𝑖𝐿,𝑃𝑃𝑃𝑃𝑃��𝑃𝑃 > 70𝑚𝑚𝑚��.

2. Primary Goal, the total (2-stage) small signal current gain, 𝑚𝑚 = 𝑖𝑖𝐿��� = 140 at 𝛽𝛽 = 200.


Simulated gain should be within 10% of this value.

3. Primary Goal: Do not exceed maximum power dissipation for any 2N2222 of 0.6 watts.

4. Primary Goal: Do not exceed maximum power dissipation for any resistor of 3 watts.

5. Secondary goal, total power supply current, IPS, should not exceed 185mA. Extra points for IPS<125mA.

6. Secondary goal, Rin of complete amplifier = 10kΩ +/- 10%.

𝑖𝑖 𝑖𝑖

7. Extra Points if your small signal gain, 𝑚𝑚 = 𝑖𝑖𝐿�, is stable with changes in β over the


range 100 𝛽𝛽 300. A stable gain is 𝑚�𝑖𝑖 ± 15% over this range in β using your β=200

value as nominal.


Resistors can have any value during the initial design. You dont need to adhere to industry standard values unless you want to (could be helpful later). If you calculate a resistor of 2 ohms or less, then you probably dont need this one.

When calculating the DC operation of your driver circuit, it could be helpful to use the typical approximation for VBEQ = 0.7V.

There will be differences between your hand calculations and the PSpice results. You will start the PSpice simulation using your hand calculated values then you will slightly adjust the resistor(s) of thedriver circuit until you achieve the required specifications in PSpice.

Assume ro = infinite (transistor small signal model)

Submission Requirements:

DESIGN: You will need the following parameters from Project #1:

1. VCC


2. R *(PSpice simulated input resistance)

3. Ai* (PSpice simulated small signal gain)

4. IPS, ICQ, VBEQ, VCEQ, and PD (PSpice simulated)

Design the input driver stage to your twostage amplifier by using the equivalent input resistance, Rin, of your Project 1 as theload to this new amplifier as shown in the figure below. You will need to translate the current gain and maximum swing specs to the equivalent circuit using the small signal gain from Project #1, Ai*. Also remove the voltage source and

100Ω source resistor used during the design of the Project 1 as shown below.

When designing the driver amp circuit, translate the specs using the following:

Current gain of Driver Amp = 140 / (Ai*)

Max. current swing of Driver Amp = 70mA / (Ai*)

After you are confident that your design meets the specifications, answer the following questions.



a. From project #1, list the selected VCC, the PSpice equivalent input resistance, Rin , and

PSpice small signal current gain, Ai*. List these parameters on a table.

b. What are your calculated DC transistor bias parameters? Make a table showing ICQ, VBEQ, VCEQ, and PD for the driver transistor.

c. What are the power rating for each of your resistors? Make a table of the resistor

values and expected power dissipation.

d. What is your total DC current, IPS, supplied from the DC voltage source (both stages for a total; use PSpice value, IPS, for project #1)?

e. What is your calculated small signal current gain, Ai? Make a table of the gain for the driver (input) stage, the power amp stage (PSpice), and the combined twostage design.

f. What is your calculated smallsignal Input Resistance, Rin, of the complete twostage amplifier? Did you achieve a calculated input resistance of 10kΩ?

g. Submit the complete schematic of your twostage design.

h. Submit a copy of your hand calculations showing important calculations and selection of all resistor values.

2) PSPICE SCHEMATIC: Submit your PSpice schematic for the complete two stage amplifier (driver and power amp connected together). On this schematic, show all values for DC voltage, DC current and DC Power for each circuit element (for OrCAD, select V, I and P from the menu). On the schematic, make sure the DC parameters are positioned to allow the display of each resistor value (no display overlaps).

3) SIMLATED DC BIAS: What are the simulated DC bias parameters, ICQ, VCEQ, VBEQ and transistor Power Dissipation, PD? Submit a table showing the simulated values for ICQ, VCEQ, VBEQ, and PD for each transistor. As a comparison and on this same table, include the values that you calculated in (b) above.

4) SIMLATED POWER SUPPLY BIAS: What is the total current delivered from the supply, IPS?

Does your circuit operate below the specification for maximum current, IPS < 185mA. Extra points for IPS<125mA.

5) SIMULATED CURRENT SWING: Simulate your twostage design in PSpice using a 1kHz

sinusoidal input. Place a current probe on the wire leading into your circuit (on the left side of the capacitor). What is the largest peaktopeak swing in the output current, iL, through the 64 ohm load resistance? To find the maximum swing, you should slowly increase the source current amplitude until the output current waveform begins to clip at the top or the bottom. Unless you have a design optimized for maximum symmetrical swing in ICQ, its most likely that the clipping will not be symmetrical. In other words, the point of clipping in the positive peak current will be different from the negative peak current. Submit a plot showing iL with clipping using a β = 200. When clipping occurs, chose the smaller of the two values and double this value, this is the peaktopeak swing in the output current, iL. Report the peaktopeak swing in the output current, iL. Did your design achieve the 70mA peak swing (140mA peak-


6) Repeat (E) for β = 100 and 300. Submit a table showing the maximum peaktopeak swing in

iL for β = 100, 200 and 300. Did you meet the peaktopeak swing under all conditions?

7) SIMULATED EFFICIENCY: Using the peak swing in load current, iL, and load resistance, 64 ohms, calculate the simulated power conversion efficiency, η? You will need the simulated

IPS in this calculation. Show all your calculations using data recorded from your simulated plots, no points for just an answer.

8) SIMULATED SMALL SIGNAL OPERATION: Using PSpice, reduce the input source amplitude until both transistors are operating in the small signal region. This current should be small enough to operate your transistor in the small signal region. Verify that the peak AC voltage across the BaseEmitter junction, vbe, is less than 2.6mV peak (<<26mV) for both transistors. In OrCAD, you can verify the voltage using the Differential Voltage Probemeasurement. If vbe is greater than 2.6mV, decrease the source current until the voltage is in the small signal range for both transistors. Record the amplitude value of the source for small signal operation. Submit plots showing of vBE for β = 200 for both transistors. Use a scale so the voltage waveforms can easily be observed.

9) SIMULATED TWOSTAGE CURRENT GAIN: Using the input source amplitude found in



part (8), what is the small signal gain, 𝑚�𝑖𝑖 = 𝑖𝑖 , at β = 200? Submit two plots showing iS and

iL under small signal conditions using a β = 200. Did you achieve a current gain, |iL/iS|, of

140 ?

10) SIMULATED GAIN VARIATION WITH β: Continue with Step (9), what is the current gain,

iL/iS, when β =100 and β =300 (plots are not required here)? Submit a table showing current



gain, 𝑚�𝑖𝑖 = 𝑖𝑖 , for β = 100, 200 and 300. What is the % variation in your gain across this range

in β (relative to β = 200)? Do you achieve a current gain variation ± 15% about the nominal

value of β = 200?

11) SIMULATED INPUT RESISTANCE: Using the source amplitude found in part (8), what is the simulated input resistance, Rin, to your amplifier? Using current and voltage test probes at the amplifier input to calculate the input resistance using PSpice by placing cursors at the peak of each sinusoidal waveform and dividing the voltage peak by the current peak (absolute

values are acceptable if required). Note that the placement of V/I probes on the schematic in circuit areas where there is DC voltage and/or DC current will result in a DC offset in the simulated measurements, if so, either move the probe or subtract the DC offset from the results. Submit a plot showing vS and iS on two separate graphs for β = 200. Submit a table showing Rin for β = 100, 200 and 300. Did you achieve an input resistance of 10 k+/−10%?

12) SIMULATED FREQUENCY RESPONSE: Perform a small signal AC analysis of the small

signal current gain over the frequency range of 100Hz to 10MHz for β=200 (note: in OrCAD, you will need to change the source to VAC component). Record the 3dB bandwidth of the current gain, |iL/iS| (use 20*log10(|iL/iS|) )? You may need to increase the upper end of the frequency range (>10MHz) if the gain has not fallen by 3dB. Submit a plot showing the current gain as a function of frequency using a log scale in the x-axis and calculated dB in the yaxis. What does this frequency range tell you about your amplifier?

13) SIMULATION USING STANDARD RESISTOR VALUES: Using your PSPICE design, substitute the standard resistors into your model. Using β=200, simulate the peakpeak current swing and small signal current gain. Submit a plot showing iL with clipping at both the positive and negative peaks using a β = 200. Submit plots of is and iL under small signal

conditions. Did your amplifier achieve the peakpeak and nominal gain specifications when the

resistors are changed? Submit a table comparing the simulated peak swing, the simulated small signal gain and input resistance with the original and standard resistors.

14) COMPONENT BILL OF MATERIALS (BOM) FOR LAB EXPERIMENT: Create the BOM, with standard resistor values (ohms), the quantities for each type of resistor and the required power spec (1/4 watt, ½ watt, 1 watt or 3 watt). Include the BOM in your project submission and also submit your BOM by email to Robert Rivera, In the email subject include class and lab section (for example, EE3124 BOM Lab Section A1