ECE 336: Microelectronics Project 2

Abstract—The purpose of this project was for the experimenters to grasp a theoretical and visual knowledge of the applications and importance of BJT differential amplifiers.

Index Terms—AC Voltage Resistor BJTTransistor DC Voltage Voltage

I. INTRODUCTION

HIS project conducted by three students currently enrolled in Electrical and Computer

Engineering course number 336 entitled Microelectronics, was performed for the purpose of understanding the physical and computational results obtained from a BJT differential amplifier. A BJT differential amplifier, according to Wikipedia, is “a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.

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The experimenters were required to design and build and simulate a BJT differential amplifier to deliver an output swing greater than or equal to six volts peak to peak. Additional requirements of the amplifier includes an CMMR greater than 60 dB, a single ended output difference mode gain greater than or equal to 80 and a single ended output common mode gain less than or equal to one tenth.

II. CIRCUIT DESIGN

A. Schematic

The circuit used the experiment was synthesized and design within the software of Mirco-Cap 10©. The software contained PSPICE capabilities, which anchored the analytical parts of the experiment. Fig 1 shows the schematic constructed for analysis. This complete synthesized circuit was used to analyze and formulate results which are expected to be more similar to the results that should be obtained from testing the actual design. This is illustrated below in Fig 1.

Fig 1: Schematic representation of the circuit used for the project.

The values of the variables used are listed below in Table 1.

V 2 R2  V CC 10V REE  

V EE -10V Q1,Q2,Q3

 2N3904

R1 Rcc1,Rcc2  Table 1: Description of values used for the circuit analysis of Fig 1.

III. THEORETICAL CALCULATIONS AND SIMULATION

A. Hand CalculationsBf 85 V BE 0.7VV T .025V V A 70V

Table 2: Assumed values used for some of the following equations.

Below are important equations used for calculating theoretical values for the experimenters BJT differential amplifier with the assistance of assumed variable depicted in table 2. To begin extrapolating variables the experimenters simplified the bottom half of the circuit as displayed in Fig. 2.

ECE 336 Project 2, BJT Differential AmplifierLevon Brassfield, James Dhoruai, Brett Stroube

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ECE 336: Microelectronics Project 2

Fig 2: Schematic representation of the lower portion of the circuit used for the project.

It is for this reason the experimenters derived the equivalent voltage and resistance shown in equations 1 and 2 respectively.

V eq=V EE∗R1

R1+R2

(1)

Req=R1∗R2

R1+R2

(2)

Additionally, to acquire the desired solution of equations 11, 12, and 13, equations 3 through 10 we needed.

I C3=

B f∗V CC−V BE−V eq

Req+(Bf +1) REE1

(3)

V EC3=V EE−I E RE−V BE (4)

I C1=I C2

=

B f

Bf +1∗I C3

2

(5)

V EC1=V EC2

=V BE−(−V CC+RCC∗I 4) (6)

r π3=

Bf∗V T

I C3

(7)

r03=

V A∗V EC3

I C3

(8)

ROUT3=r03(1+¿

Bf∗REE1

Req+r π 3+REE1

) (9)

AV =gm∗RCC

2 (10)

Add=gm∗RCC (11)

Acc=RCC( 1Bf r 03

−1

2REE1) (12)

CMRR=gm1∗ROUT 3

= | Add

2Acc

| (13)

A. Narrative of its Operation

The experiment was to set up a BJT differential amplifier as per simulations. Testing was carried out separately for each transistor and resistor to check for correctness. The experimenters achieved this be using an Oscilloscope.

Upon the completion the experimenters then applied a differential voltage signal at 1kHz to each input equal in magnitude but opposite in polarity. Subsequently, measurements for the output voltage and the differential mode voltage gain resulted in 25mV and 6.7Vdc respectively.

Lastly, a common mode signal at 1kHz (equal magnitude and same polarity), was applied to the inputs. As a result, the output voltage and common mode voltage gain were recorded to be 1V and 5.61Vdc respectively.

B. Simulation

Below is a graph of probe the transit analysis for sinusoidals retrieved from the simulated circuit.

Fig 3: Ac analysis of terminal gains

C. Actual

Fig 4: Broadboard with implementation of circuit designed in Fig 1.

After every piece of the circuit was tested for accuracy and constructed, the experimenters began to us a voltmeter to measure the DC and AC voltage across the load and transistors, to compare it with their hand calculations.

The experimenters ultimately got their CMRR from equation 13 to be 91.7dB.

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ECE 336: Microelectronics Project 2

IV. COMPONENTS

The experiment required the following equipment 3-2N3904 transistors  A Solderless breadboard  Alligator clips 22 Gauge Wires Resistors Oscilloscope Voltmeter

The only difference between the original simulation and the actual circuit is that the power Transistor blew up and a separate 2N3904 transistor was put in its place.

V. CONCLUSION

This project took and great amount of time for the experimenters. The efforts given were collaborative and congruently symmetric. The project completed was outstanding to the group as the percent difference seemed to be in their favor. The experimenters had a tough time going from a simulate version of the circuit to the actual.

APPENDIX

Attached in the Appendices are hand calculations and a signed documentation of the variables and values obtain trough testing. These hand calculations where used by the experimenters to evaluate and set values of resistors and transistors until the desired gain was fashioned.

ACKNOWLEDGMENT

The experimenters would like to thank the Lab assistants for all of their help and guidance.

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