1. THE BJT (BIPOLAR JUNCTION TRANSISTOR) INTRODUCTION 2015-1 Prof. Gustavo Patiño. M.Sc. Ph.D MJ 12- 14 07-05-2015

2. THE BJT: THE BASICS  The transistor (name itself) comes from the phrase “transferring and electrical signal across a resistor”.  It is a three-terminal device that is capable of controlling current flow.  A small current at the input of the device can control larger currents at its output. Thus, we are able to build circuits that can amplify.

3. PHYSICAL STRUCTURE OF TRANSISTORS

4. BJT DEFINED BY PN JUNCTIONS  Junction: Thin layer where p-type and n-type materials meet.  Terminals: E - Emitter - Injects electrons (or holes) B - Base - Controls current flow C - Collector - Collects electrons (or holes)

5. TRANSISTOR OPERATION: CURRENT FLOW  Current Flow in an NPN transistor biased to operate in the active mode:  For an NPN emitter electrons are the majority carriers (n-type material has an excess of free electrons).  Electrons begin to move from E to B when a forward voltage VBE is applied (VBE ≥ 0.7V).  A tiny fraction of this injected stream of electrons are recombined in the p-type base region and flow out of the B terminal.  But the majority of the electrons are attracted to the n-type collector, because of the larger (+) voltage at the C terminal.

6. TRANSISTOR OPERATION: CURRENT FLOW (2)

7. BJT OPERATING REGIONS Operating Region ParametersMode Cut Off V BE < V cut-in V CE > V supply I B = I C = 0 Switch OFF Linear V BE = V cut-in V sat < V CE < V supply I C = β*I B Amplification Saturated V BE = V cut-in, V CE < V sat I B > I C,max /β, I C,max > 0 Switch ON

8. I-V CHARACTERISTICS OF BJT BJT OPERATING REGIONS

9. BJT OPERATING REGIONS (2) ModeEmitter-Base JunctionCollector-Base Junction ACTIVE (partially ON)FORWARD REVERSE (useful for amplification) CUTOFF (OFF)REVERSE REVERSE (useful for switching in digital circuits) SATURATION (fully ON)FORWARD FORWARD (useful for switching in digital circuits) Operating region = Operating mode

10. ALGORITHM TO DETERMINE OPERATION REGION FOR NPN TRANSISTOR

11. TRANSISTOR SYMBOLS AND CONVENTIONS

12. TRANSISTOR OPERATION: BJT PARAMETERS

13. TRANSISTOR OPERATION: CURRENT GAIN

14. TRANSISTOR OPERATION: CURRENT GAIN (2)

15. ACTIVE MODE OPERATION Need to apply voltage sources as follows to get current flow in the active region:It is usually easiest to analyze a transistor circuit directly on the circuit diagram.Why? Because it shows direction of current flow and relative voltages.

16. SUMMARY

17. CONCEPTUAL CIRCUIT FOR MEASURING THE I C -V CE CHARACTERISTICS OF THE BJT.

18. THE TRANSISTOR AS A SWITCH  While there are limitations as to what we can switch on and off, transistor switches offer lower cost and substantial reliability over conventional mechanical relays.  The secret to making a transistor switch work properly is to get the transistor in a saturation state.

19. THE TRANSISTOR AS A AMPLIFIER Transistor Connections Because an amplifier must have two input and two output terminals, a transistor used as an amplifier must have one of its three terminals common to both input and output as shown on the right. The choice of which terminal is used as the common connection has a marked effect on the performance of the amplifier. There are three connection modes: Common Emitter Mode. Common Collector Mode. Common Base Mode.

20. THE TRANSISTOR AS AN AMPLIFIER ParameterCommon EmitterCommon CollectorCommon Base Voltage gain AvHigh (about 100)Unity (1)Medium (10-50) Current GainHigh (50 - 800) Less than unity (<1) Input Impedance Medium (about 3 to 5k) High (several k)Low (about 50R) Output Impedance Medium, Approx = Load resistor value Low (a few ohms)High (about 1M) Summary of the three types transistor connection

21. DC OPERATION OF THE COMMON-EMMITER AMPLIFIER

22. GRAPHICAL CONSTRUCTION FOR THE DETERMINATION OF THE DC BASE CURRENT IN THE CIRCUIT

23. GRAPHICAL CONSTRUCTION FOR DETERMINING THE DC COLLECTOR CURRENT I C AND THE COLLECTOR-TO-EMMITER VOLTAGE V CE

24. OPERATION POINT AND SIGNAL COMPONENTS

25. DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES 1. Write down what you know on the diagram (voltages, currents, resistor values). 2. Put "?" next to the quantities that you want to find. 3. Take it step by step. 4. Look at the picture and solve for something that is easy to find, then write your answer on the diagram in the appropriate place. 5. Repeat step 4 until all unknowns have been found. 6. Most of the problems can be solved with just Ohm's law and the simple transistor relationships, for example, IC = βI B.

26. DC ANALYSIS: A FEW SIMPLE ANALYSIS RULES (…CONT) 7. When done, do a "sanity check" on the resultant voltages and currents: Do voltages fall from highest to lowest starting at the top of the diagram? Do your answers make sense? 8. In many cases we do not need to be very precise, so we use simple models. If more precision is required, we can use SPICE, MULTISIM or any other software and get more exact values for unknowns. Just remember that resistor tolerances are 1% (or even 5%), so the increased precision isn't always necessary. In addition, transistor β can vary widely. 9. Other Hints: "Assume that β is large" means to ignore the base current. For BJT in the active region, unless given, assume VBE ≈ 0.7V.

27. EJERCICIO PARA LA PRÓXIMA CLASE Encuentre todas las corrientes y voltajes en el siguiente circuito:

28. BIBLIOGRAPHY  http://people.senecac.on.ca/john.kawenka /EDV255/bjt.html http://people.senecac.on.ca/john.kawenka /EDV255/bjt.html  Sedra/Smith. Microelectronics Circuits. Fourth Edition.

29. APORTE DEL ESTUDIANTE  Para el estudiante:  En tu tiempo extra-clase, de qué manera puedes complementar el contenido dado en esta clase ?  Qué información adicional complementa y ayuda a comprender mejor el contenido de estas diapositivas ?  Qué preguntas te surgen de esta clase?  Qué respuestas le das a dichas preguntas?  Busca más bibliografía e información adicional que complemente tus respuestas y el contenido de esta clase.  Consulta oportunamente al profesor del curso para complementar tus respuestas y resolver tus dudas restantes. Electrónica Analógica I. Facultad de Ingeniería. Universidad de Antioquia. 2015-1