Electrónica Análoga I Prof. Gustavo Patiño. M.Sc, Ph.D. MJ

 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.

 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)

 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.

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

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

 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.

 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.

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.

ParameterCommon EmitterCommon CollectorCommon Base Voltage gain AvHigh (about 100)Unity (1)Medium (10-50) Current GainHigh ( ) 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

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, I C = βI B.

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.

Encuentre todas las corrientes y voltajes en el siguiente circuito:

 DV255/bjt.html DV255/bjt.html  Sedra/Smith. Microelectronics Circuits. Fourth Edition.