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Trans across

istor resistor

The transistor can be thought of as a device whoseresistance (across collector-emitter) varies with inputcurrent (at the base).

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A water pipe analogy The collector (C) collects

water at the top, theemitter (E) emits waterat the bottom, and theflow of water current iscontrolled by a small tapand valve, similar to thebase lead (B) of atransistor.

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Water Pipe model (cont)

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When the tap is jammed on (valve vertical), maximumwater flows. Analogous to transistor saturation.

IC = IE at all times.

When the tap is jammed off (valve horizontal), nocurrent flows. Analogous to transistor cut off.

These two states are used in all digital circuitry usingtransistors. Only ON or OFF (1-0) states are possible.

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Analogue Transistor Action If we turn the tap half on

and then continuouslytwist the tap clockwiseand anticlockwise, thenthe change in watercurrent flowing fromcollector to emitter will

be proportional to thetwisting action.

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This is analogous to a transistor operating in the linearregion. Small changes in the base current cause muchlarger but directly proportional changes in the currentflowing from collector to emitter.

The transistor can be biased using a DC power supplyand resistors to act as an amplifier.

Small AC signals are applied via the input capacitor,which allows AC but not DC to flow through it.

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This is what it looks like on

breadboard

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Two main categories of transistors: bipolar junction transistors(BJTs) and field effect transistors (FETs).

Transistors have 3 terminals where the application ofcurrent (BJT) or voltage (FET) to the input terminalincreases the amount of charge in the active region.

The physics of "transistor action" is quite different for theBJT and FET.

In analog circuits, transistors are used in amplifiers andlinear regulated power supplies. In digital circuits they function as electrical switches,

including logic gates, random access memory (RAM),and microprocessors.

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Terminals and Operations Three terminals:

Base (B): very thin and lightly doped central region (littlerecombination).

Emitter (E) and collector (C) are two outer regionssandwiching B.

Normal operation (linear or active region): B-E junction forward biased; B-C junction reverse biased. The emitter emits (injects) majority charge into base region

and because the base very thin, most will ultimately reach thecollector. The emitter is highly doped while the collector is lightly

doped. The collector is usually at higher voltage than the emitter

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Transistors

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Circuit Symbol

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Circuit Configuration

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Common-emitter

It is called the common-emitter configuration because (ignoring the power

supply battery) both the signal source and the load share the emitter lead as

a common connection point

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Common-collector aAlso called an emitter foll

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ctdAlso called as emitter follower as the output is taken

from emitter resistor, used as an impedance matchingdevice, since its input impedance is much higher thanoutput impedance.

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Common-base

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ctd This configuration is more complex than the other

two, and is less common due to its strange

operating characteristics. Used for high frequency applications because the

base separates the input and output, minimizingoscillations at high frequency. It has a high voltage

gain, relatively low input impedance and highoutput impedance compared to the commoncollector.

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NPN Characteristic Curves

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PNP Characteristic Curves

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Common Emitter Characteristics

BCE III

coEC III

COBCC I)II

-1

ICOC BII

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1

-1

III

CO

BC

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Ideal CE Input (Base) Characteristics

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Ideal CE Output (Collector) Characteristics

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Avoid thissaturation regionwhere we try toforward bias both

junctions

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IDEAL CE OUTPUT

Avoid this cut-off region where we try to reversebias both junctions (IC approximately 0)

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Actual ce output characteristicsIB =

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Salient features are:

The finite slope of the plots (IC depends on VCE)

A limit on the power that can be dissipated. The curves are not equally spaced (i.e varies with

base current, IB).

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Operation Mode

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Operation ModeActive:

Most important mode, e.g. for amplifier operation.

The region where current curves are practically flat.

Saturation: Barrier potential of the junctions cancel each other out

causing a virtual short.

Ideal transistor behaves like a closed switch.

Cutoff: Current reduced to zero

Ideal transistor behaves like an open switch.

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BJT in Active Mode

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Active Mode Operation

Forward bias of EBJ injects electrons from emitter into base (smallnumber of holes injected from base into emitter)

Most electrons shoot through the base into the collector across thereverse bias junction

Some electrons recombine with majority carrier in (P-type) baseregion

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Operation Mode

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Bringing it Together

Type CommonBase

CommonEmitter

CommonCollector

Relationbetweeninput/output

phase

0 180 0

Voltage Gain High Medium Low

Current Gain Low () Medium (b) High (g)

Power Gain Low High Medium

Input Z Low Medium High

Output Z High Medium Low

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Load line

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Load line-To draw load line find saturation current and cut off voltage.After plotting these values on the vertical and horizontal axis, a line isdrawn joining these two points, which represents DC load line . Itrepresents all possible combinations of the collector current and

collector voltage for the given load resistor Rc.

Saturation point-the point at which load line intersects thecharacteristic curve near the collector current axis is saturation point.

At this point current is maximum and the voltage across the collector isminimum for a given value of load.

Ic (sat)= Vcc/Rc

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ctd Cut off point- the point where the load line intersects cut off region of

collector curves is refered to as the cut off point. At this point collectorcurrent is approximately zero and emitter is grounded for fixed biascircuit.

Vce(cut)= Vc =Vcc

Operating point- the Q-point for a transistor amplifier circuit is thepoint along its operating region in a quiescent , where no input signal

gets amplified.

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