Transistors Pres

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Transcript of Transistors Pres

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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), maximum water 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 continuously twist 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 supply and 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 of current (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-base

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NPN Characteristic Curves

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PNP Characteristic Curves

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Common Emitter Characteristics

BCE III

coEC IIαI

COBCC I)IΙαI

  

  

  

  

α-1

ICOC B I  I 

 

 

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α1

αβ

 

  

 

α-1

IIβI

CO

BC

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Ideal CE Input (Base) Characteristics

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Ideal CE Output (Collector) Characteristics

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Operation Mode

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Operation Mode Active:

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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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