i. transistor as a switch

ELECTRONICS ENGINEERING III

Engr. Robert G. de LunaECE Instructor, De La Salle Lipa

2

Introducing Transistors

• This is the symbol for an NPN transistor.

• Transistors are process devices.

Electronics Engineering III by Engr. Robert G. de Luna

3

Transistor Terminals

• Transistors have three terminals:

Collector

Emitter

Base


4

Transistor as a Switch• Transistors can be used as switches.

• Transistors can eitherconduct or not conduct current.

• That is, transistors can either be on or off.

Transistor Switch


5

How Transistors Work

• Switching iscontrolled bythe voltagebetween theBase and theEmitter.

Collector

Emitter

Base

• When VBE < 0.7V the transistor switches off andno current flows between the Collector and the Emitter.

• When VBE ≥ 0.7V the transistor switches on andcurrent flows between the Collector and the Emitter.


The transistor can be madeto operate as a switch bydesigning the associatedcircuit so that the transistoris either in the cut-offregion or in the saturationregion.

6Electronics Engineering III by Engr. Robert G. de Luna

The transistor can be operated in three modes:

A. Cutoff RegionB. Active RegionC. Saturation Region


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When IB=0 µA, the transistor is inthe cut-off region of its operation.This is drawn with the base leadopen, resulting in a base currentof zero.

Under this condition, there is avery small amount of collectorleakage current, ICEO, due mainlyto thermally produced carriers.


Because ICEO is extremely small,it will usually be neglected incircuit analysis so that

VCE = VCC

In cut-off, both the base-emitterand the base-collector junctionsare reverse-biased.


When the base-emitter junction becomesforward-biased and the base current isincreased, the collector current alsoincreases and VCE decreases as a resultof more drop across the collectorresistor.

When VCE reaches its saturation value,VCE(SAT), the base-collector junctionbecomes forward biased and IC canincrease no further even with a continuedincrease in IB.


At the point of saturation, therelation IC=βIB is no longervalid.

VCE(SAT) for a transistor occurssomewhere below the knee ofthe collector curves, and it isusually only a few tenths of avolt for silicon transistors.


http://people.deas.harvard.edu/~jones/es154/lectures/lecture_3/collector_current/IcVce_1b.gif�

When the transistor is in the cut-off region, the base-emitterjunction is reversed-biased.

Neglecting leakage current, all ofthe currents are zero and VCE isequal to VCC.


When the transistor is in thesaturation region, the base-emitterjunction is forward-biased.

There is enough base current toproduce a maximum collector current.

The formula for collector saturationcurrent is


CC CE(SAT)C(SAT)

C

V – VI =

R

Since VCE(SAT) is very smallcompared to VCC, it can usually beneglected.

The minimum value of base currentneeded to produce saturation is


C SATB MIN

II =β

Consider the given circuit;

As the input starts positive, currentbegins to flow in the base and thetransistor passes into the active region.

This causes current to flow in thecollector circuit and this current flowsthrough resistor RC and so the outputbegins to fall.


Output

Input

The base-emitter junction is wellforward-biased to the currentthrough the resistor RB and thetransistor will be saturated.

When the voltage at the input goesback to 0 Volt, the process isreversed.

The transistor passed fromsaturation through the active regionand is again cutoff.


Output

Input

Therefore,

Turn-OnHigh Collector CurrentLow Collector Voltage

Turn-OffHigh Collector VoltageLow Collector Current


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Transistor Switching Example

• When VBE is less than 0.7V the transistor is offand the lamp does not light.

• When VBE is greater than 0.7V the transistor is onand the lamp lights.

X

VariableVoltageSupply

12V


A. Given the circuit, what is VCEwhen VIN = 0 V?

B. What minimum value of IB isrequired to saturate this transistor ifβ = 200? Neglect VCE(SAT).

C. Calculate the maximum value ofRB when VIN = 5 V.


Determine whether or not the transistor is in saturation.Assume β = 25 and VCE(SAT) = 0.2 V.


A. The LED requires 30 mA to emit asufficient level of light. Therefore,the collector current should beapproximately 30 mA. For the givencircuit values, determine theamplitude of the square wave inputvoltage necessary to make sure thatthe transistor saturates. Use doublethe minimum value of base currentas a safety margin to ensuresaturation. Assume that β = 50 andVCE(SAT) = 0.3 V


B. If you change the LED to one thatrequires 50 mA for a specified lightemission and you can not increasethe input amplitude above 5 Volts orVCC above 9 Volts, how would youmodify the circuit? Specify thecomponent(s) to be changed andthe value(s).


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END


i. transistor as a switch

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