transistor as switch

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Analogue Electronics

Project


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Signal Switches

A mechanical switch has the desirable characteristics of practically zero resistance when ON,

and practically infinite resistance when OFF, providing a complete break in the conducting

path. However, it has to be operated mechanically and slowly, bounces when closed, and

deteriorates with use. ransistor switch, which is close to ideal and can handle large currents,

but nevertheless has a finite voltage across it when closed, and does not give perfect isolation

when open. On the other hand, it operates very rapidly, and does not deteriorate with use.

Another class of switches can be called signal or analog switches. Here, we do not handle

power but signal levels, and what is desired is a high resistance, almost perfect isolation,

when OFF. A reasonable amount of resistance can be tolerated in the ON state, up to a few

hundred ohms or so. he switch should be able to control signals at any reasonable voltage

level !which is why they are called analog" and not be tied to ground or the positive supply.

Date:

/3/2015

Submitted !: ilal Sarwar "1##0#


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i$olar %unction &ransistor as a Switch

#olid state switches are one of the main applications for the use of transistors, and transistor

switches can be used for controlling high power devices such as motors, solenoids or lamps,

but they can also used in digital electronics and logic gate circuits.

$f the circuit uses the %ipolar ransistor as a #witch, then the biasing of the transistor, either

N&N or &N& is arranged to operate the transistor at both sides of the ' $() * characteristics

curves we have seen previously.

he areas of operation for a ransistor #witch are known as the #aturation +egion andthe ut(off +egion. his means then that we can ignore the operating -(point biasing and

voltage divider circuitry reuired for amplification, and use the transistor as a switch by

driving it back and forth between its 'fully(OFF* !cut(off" and 'fully(ON* !saturation"

regions as shown below.

Operating +egions

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he pink shaded area at the bottom of the curves represents the 'ut(off* region while the

blue area to the left represents the '#aturation* region of the transistor. %oth these transistor

regions are defined as/

0. ut(off +egion

Here the operating conditions of the transistor are zero input base current ! $% ", zero output

collector current ! $ " and ma1imum collector voltage ! )2 " which results in a large

depletion layer and no current flowing through the device. herefore the transistor is

switched 'Fully(OFF*.

ut(off haracteristics

3 he input and %ase are grounded ! 4v "

3 %ase(2mitter voltage )%2 5 4.6v

3 %ase(2mitter 7unction is reverse biased

3 %ase(ollector 7unction is reverse biased

3 ransistor is 'fully(OFF* ! ut(off region "

3 No ollector current flows ! $ 8 4 "

3 )O9 8 )2 8 ) 8 *0:

3 ransistor operates as an 'open switch*

hen we can define the 'cut(off region* or 'OFF mode* when using a bipolar transistor as a

switch as being, both 7unctions reverse biased, )% 5 4.6v and $ 8 4. For a &N& transistor, the

2mitter potential must be negative with respect to the %ase.

;. #aturation +egion

Here the transistor will be biased so that the ma1imum amount of base current is applied,

resulting in ma1imum collector current resulting in the minimum collector emitter voltage

drop which results in the depletion layer being as small as possible and ma1imum current

flowing through the transistor. herefore the transistor is switched 'Fully(ON*.

#aturation haracteristics


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3 he input and %ase are connected to )

3 %ase(2mitter voltage )%2 < 4.6v

3 %ase(2mitter 7unction is forward biased

3 %ase(ollector 7unction is forward biased

3 ransistor is 'fully(ON* ! saturation region "

3 =a1 ollector current flows ! $ 8 )cc>+ ? "

3 )2 8 4 ! ideal saturation "

3 )O9 8 )2 8 *4:

3 ransistor operates as a 'closed switch*

hen we can define the 'saturation region* or 'ON mode* when using a bipolar transistor as

a switch as being, both 7unctions forward biased, )% < 4.6v and $ 8 =a1imum. For a &N&

transistor, the 2mitter potential must be positive with respect to the %ase.

hen the transistor operates as a 'single(pole single(throw* !#" solid state switch. @ith a

zero signal applied to the %ase of the transistor it turns 'OFF* acting like an open switch andzero collector current flows. @ith a positive signal applied to the %ase of the transistor it

turns 'ON* acting like a closed switch and ma1imum circuit current flows through the

device.

'ircuit (llustrating Switch &urned )*++,


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'ircuit (llustrating Switch &urned )*-,


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%& Switch Summar!:

• % transistor switches can be used to switch and control lamps, relays or even

motors.

• @hen using the bipolar transistor as a switch they must be either 'fully(OFF* or

'fully(ON*.

• ransistors that are fully 'ON* are said to be in their Saturation region.

• ransistors that are fully 'OFF* are said to be in their 'ut"o.. region.

• @hen using the transistor as a switch, a small %ase current controls a much larger

ollector load current.

• @hen using transistors to switch inductive loads such as relays and solenoids, a

'Flywheel Biode* is used.

• @hen large currents or voltages need to be controlled, Darlington &ransistors can

be used.


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&he *S+E& as a Switch

2nhancement(mode =O#F2 !e(=O#F2" operates using a positive input voltage and has

an e1tremely high input resistance !almost infinite" making it possible to interface withnearly any logic gate or driver capable of producing a positive output. Also, due to this very

high input !Cate" resistance we can parallel together many different =O#F2s until we

achieve the current handling limit reuired.

%ecause of the e1tremely high input or gate resistance that the =O#F2 has, its very fast

switching speeds and the ease at which they can be driven makes them ideal to interface with

op(amps or standard logic gates. However, care must be taken to ensure that the gate(source

input voltage is correctly chosen because when using the =O#F2 as a switch the device

must obtain a low + B#!on"channel resistance in proportion to this input gate voltage.

he operation of the 2nhancement(mode =O#F2, or e(=O#F2, can best be

described using its $() characteristics curves shown below. @hen the input voltage,

! )$N " to the gate of the transistor is zero, the =O#F2 conducts virtually no current

and the output voltage ! )O9 " is eual to the supply voltage )BB. #o the =O#F2 is

'fully(OFF* and in its 'cut(off* region.

=O#F2 haracteristics urves

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he minimum ON(state gate voltage reuired to ensure that the =O#F2 remains

fully(ON when carrying the selected drain current can be determined from the )($

transfer curves above. @hen )$Nis H$CH or eual to )BB, the =O#F2 -(point

moves to point A along the load line. he drain current$B increases to its

ma1imum value due to a reduction in the channel resistance. $B becomes a

constant value independent of )BB, and is dependent only on )C#. herefore, the

transistor behaves like a closed switch but the channel ON(resistance does notreduce fully to zero due to its+ B#!on" value, but gets very small.

?ikewise, when )$N is ?O@ or reduced to zero, the =O#F2 -(point moves

from point A to point % along the load line. he channel resistance is very high so

the transistor acts like an open circuit and no current flows through the channel.

#o if the gate voltage of the =O#F2 toggles between two values, H$CH and

?O@ the =O#F2 will behave as a 'single(pole single(throw* !#" solid state

switch and this action is defined as/

0. ut(off +egion

Here the operating conditions of the transistor are zero input gate voltage ! )$N ",

zero drain current$B and output voltage )B# 8 )BB. herefore the enhancement

type =O#F2 is switched 'Fully(OFF*.

ut(off haracteristics

• 3 he input and Cate are grounded ! 4v "

• 3 Cate(source voltage less than threshold

voltage )C# 5 )H

• 3 =O#F2 is 'fully(OFF* ! ut(off region "

• 3 No Brain current flows ! $B 8 4 "

• 3 )O9 8 )B# 8 )BB 8 *0:

• 3 =O#F2 operates as an 'open switch*

hen we can define the 'cut(off region* or 'OFF mode* when using an e(

=O#F2 as a switch as being, gate voltage, )C# 5 )H and $B 8 4. For a &(

channel enhancement =O#F2, the Cate potential must be more positive with

respect to the #ource.


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;. #aturation +egion

$n the saturation or linear region, the transistor will be biased so that the

ma1imum amount of gate voltage is applied to the device which results in thechannel resistance + B#!on being as small as possible with ma1imum drain current

flowing through the =O#F2 switch. herefore the enhancement type =O#F2

is switched 'Fully(ON*.

#aturation haracteristics

• 3 he input and Cate are connected to )BB

• 3 Cate(source voltage is much greater than

threshold voltage )C# < )H

• 3 =O#F2 is 'fully(ON* ! saturation region

"

•

3 =a1 Brain current flows ! $B 8 )BB > + ? "

• 3 )B# 8 4) !ideal saturation"

• 3 =in channel resistance + B#!on" 5 4.0D

• 3 )O9 8 )B# 8 4.;)≅ due to + B#!on"

• 3 =O#F2 operates as a low resistance

'closed switch*

hen we can define the 'saturation region* or 'ON mode* when using an e(

=O#F2 as a switch as gate(source voltage, )C# < )H and $B 8 =a1imum. For a

&(channel enhancement =O#F2, the Cate potential must be more negative with

respect to the #ource.

%y applying a suitable drive voltage to the gate of an F2, the resistance of thedrain(source channel, + B#!on" can be varied from an 'OFF(resistance* of many

hundreds of kDEs, effectively an open circuit, to an 'ON(resistance* of less than

0D, effectively a short circuit.


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@hen using the =O#F2 as a switch we can drive the =O#F2 to turn 'ON*

faster or slower, or pass high or low currents. his ability to turn the power

=O#F2 'ON* and 'OFF* allows the device to be used as a very efficient switch

with switching speeds much faster than standard bipolar 7unction transistors.

Sim$le Power *S+E& otor 'ontroller:

As the motor load is inductive, a simple diode is connected across the inductive load to

dissipate any back emf generated by the motor when the =O#F2 turns it 'OFF*. A

clamping network formed by a diode in series with the diode can also be used to allow for

faster switching and better control of the peak reverse voltage and drop(out time.


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%+E& As a Switch

%E+& as Analog Switch:

F2 can be used as an analog switch as shown in figure below $t is the ma7or application of

a F2. he idea is to use two points on the load line/ cut off and saturation. @hen F2 is

cut off, it is like an open switch. @hen it is saturated, it is like a closed switch.

@hen )C# 84, the F2 is saturated and operates at the upper end of the load line. @hen)C# is eual to or more negative than )C#!off" , it is cut off and operates at lower end of the

load line !open and closed switch".his is shown in .ig .

Only these two points are used for operation when used as a switch. he F2 is normally

saturated well below the knee of the drain curve. For this reason the drain current is much

smaller than $B## .

+E& as a Shunt Switch:

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F2 can be used as a shunt switch as shown in .igure. @hen )cont84, the F is saturated and

the switch is closed @hen )cont is more negative F2 is like an open switch. he euivalent

circuit is also shown in .igure.

+E& as a series switch:

F2 can also be used as series switch as shown in .igure. @hen control is zero, the F2 is a

closed switch. @hen )con8 negative, the F2 is an open switch. $t is better than shunt switch.

'onclusion:

%s are characterized by linear current transfer function between the collector current and

the base current. hey have much larger transconductance and they can achieve much higher

input signal gain thanks to their current control. $n addition, they have higher speeds and

higher ma1imum operating freuency. onseuently , they are preferred in the amplifier

circuits and in the linear integrated circuits as well as high freuency and high power

applications. @hen %s are operated as switches, they consume appreciable power and

therefore they are less suitable in )?#$ integrated circuits. hey are used in very high speed

logic circits such as ? and 2?.hey consume more area on the chip than the mosfet

transistors.

he F2s are characterized by high input impedance and some types of F2# operate as a


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relay such as the enhancement =O#F2# making them superior as switches.heir nonlinear

transfer characteristics between the drain current and the gate to source voltage and their

smaller trans conductance makes them less suitable in amplifier circuits. herefore, we see

that the dominating logic family for implementing memories, &9s and B#&s are made of

=O# transistors especially the complementary =O# transistors which have good logic

performance parameters.he static power consumption of the =O# is negligible.

$n summary, the applications of a device and its dominance in some applications stem from

its characteristics , availability, cost and familiarity.

transistor as switch

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