Chapter 4 ELECTRONICS4.3UNDERSTANDING TRANSISTORSA student is able todescribe a transistor in terms of its terminals.describe how a transistor can be used as a current amplier.describe how a transistor can be used as an automatic switch.4.3 TRANSISTORSA transistor is a combination of two back-to-back semiconductor diodes.4.3 TRANSISTORSThere are two t!pes of transistor"#$ n%p%n transistor"&$ p%n%p transistor4.3 TRANSISTORSA transistor is a semiconductor de'ice that can perform the followin( electrical functions.a. Act as an electrical switch in a circuit.b. Act as an amli!er of current in a circuit.c. Re"#late the c#rrent in a circuit.4.3 TRANSISTORSStr#ct#re$ schematic %ia"ram an% s&mbol o' transistorA transistor has tree leads) the leads are labelled(. )ase "*$ + Control the ,ow of char(e carriers from E to C*. +ollector "C$ + Recei'e char(e carriers from E3. Emitter "E$ + Suppl! char(e carriers to C4.3 TRANSISTORS,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORSThe terminals of a transistor must be connected to the poles of a batter! correctl! to a'oid dama(e to the transistor.,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORSTo connect a n%p%n transistor- the ne(ati'e pole of the batter! is .oined to E and its positi'e pole to C./orward%biasedRe'erse%biasedC B EI I I + =EC**ase circuit Collector circuit,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORS0hen the circuit is switched on- the base emitter .unction is forward%biased. The ma.orit! char(e carriers- electrons ,ow from E to *./orward%biasedRe'erse%biasedC B EI I I + =EC**ase circuit Collector circuit,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORS1er! few electrons combine with holes at *. Some of the remainin( electrons ,ow bac2 to the base circuit and the rest into the collector circuit./orward%biasedRe'erse%biasedC B EI I I + =EC**ase circuit Collector circuit,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORSInstead electrons are attracted across the * due to hi(her potential di3erence at the collector terminal. /orward%biasedRe'erse%biasedC B EI I I + =EC**ase circuit Collector circuit,echanism o' +har"e +arrier -low in Transistor4.3 TRANSISTORSTo connect a p%n%p transistor- the positi'e pole of the batter! is .oined to E and its ne(ati'e pole to C./orward%biasedRe'erse%biasedC B EI I I + =EC**ase circuit Collector circuitTransistor as a +#rrent Amli!er4.3 TRANSISTORSA transistor functions as a current amplier b! allowin" a small c#rrent to control a lar"er c#rrent.Transistor as a +#rrent Amli!er4.3 TRANSISTORSA small chan(e in the base current- Ib. will cause a bi( chan(e in the collector current- Ic. The current amplifcation can be calculated as follows4bcIIion Amplificat Current =4.3 TRANSISTORS A small current I* ,owin( throu(h the base "*$ will combine with IC to produce a lar(e current IE ,owin( throu(h the emitter.C B EI I I + =IB 5 6.6& mAIC 5 6.78 mAIE 5 #.66 mATransistor as a +#rrent Amli!er4.3 TRANSISTORSThe switchin( action of a transistor is produced b! 'ar!in( the 'olta(e applied to the base so that the transistor is either turned ON or O--.Transistor as an A#tomatic Switch4.3 TRANSISTORS 0hen R# 5 6 - the base 'olta(e VR# 5 6. The base current does not ,ow and the collector current also does not ,ow. Ib 5 6 and Ic 5 6Turn O-- modeTransistor as an A#tomatic Switch4.3 TRANSISTORS0hen the resistance of R# is increased- the base 'olta(e will increase until the base .olta"e e/cee%s a certain minim#m .al#e- the base c#rrent 0ows an% ca#se a lar"e collector c#rrent 0ows.Turn ON modeTransistor as an A#tomatic Switch4.3 TRANSISTORS/rom the wor2in( principles abo'e- we conclude that a transistor functions as an automatic switch- so that the transistor turned ON or O//.Transistor as an A#tomatic Switch4.3 TRANSISTORSIn bri(ht li(ht- the li(ht%dependent resistor"L9R$ has a 'er! low resistance. Therefore the potential di3erence across L9R is low and hence the potential di3erence across resistor R is hi(h. Transistor As A 1i"ht +ontrolle% Switch4.3 TRANSISTORSThe base current ,ows and cause a lar(e collector current ,ows. The bulb li(hts up.Transistor As A 1i"ht +ontrolle% Switch4.3 TRANSISTORSIn dar2ness - the li(ht%dependent resistor"L9R$ has a 'er! hi(h resistance. Therefore the potential di3erence across L9R is hi(h and hence the potential di3erence across resistor R is low. Transistor As A 1i"ht +ontrolle% Switch4.3 TRANSISTORSThe base current does not ,ow and cause the collector current does not ,ow. The bulb not li(hts up.Transistor As A 1i"ht +ontrolle% Switch4.3 TRANSISTORSIf the positions of the L9R and R are interchan(ed- the bulb is switched on in the dar2 and o3 in the bri(ht li(ht.Transistor As A 1i"ht +ontrolle% Switch4.3 TRANSISTORS0hen the thermistor is cold- it has a lar(er resistance than R. Therefore the potential di3erence across thermistor is hi(h and hence the potential di3erence across resistor R is low. Transistor As A Temerat#re +ontrolle% Switch4.3 TRANSISTORSThe base current does not ,ow and cause the collector current does not ,ow.The bulb not li(hts up.0hen the temperature rises- the resistance of thermistor falls and the bulb li(hts up.Transistor As A Temerat#re +ontrolle% Switch4.3 TRANSISTORS2otential Di.i%er4.3 TRANSISTORShttps4::www.!outube.com:watch;'5/O9/owm9f'<