Chapter Seven

Crystal Diodes

IntroductionA pn junction is known as a semi-conductor diode or crystal

diode. The outstanding property of a crystal diode (or pn junction) is that it conducts current easily when forward biased and practically no current flows when it is reversed biased. This unilateral conduction characteristic of a crystal diode is similar to that of a vacuum diode. Therefore, like a vacuum diode, a crystal diode can accomplish the job of rectification change alternating to direct current. However, crystal diodes are finding favour these days because they are smaller in size, cheap and robust and usually operate with greater efficiency.

A crystal diode is usually represented by the symbol of an arrow with bar. It has two terminals. While using a crystal diode, it is often necessary to know which end is arrow and which end is bar? For this purpose, there are two most commonly used methods. Some manufacturers actually paint the symbol on the body of the diode. BY 127, BY 114 crystal diodes manufactured by BEL. Sometimes red and blue marks are used on the body of the crystal diode. Red mark denotes arrow and blue mark indicates bar OA 80 crystal diodes.

Resistance of crystal diode.We know that a forward biased diode conducts easily whereas a reverse biased diode practically conducts no current. It means that forward resistance of a diode is quite small as compared to its reverse resistance. The forward resistance of a diode generally ranges from 1 to 25 ohms whereas reverse resistance is usually of the order of several megaohms. As a crystal diode utilizes the forward characteristic for rendering rectification, therefore, forward resistance is a quantity of our interest. It may be mentioned here that ratio of reverse to forward resistance is very large in silicon diodes that in germanium diodes. For this reason silicon diodes are preferred to germanium diodes.

Important terms. While discussing diode circuits, the following terms are generally used.

Forward current. It is the current flowing through a forward biased diode. Every diode has a maximum value of forward current that it can safely carry, if this value is exceeded, the diode may be destroyed due to excessive heat.

Peak inverse voltage (PIV). It is the maximum reverse voltage that a diode can withstand without destroying the junction. If the reverse voltage across a diode exceeds this value, the reverse current increases sharply and breaks down the junction due to excessive heat. Peak inverse voltage is extremely important when diode is used as a rectifier.

Reverse current. It is the current that flows through a reverse biased diode. This current is due to the minority carriers, under normal operating voltages, the reverse current is quite small; its value is extremely small.

Advantages of crystal diodes. 1. A crystal diode has the following advantages over

vacuum diode:2. It has a very small size and greater mechanical strength3. No heater is required. Hence power efficiency of crystal diode is very high.4. The life of a crystal diode is much more than that of a vacuum diode.5. A crystal diode does not require warm-up time.6. The voltage and current requirements of a crystal diode are much smaller than that of a vacuum diode.7. It has a small junction capacitance. This helps to avoid bypassing high-frequency signals when the crystal diode is in the non-conducting part of the input cycle.Note. A crystal diode is very much temperature dependent. A small change in temperature can drastically change its characteristics. It may be noted that for high voltage and power applications, vacuum diodes are preferred to crystal diodes.  

CRYSTAL DIODES ARE CAPABLE OF ACHIEVING RECTIFICATION IN A FASHION COMPORTABLE AND OFTEN SUPERIOR TO THAT REALISED BY VACUUM DIODES

HALF-WAVE RECTIFIER

A HALF-WAVE RECTIFIER EMPLOYS A SINGLE CRYSTAL DIODE AND CONDUCTS POSITIVE IS HALF CYCLES CURRENT DURING OF INPUT A.C

SUPPOSE AN A.C. SUPPLY v=VmSin 0 IS APPLIED TO A CRYSTAL DIODE HALF-WAVE RECTIFIER.LET rf AND RL BE THE FORWARD RESISTANCE AND LOAD RESISTANCE RESPECTIVELY.

THE VARIOUS CIRCUIT VALUESARE:

VmIm= rf+RL

Idc= Im n Ir.m.s= Im 2

P dc= Idc x RLP ac= I r.m.s x (rf+RL)

EFFICIENCY OF RECTIFICATIONN= P dc = 0.406 P ac I+rf/RL

THE EFFICIENCY WILL BE MAXIMUM WHEN RL>rf.

FULL-WAVE RECTIFIER

A FULL-WAVE RECTIFIER EMPLOYS TWO DIODES AND CONDUCTS THROUGH LOAD IN THE SAME DIRECTION FOR BOTH HALF-CYCLES OF INPUT A.C VOLTAGE.

SUPPOSE AN a.c. SUPPLY v=Vm Sin 0 IS BEING USED FOR FULL-WAVE RECTIFICATION. IF rf AND RL ARE THE DIODE RESISTANCE AND LOAD RESISTANCE RESPECTIVELY;

THEN THE VARIOUS CIRCUIT VALUES ARE:

Im = Vm rf+RL

Idc = 2Im n

Iac = Im 2

Pdc = I dc x RLPac = I ac x (rf+RL)

EFFICIENCY OF RECTIFICATION

n = P dc P ac

= 0.812 I+rf/RL

THE EFFICIENCY WILL BE MAXIMUM WHEN RL>rfMAXIMUM n = 81.2%

TRANSISTORS

•C O N S I S T S O F 2 P N J U N C T I O N S F O R M E D BY S A N D W I C H I N G E I T H E R P- T Y P E O R N - T Y P E S E M I C O N D U C T O R B E T W E E N A PA I R O F O P P O S I T E T Y P E S .• AC C O R D I N G LY ; T H E R E A R E 2 T Y P E S O F T RA N S I S T O R S , N A M E LY: ( A ) N - P- N T RA N S I S T O R A N D ( B ) P- N - P T RA N S I S T O R.•T H U S A T RA N S I S T O R ( N - P- N O R P- N - P ) H A S 3 S E C T I O N S O F D O P E D S E M I C O N D U C T O R S .•T H E S E C T I O N O N O N E S I D E I S T H E E M I T T E R A N D S E C T I O N O N T H E O P P O S I T E S I D E I S T H E C O L L E C T O R .•T H E M I D D L E S E C T I O N I S C A L L E D T H E B A S E A N D F O R M S 2 J U N C T I O N S B E T W E E N E M I TT E R A N D C O L L E C T O R.

INTRODUCTION:

•T H E I N P U T C I RC U I T ( I . E . E M I T T E R - B A S E J U N C T I O N F O R A C O M M O N B A S E A R R A N G E M E N T ) H A S A L O W R E S I S TA N C E B E C A U S E A F O R WA R D B I A S W H E R E A S O U T P U T C I RC U I T ( I . E . C O L L E C T O R - B A S E J U N C T I O N ) H A S H I G H R E S I S TA N C E D U E T O R E V E R S E B I A S . •T H E I N P U T E M I T T E R C U R R E N T A L M O S T E N T I R E LY F L O W S I N T H E C O L L E C T O R C I RC U I T , T H E R E F O R E , A T R A N S I S T O R T R A N S F E R T H E I N P U T S I G N A L C U R R E N T F R O M A L O W R E S I S TA N C E C I RC U I T T O A H I G H R E S I S TA N C E C I RC U I T. T H I S I S T H E K E Y FA C T O R R E S P O N S I B L E F O R T H E A M P L I F Y I N G C A PA B I L I T Y O F T H E T R A N S I S T O R . T H E F O L L O W I N G P O I N T S A R E W O RT H N O T I N G :a. A T R A N S I S T O R T R A N S F E R S I G N A L C U R R E N T F R O M A L O W

R E S I S TA N C E T O A H I G H R E S I S TA N C E C I RC U I T. T H E P R E F I X ‘ T R A N S ’ M E A N S T H E S I G N A L T R A N S F E R P R O P E RT Y O F T H E D E V I C E W H I L E ‘ I S T O R ’ C L A S S I F I E S I T A S A S O L I D E L E M E N T I N T H E S A M E G E N E R A L FA M I LY W I T H R E S I S T O R S .

b. A T R A N S I S T O R I S A C U R R E N T O P E R AT E D D E V I C E I . E . I N P U T C U R R E N T C O N T R O L S T H E O U T P U T C U R R E N T. T H I S I S I N C O N T R A S T T O A VA C U U M T U B E , W H E R E I N P U T V O LTA G E C O N T R O L S T H E O U T P U T C U R R E N T.

TRANSISTOR AS AN AMPLIFIER

- THERE ARE 3 LEADS IN A TRANSISTOR VIZ. EMITTER, BASE AND COLLECTOR. ACCORDINGLY, A TRANSISTOR CAN BE CONNECTED IN A CIRCUIT IN THE FOLLOWING 3 WAYS:a. COMMON BASE CONNECTIONb. COMMON EITHER CONNECTIONc. COMMON COLLECTOR CONNECTION

I T MAY NOTED HERE THAT REGARDLESS OF THE C IRCU IT CONNECT ION, EM ITTER IS ALWAYS FORWARD B IASED WHILE THE COLLECTOR ALWAYS HAS A REVERSE B IAS .

TRANSISTOR CONNECTIONS

•C O M M O N B A S E C O N N E C T I O N – I N T H I S C I RC U I T-A R R A N G E M E N T , I N P U T I S A P P L I E D B E T W E E N E M I T T E R A N D B A S E A N D O U T P U T I S TA K E N F R O M C O L L E C T O R A N D B A S E . T H E F O L L O W I N G P O I N T S A R E W O RT H N O T I N G I N T H I S C O N N E C T I O N :A . T H E C U R R E N T A M P L I F I C AT I O N FA C T O R Α I N T H I S A R R A N G E M E N T I S T H E R AT I O O F C H A N G E I N O U T P U T C U R R E N T T O T H E C H A N G E I N I N P U T C U R R E N T I . E . ∆ I C

Α =

∆ I E O B V I O U S LY , T H E V A L U E O F Α I S L E S S T H A N U N I T Y. T H I S V A L U E C A N I N C R E A S E D ( B U T N O T M O R E T H A N U N I T Y ) B Y D E C R E A S I N G T H E B A S E C U R R E N T I . E . B Y M A K I N G T H E B A S E T H I N A N D D O P I N G I T L I G H T LY.

B . T H E C O L L E C T O R C U R R E N T C O N S I S T S O F 2 PA RT S V I Z . T H AT PA RT O F E M I T T E R C U R R E N T W H I C H R E A C H E S T H E C O L L E C T O R ( I . E . Α I € ) A N D T H E L E A K A G E C U R R E N T I C B O .

I C = Α I E + I C B O

IT MAY BE NOTED HERE THAT REGARDLESS OF THE CIRCUIT CONNECTION, EMITTER IS ALWAYS FORWARD BIASED WHILE THE COLLECTOR ALWAYS HAS A REVERSE BIAS.

T H E C U R R E N T I C B O I S T H E L E A KAG E - C U R R E N T T H AT F LO W S AC R O S S C O L L E C T O R-B A S E J U N C T I O N D U E T O M I N O R I T Y C A R R I E R S . T H E S U F F I X C BO M E A N S C O L L E C T O R T O B A S E J U N C T I O N W I T H E M I TT E R O P E N .C . I N P U T R E S I S TA N C E , ∆ V E B

Α = AT C O N S TA N T V C

∆ I E

T H E C U R R E N T I C B O I S L E A KAG E - C U R R E N T T H AT F LO W S AC R O S S C O L L E C T O R-B A S E J U N C T I O N D U E T O M I N O R I T Y C A R R I E R S . T H E S U F F I X C BO M E A N S C O L L E C T O R T O B A S E J U N C T I O N W I T H E M I TT E R O P E N . A S T H E I N P U T C I RC U I T I S F O RWA R D B I A S E D , A S M A L L V E B I S S U F F I C I E N T T O P R O D U C E A L A R G E F LO W O F E M I TT E R C U R R E N T. T H E R E F O R E , I N P U T R E S I S TA N C E I S Q U I T E S M A L L , O F T H E O R D E R O F A F E W O H M S . D . O U T P U T R E S I S TA N C E , ∆ V C B

R O =

∆ I C

 

A S T H E O U T P U T C I RC U I T I S R E V E R S E B I A S E D , T H E C H A N G E I N C O L L E C T O R C U R R E N T I C I S E XT R E M E LY S M A L L D U E T O T H E C H A N G E I N V C B .

I I . C O M M O N - E M I T T E R - C O N N E C T I O N - I N T H I S C I RC U I T A R RA N G E M E N T , I N P U T I S A P P L I E D B E T W E E N B A S E A N D E M I T T E R A N D O U T P U T I S TA K E N F R O M C O L L E C T O R A N D E M I T T E R . T H E F O L LO W I N G P O I N T S A R E W O RT H N O T I N G I N T H I S C O N N E C T I O N : A . T H E O U T P U T C U R R E N T I S I C A N D I N P U T C U R R E N T I S I B S O T H AT C U R R E N T A M P L I F I C AT I O N FA C T O R Β I S G I V E N BY: ∆ I C

Β = ∆ I B

B . T H E C O L L E C T O R C U R R E N T I N T H I S A R RA N G E M E N T C A N B E E X P R E S S E D I N T H E F O L LO W I N G D I F F E R E N T WAY S : Α 1 I C = I B + I C B O

1 – Α 1 – Α  Α I C = I B + I C E O

1 – Α  I C = Β I B + I C E O

 

C . I N P U T R E S I S TA N C E , ∆ V B E AT C O N S TA N T V C E

R I =

∆ I B

 D . O U T P U T R E S I S TA N C E , ∆ V C E AT C O N S TA N T I B

R O =

∆ I C

E . T H E C O M M O N E M I TT E R A R RA N G E M E N T I S T H E M O S T W I D E LY U S E D I N P RAC T I C E . I T I S U S E D I N A BO U T 9 0 T O 9 5 P E R C E N T O F A L L T RA N S I S T O R A P P L I C AT I O N S . T H E M A I N R E A S O N S F O R T H E W I D E S P R E A D U S E O F T H I S A R RA N G E M E N T A R E : H I G H C U R R E N T G A I N , H I G H VO LTAG E A N D P O W E R G A I N A N D M O D E RAT E O U T P U T T O I N P U T I M P E DA N C E RAT I O. I I I . COMMON-COLLECTOR-CONNECT ION – I N T H I S C I RC U I T A R RA N G E M E N T , I N P U T I S A P P L I E D B E T W E E N B A S E A N D C O L L E C T O R W H I L E O U T P U T I S TA K E N F R O M E M I TT E R A N D C O L L E C T O R. 

TRANSISTORS VERSUS VACUUM TUBES•I T I S D E S I RA B L E T O C O M PA R E T RA N S I S T O R S A N D VAC U U M T U B E S A S A M P L I F Y I N G D E V I C E S :•I N A VAC U U M T U B E , T H E C U R R E N T C A R R I E R S A R E A LWAY S E L E C T R O N S P R O D U C E D BY T H E R M I O N I C E M I S S I O N•A VAC U U M T U B E I S A VO LTAG E O P E RAT E D D E V I C E I . E . G R I D VO LTAG E C O N T R O L S T H E P L AT E C U R R E N T.•F O R M O S T T U B E A P P L I C AT I O N S , G R I D C U R R E N T D O E S N O T F LO W B E C A U S E G R I D I S B I A S E D N E G AT I V E W. R . T. C AT H O D E .•T H E I N P U T R E S I S TA N C E O F A VAC U U M T U B E I S V E RY H I G H B E C A U S E N O G R I D C U R R E N T F LO W S•I N A T U B E , P L AT E I S A LWAY S P O S I T I V E W. R . T. E M I TT E R , D E P E N D I N G U P O N W H E T H E R T H E T RA N S I S T O R , I S N P N O R P N P.•I N A T RA N S I S T O R ( C E A R RA N G E M E N T ) , I C I S T H E M A I N C U R R E N T A N D I B I S T H E C O N T R O L C U R R E N T.

•T H E O U T P U T C I RC U I T O F A T U B E I S N O R M A L LY B I A S E D I N T H E F O RWA R D D I R E C T I O N , P E R M I TT I N G C U R R E N T T O F LO W E A S I LY.