Chapter Seven Crystal Diodes Introduction A pn junction

Chapter Seven Crystal Diodes

Introduction A 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 strength 3. 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=Vm Sin 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: Vm Im= rf+RL Idc= Im n Ir. m. s= Im 2 P dc= Idc x RL P ac= I r. m. s x (rf+RL)

EFFICIENCY OF RECTIFICATION N= 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 = 2 Im n Iac = Im 2 Pdc = I dc x RL Pac = 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>rf MAXIMUM n = 81. 2%

TRANSISTOR S

INTRODUCTION: • CONSISTS OF 2 PN JUNCTIONS FORMED BY SANDWICHING EITHER P- TYPE OR N- TYPE SEMICONDUCTOR BETWEEN A PAIR OF OPPOSITE TYPES. • ACCORDINGLY; THERE ARE 2 TYPES OF TRANSISTORS, NAMELY: (A) N-P-N TRANSISTOR AND (B) P-N-P TRANSISTOR. • THUS A TRANSISTOR (N-P-N OR P-N-P) HAS 3 SECTIONS OF DOPED SEMICONDUCTORS. • THE SECTION ON ONE SIDE IS THE EMITTER AND SECTION ON THE OPPOSITE SIDE IS THE COLLECTOR. • THE MIDDLE SECTION IS CALLED THE BASE AND FORMS 2 JUNCTIONS BETWEEN EMITTER AND COLLECTOR.

TRANSISTOR AS AN AMPLIFIER • THE INPUT CIRCUIT (I. E. EMITTER- BASE JUNCTION FOR A COMMON BASE ARRANGEMENT) HAS A LOW RESISTANCE BECAUSE A FORWARD BIAS WHEREAS OUTPUT CIRCUIT (I. E. COLLECTOR- BASE JUNCTION) HAS HIGH RESISTANCE DUE TO REVERSE BIAS. • THE INPUT EMITTER CURRENT ALMOST ENTIRELY FLOWS IN THE COLLECTOR CIRCUIT, THEREFORE, A TRANSISTOR TRANSFER THE INPUT SIGNAL CURRENT FROM A LOW RESISTANCE CIRCUIT TO A HIGH RESISTANCE CIRCUIT. THIS IS THE KEY FACTOR RESPONSIBLE FOR THE AMPLIFYING CAPABILITY OF THE TRANSISTOR. THE FOLLOWING POINTS ARE WORTH NOTING: a. A TRANSISTOR TRANSFER SIGNAL CURRENT FROM A LOW RESISTANCE TO A HIGH RESISTANCE CIRCUIT. THE PREFIX ‘TRANS’ MEANS THE SIGNAL TRANSFER PROPERTY OF THE DEVICE WHILE ‘ISTOR’ CLASSIFIES IT AS A SOLID ELEMENT IN THE SAME GENERAL FAMILY WITH RESISTORS. b. A TRANSISTOR IS A CURRENT OPERATED DEVICE I. E. INPUT CURRENT CONTROLS THE OUTPUT CURRENT. THIS IS IN CONTRAST TO A VACUUM TUBE, WHERE INPUT VOLTAGE CONTROLS THE OUTPUT CURRENT.

TRANSISTOR CONNECTIONS - 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 CONNECTION b. COMMON EITHER CONNECTION c. COMMON COLLECTOR CONNECTION IT MAY NOTED HERE THAT REGARDLESS OF THE CIRCUIT CONNECTION, EMITTER IS ALWAYS FORWARD BIASED WHILE THE COLLECTOR ALWAYS HAS A REVERSE BIAS.

CIRCUIT CONNECTION, EMITTER IS ALWAYS FORWARD BIASED WHILE THE COLLECTOR ALWAYS HAS A REVERSE BIAS. • COMMON BASE CONNECTION – IN THIS CIRCUITARRANGEMENT, INPUT IS APPLIED BETWEEN EMITTER AND BASE AND OUTPUT IS TAKEN FROM COLLECTOR AND BASE. THE FOLLOWING POINTS ARE WORTH NOTING IN THIS CONNECTION: A. THE CURRENT AMPLIFICATION FACTOR Α IN THIS ARRANGEMENT IS THE RATIO OF CHANGE IN OUTPUT CURRENT TO THE CHANGE IN INPUT CURRENT I. E. ∆IC Α = ∆IE OBVIOUSLY, THE VALUE OF Α IS LESS THAN UNITY. THIS VALUE CAN INCREASED (BUT NOT MORE THAN UNITY) BY DECREASING THE BASE CURRENT I. E. BY MAKING THE BASE THIN AND DOPING IT LIGHTLY. B. THE COLLECTOR CURRENT CONSISTS OF 2 PARTS VIZ. THAT PART OF EMITTER CURRENT WHICH REACHES THE COLLECTOR (I. E. ΑI€) AND THE LEAKAGE CURRENT ICBO. IC = Α IE + ICBO

THE CURRENT ICBO IS THE LEAKAGE-CURRENT THAT FLOWS ACROSS COLLECTOR-BASE JUNCTION DUE TO MINORITY CARRIERS. THE SUFFIX CBO MEANS COLLECTOR TO BASE JUNCTION WITH EMITTER OPEN. C. INPUT RESISTANCE, ∆ VEB Α = AT CONSTANT VC ∆ IE THE CURRENT ICBO IS LEAKAGE-CURRENT THAT FLOWS ACROSS COLLECTOR-BASE JUNCTION DUE TO MINORITY CARRIERS. THE SUFFIX CBO MEANS COLLECTOR TO BASE JUNCTION WITH EMITTER OPEN. AS THE INPUT CIRCUIT IS FORWARD BIASED, A SMALL VEB IS SUFFICIENT TO PRODUCE A LARGE FLOW OF EMITTER CURRENT. THEREFORE, INPUT RESISTANCE IS QUITE SMALL, OF THE ORDER OF A FEW OHMS. D. OUTPUT RESISTANCE, ∆ VCB

II. COMMON-EMITTER-CONNECTION- IN THIS CIRCUIT ARRANGEMENT, INPUT IS APPLIED BETWEEN BASE AND EMITTER AND OUTPUT IS TAKEN FROM COLLECTOR AND EMITTER. THE FOLLOWING POINTS ARE WORTH NOTING IN THIS CONNECTION: A. THE OUTPUT CURRENT IS IC AND INPUT CURRENT IS IB SO THAT CURRENT AMPLIFICATION FACTOR Β IS GIVEN BY: ∆ IC Β = ∆ IB B. THE COLLECTOR CURRENT IN THIS ARRANGEMENT CAN BE EXPRESSED IN THE FOLLOWING DIFFERENT WAYS: Α 1 IC = IB + ICBO 1 – Α Α IC = 1 – Α ΒIB IB + ICEO

C. INPUT RESISTANCE, CONSTANT VCE RI = D. OUTPUT RESISTANCE, CONSTANT IB RO = ∆ VBE AT ∆ IB ∆ VCE ∆ IC AT E. THE COMMON EMITTER ARRANGEMENT IS THE MOST WIDELY USED IN PRACTICE. IT IS USED IN ABOUT 90 TO 95 PER CENT OF ALL TRANSISTOR APPLICATIONS. THE MAIN REASONS FOR THE WIDESPREAD USE OF THIS ARRANGEMENT ARE: HIGH CURRENT GAIN, HIGH VOLTAGE AND POWER GAIN AND MODERATE OUTPUT TO INPUT IMPEDANCE RATIO. III. COMMON-COLLECTOR-CONNECTION – IN THIS CIRCUIT ARRANGEMENT, INPUT IS APPLIED BETWEEN BASE AND COLLECTOR WHILE OUTPUT IS TAKEN FROM EMITTER

TRANSISTORS VERSUS VACUUM TUBES • IT IS DESIRABLE TO COMPARE TRANSISTORS AND VACUUM TUBES AS AMPLIFYING DEVICES: • IN A VACUUM TUBE, THE CURRENT CARRIERS ARE ALWAYS ELECTRONS PRODUCED BY THERMIONIC EMISSION • A VACUUM TUBE IS A VOLTAGE OPERATED DEVICE I. E. GRID VOLTAGE CONTROLS THE PLATE CURRENT. • FOR MOST TUBE APPLICATIONS, GRID CURRENT DOES NOT FLOW BECAUSE GRID IS BIASED NEGATIVE W. R. T. CATHODE. • THE INPUT RESISTANCE OF A VACUUM TUBE IS VERY HIGH BECAUSE NO GRID CURRENT FLOWS • IN A TUBE, PLATE IS ALWAYS POSITIVE W. R. T. EMITTER, DEPENDING UPON WHETHER THE TRANSISTOR, IS NPN OR PNP. • IN A TRANSISTOR (CE ARRANGEMENT), IC IS THE MAIN CURRENT AND IB IS THE CONTROL CURRENT. • THE OUTPUT CIRCUIT OF A TUBE IS NORMALLY BIASED IN THE FORWARD DIRECTION, PERMITTING CURRENT TO FLOW EASILY.