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 = 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 RL Pac = I ac x (rf+RL)

EFFICIENCY OF RECTIFICATION n = P dc P ac = I+rf/RL THE EFFICIENCY WILL BE MAXIMUM WHEN RL>rf MAXIMUM n = 81.2%

TRANSISTORS

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. INTRODUCTION:

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 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 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. TRANSISTOR CONNECTIONS

COMMON BASE CONNECTION – IN THIS CIRCUIT-ARRANGEMENT, 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. ∆I C Α = ∆I E 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 I CBO. I C = Α I E + I CBO IT MAY BE NOTED HERE THAT REGARDLESS OF THE CIRCUIT CONNECTION, EMITTER IS ALWAYS FORWARD BIASED WHILE THE COLLECTOR ALWAYS HAS A REVERSE BIAS.

THE CURRENT I CBO 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, ∆ V EB Α = AT CONSTANT V C ∆ I E THE CURRENT I CBO 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 V EB 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, ∆ V CB R O = ∆ I C AS THE OUTPUT CIRCUIT IS REVERSE BIASED, THE CHANGE IN COLLECTOR CURRENT I C IS EXTREMELY SMALL DUE TO THE CHANGE IN V CB.

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 I C AND INPUT CURRENT IS I B SO THAT CURRENT AMPLIFICATION FACTOR Β IS GIVEN BY: ∆ I C Β = ∆ I B B. THE COLLECTOR CURRENT IN THIS ARRANGEMENT CAN BE EXPRESSED IN THE FOLLOWING DIFFERENT WAYS: Α 1 I C = I B + I CBO 1 – Α 1 – Α Α I C = I B + I CEO 1 – Α I C = ΒI B + I CEO

C. INPUT RESISTANCE, ∆ V BE AT CONSTANT V CE R I = ∆ I B D. OUTPUT RESISTANCE, ∆ V CE AT CONSTANT I B R O = ∆ I C 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 AND COLLECTOR.

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), I C IS THE MAIN CURRENT AND I B IS THE CONTROL CURRENT. THE OUTPUT CIRCUIT OF A TUBE IS NORMALLY BIASED IN THE FORWARD DIRECTION, PERMITTING CURRENT TO FLOW EASILY.