Diode and Diode Circuit• All materials can be classified (electrically) into three categories: » Conductors. » Insulators. » Semiconductors• Conductors easily allow current to pass through them.• Insulators do not allow current to pass through them. Semiconductors are a group of material that posses the property of neither insulator nor good conductor, but somewhere in between Example of semiconductors are silicon (Si) and germanium (Ge).• Pure semiconductors are poor conductors, because the low number of free electrons. However, the resistivity can be reduced (so that it conducts more current) by putting in impurities into the pure semiconductors. The process of introducing a small amount of impurities (during manufacturing) into the semiconductors is called doping.
DOPING•• The type of material that is added to the pure semiconductor will determine whether it will become n-type or p-type semiconductor.• N-type semiconductor is produced if the impurity is either phosphorus (P), arsenic (As), or antimony (Sb) - all from group 5 of periodic table. The introduction of either one of these impurities into a pure semiconductor produces more free electron in the semiconductor.• P-type semiconductor is produced if the impurity is either aluminium (Al), boron (B), or gallium (Ga) - all from group 3 of periodic table. The introduction of either one of these impurities into a pure semiconductor produces more "hole" in the semiconductor. A hole is a condition where there is absence of one electron, which gives the effect of more positive charge.
PN JUNCTION• A p-n junction is piece of semiconductor material in which part of the material is p-type and part is n-type. In order to examine the charge situation, assume that separate blocks of p-type and n-type materials are pushed together. Also assume that a hole is a positive charge carrier and that an electron is a negative charge carrier.• At the junction, the donated electrons in the n-type material, called majority carriers, diffuse into the p-type material and the acceptor holes in the p-type material diffuse into the n-type material as shown by the arrows in Figure 2.2.• Because the n-type material has lost electrons, it acquires a positive potential with respect to the p-type material and thus tends to prevent further movement of electrons.• The p-type material has gained electrons and becomes negatively charged with respect to the n-type material and hence tends to retain holes. Thus after a short while, the movement of electrons and holes stops due to the potential difference across the junction, called the contact potential.• The area in the region of the junction becomes depleted of holes and electrons due to electron-hole recombinations, and is called a depletion layer, as shown in Figure 2.3.
Transistors• Transistors often involve power transfer and are usually manufactured from silicon (resistor) material.• The name ‘transistor’ derives from TRANSfer and resISTOR.• The general form of a transistor is a crystal (usually silicon) in which two pn junctions are formed.• The junctions can be npn or pnp.• The basic transistor has three electrode regions within the one crystal structure (compared to two in the pn junction diode).• These regions in a transistor are termed as base, collector, and emitter and that there will be three connection terminals• This form of transistor if often termed a junction transistor or bipolar transistor.
Silicon Controlled Rectifier• Silicon Controlled Rectifier (SCR).• Thyristor is used for requiring high speed & high power switching.• Handle V & I up to 1 kV & 1000A• Anode : high +ve voltage with relative to cathode & gate at small +ve potential w.r.t cathode.
Zener DiodeWith the application of sufficient reverse voltage, a p-n junction will experiencea rapid avalanche breakdown and conduct current in the reverse direction. Zener Regulator The constant reverse voltage of the zener diode makes it a valuable component for the regulation of the output voltage. The current through the zener will change to keep the voltage at within the limits of the threshold current and the maximum power it can dissipate
Is Vo Vz Vs IsRs Vo Vs Vo Is Rs Vo IL RL KCL : Is Iz Io Iz Is Io(Any components in parallel with Zener, it will follow Vz)
Is Vo VCE Vz VBE Ic IL Vs IsRs Vz VBE IsRs Vo Vs Vo Is Rs Vo IL RL Is Iz Ic I L Basic Idea: IZ = IB Ic Is I L , as Iz small compare to IcVs = VRS + Vo IC = βIB Proton Saga 1.5L When Vo↑, VBE ↑, IB ↑, IC ↑, IL↓, Vo normal
Iswara VB VZ VBE VO Vo ↑, VBE↓, IB↓, VO VZ VBE , VZ constant(IC=IL)↓, Vo normal VO VB IL Vo ↓, VBE ↑, IB↑, RL(IC=IL)↑, Vo normal PD I CVCE I L (VS VO ) in which I C I L
IR•In normal operation, VB = VZ•The current flowing through resistor R is: VB VZ VBE VO VS VR VZ VS VZ VR VS VZ , I R R VS VZ , I R R IR IB IZ•For a fixed Vs (and also Vz), IR is a fixed value•When IB increases, Iz will decrease•In order to get a good regulation, Iz must be larger thana minimum value IZK over the rated range of load current
Darlington TransistorsDarlington transistor combines two bipolar transistors in “Darlington pair")in a single device so that the current amplified by the first is amplified furtherby the second transistor.This gives it high current gain and takes up less space than using two discretetransistors in the same configuration. I C1 I B 2 β1 I B1 I C 2 β2 I B 2 β2 β1 I B1 A typical modern device has a current gain of 1000 or more, so that only a tiny base current is required to make the pair switch on. Example: Typical Darlington transistor has current gain of 1000. If input current is 10mA, means that output current, IC = 10mA x 1000 = 10A
Conclusion:Darlington transistor required less base current, IB to produced the requiredamount of IL. So less Iz drawn away from zener diode and the stabilityof the circuit can be maintained.
Concept: Depends on the voltage different between V+ and V- When Vo↑, VR2 ↑, (V+ -V-)↓, IB↓, (IC=IL)↓, Vo normal
VR2 = VBE2 + VZ (constant)IR3 = IC2 + IB1 Concept: Depends on the VBE2 When Vo↑, VR2 ↑, VBE2↑, IC2↑, IB1↓, IC1↓, Vo normal
New equivalent circuit when the output is accidentally shorted 18v Analysis: Condition under short circuit Vo = 0V, no feedback voltage, VBE = 0V 200Ω Q2 off. Now: VS VBE 18 0.7 I R3 86.5mA R3 200 SINCE , POWER DISSIPATION I R 3 I B1 PD VCE1 I C1 So, I C1 I B1 100 86.5mA 8.65 A PD (VS 0) I C1 PD 18V 8.65 A PD 155.7W
Equivalent Circuit Under Short Circuit Condition Transistor turns ON before output load short circuit: Concept: More short; more current shunt awayUnder Short Circuit Condition:Most of the output voltage now dropped across RCS. VRB becomesLarge and shunt away most of the current from IR3. RB VBE ( )VRCS RA RB