Q.131 Write the circuit of a current-to-voltage converter using an OPAMP and explain its operation.  

  Ans :          

     A device that produces a voltage proportional to input signal current is called a current to voltage converter. There is a virtual ground at the inverting input terminal, current flowing through RS is zero, and, therefore, the entire input current IS flows through the load resistor RL resulting in the output voltage given as Vout = - IS. RL   The above equation clearly indicates that the output voltage is directly proportional to the input current IS.  

Q.132 Define the following terms as used in IC fabrication:    (i)  Chip                        (ii)   Diffusion                  (iii)  Etching. (4)  

  Ans :   (i) Chip - An integrated circuit IC is one in which all active and passive components are automatically part of a small semiconductor chip.   (ii) Diffusion - is the process of introduction of controlled amount of dopant atoms into the semiconductor .Diffusion alters the type of conductivity of the semiconductor. In silicon integrated circuit processing diffusion is used to form bases, emitters and resistors in bipolar technology and source and drain regions of MOSFET’s in MOS technology. Commonly used diffusion methods are diffusion from a chemical source, diffusion from a doped oxide source.     (iii) Etching – Selective removal of material in silicon IC process is known etching. The process may be chemical or physical. By physical means etching can be done by the kinetic energy associated with the bombarding ions in the ion stream or plasma. Etching can be classified as dry and wet etching. In case of dry-etching the wafer is bombarded by ions radicals or atoms in the vapour phase. In wet etching liquid chemicals are used.    

Q.133 Briefly explain a Schottky diode.  (4)  

  Ans :     

  The reverse recovery time is short in small signal diodes that its effect cannot be noticed at frequencies below 10MHZ or so. It becomes very important well above 10 MHZ.    The solution is a special purpose device called a Schottky diode. Such a diode has no depletion layer eliminating the stored charges at the junction. Due to the lack of charge storage, the Schottky diode can switch off faster than ordinary diode. It’s construction is very different from the normal PN junction in that a metal semiconductor junction is developed.     Q.134 What is an inductor? Explain briefly various types of fixed inductors employed in electronic industry. What is the role of variable inductors in radio receiver?  (6)    

  Ans :   An inductor has been defined as a physical device which is capable of storing energy by virtue of a current flowing through it.    In case of an inductor current does not change instantaneously. It offers high impedance to ac but very low  impedance to dc. It blocks ac signal but passes dc signal.   Inductors can be classified into filter chokes audio frequency chokes and radio frequency chokes. 

  Filter choke has many turns of fine wire wound on an iron core made of laminated sheets of E and I shapes and is used in smoothing the pulsating current produced by rectifying ac into dc.   Audio frequency chokes are used to provide high impedance to audio frequencies.   Radio frequency chokes are employed to block the radio frequency.   Variable inductors – Tuning circuits, phase shifting and switching of bands in amplifier sometimes require a variable inductance.  

Q.135 Differentiate between a current source and a voltage source. Give their graphical representations. How can they be converted from one another? Determine the current flowing through 7Ω resistor in the circuit shown in Fig. 2 by using source transformation technique.  (10)         

  Ans :   Any device that produces voltage output continuously is known as voltage source. It’s basic purpose is to supply power to load connected across it.    

                                                                    

                                                                                   

                                       Dc voltage source       ac voltage source         

Ideal current source    Constant current source,  a source that supplies a constant current to a load even if its impedance varies.    It should be noted that a voltage source series resistance combination is equivalent to a current source parallel resistance combination if, and only if their respective open circuit voltages are equal to respective short circuit currents are equal.  

Q.136 “As regards conduction of current in concerned, a semiconductor is bipolar in nature whereas a metal is unipolar”-Justify (or) nullify the above statement. (7)  

  Ans :   In semiconductors both holes and electrons take part in conduction. This is the reason that these are bipolar in nature.   In conducting materials there is no forbidden gap. The orbits in the conduction band are very large. An electron in the conduction band experiences almost negligible nuclear attraction. In fact an electron in the conduction band does not belong to any particular atom but it moves randomly through out the solid.  

Q.137 Explain what do you understand by intrinsic, N-type and P-type semiconductors. Discuss the position of Fermi level in each case.  (9)  

  Ans :   An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form.   When a small amount of pentavalent impurity such as arsenic, antimony or phosphorous is added to a pure semiconductor crystal during crystal growth the resulting crystal is called the N-type extrinsic semiconductor.   When a small amount of trivalent impurity such as boron, gallium, indium or aluminium is added to a pure semiconductor crystal during the crystal growth, the resulting crystal is called the P-type extrinsic semiconductor.   

Q.138 Discuss the reasons for the existence of a depletion layer in a P-N junction. Relate it to the rectifying properties of a P-N junction.  (10)  

  Ans :   On the formation of P-N junction some of the holes from P-type material tend to diffuse across the boundary into N-type material and some of the free electrons similarly diffuse into the P-type material. This happens due to density gradient (as concentration of holes is higher on P-side than that on N-side and concentration of electrons is higher on N-side than that on P-side.) This process is known as diffusion.   As a result of the displacement of the chargers, an electric field appears across the junction. Equilibrium is established when the field becomes large enough to restrain the process of diffusion. The electric charges are confined to the neighbourhood of the junction, and consists of immobile ions. The initial diffusion of charge carriers creates a barrier potential at the junction. The region around the junction is completely ionised. As a result there are no free electrons on the N-sides nor the holes on the P-side. Since the region around the junction is depleted of mobile charges it is called the depletion region, the space charge region or transition region.  

Q.139 What is a Zener diode? Explain, with the help of a circuit diagram. How Zener diode can be used as a voltage regulator?  (6)  

  Ans :   Zener diode also sometimes called the breakdown diode is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition.     

  Zener diode symbol Zener diode used as a Voltage regulator  

  Voltage regulation is a measure of a circuit‘s ability to maintain a constant output voltage even when either input voltage or load current varies.   A resistor RS is necessary to limit the reverse current through the diode to a safer value. The voltage source VS and resistor RS are selected that the diode operates in the breakdown region. The diode voltage in this region which is also the voltage across the load RL is called Zener Voltage VZ and the diode current is called the Zener current IZ.   As long as voltage across the load resistor RL is less than the breakdown voltage VZ the zener diode does conduct. The resistors RS and RL constitute a potential divider across VS. At an increased supply voltage VS the voltage drop across load resistor becomes greater than the zener breakdown voltage. It then operates in the breakdown region. The series resistor RS limits the zener current IZ from exceeding its rated maximum value because 

zener current is given as IZ = S, SZ R VV −, so IS = IZ +IL 

  When zener diode operates in its breakdown region the voltage across it VZ remains fairly constant even though the current IZ flowing through it may vary considerably.  

Q.140 Explain the operation of JFET as an analog switch. (7)  

Ans :             

JFET as am Analog Switch  

 When no gate voltage is applied to the FET ie. VGS =0, FET becomes saturated and it behaves like a small resistance usually of the value of less than 100Ω and, therefore, output 

voltage becomes equal to N I DDS DS OUT V RONR R V ()+ =   Since RD>>RDS(ON) so VOUT can be taken equal to zero.. When a negative voltage equal to VGS(OFF) is applied to the gate , the FET operates in the cut-off region and it acts like a very high resistance usually of some mega ohms. Hence output voltage becomes nearly equal to input voltage.  

Q.141 Compare the characteristics of CB, CE and CC configurations of a transistor. Draw the circuit of a common collector transistor configuration and explain its operation. Also derive the relation between γ and α current amplification factors. (9)  

  Ans :    Characteristics Common Base Common Emitter Common Collector Input impedance Low (≅100Ω) Medium (≅800Ω) Very high (≅750kΩ) Output impedance Very high (≅500kΩ) High (≅50kΩ) Low (≅50Ω) Current gain Less than unity High(≅80) High(≅100) Voltage gain About 150 ≅500 Less than unity Leakage current ≅5µA Very large ≅500µA Very large        Common collector configuration         

CC-NPN Transistor   In this arrangement base current IB flows in the input circuit and emitter current IE flows in the output circuit. So, change in emitter current ∆ IE to change in base current ∆ IB gives the current amplification factor γ      IC= α IE+ ICBO   and IE= IB + IC   ∴ IE= IB+ α IE+ ICBO      IE(1- α ) = IB+ ICBO  This configuration primarily is used for impedance matching.  

Q.142 Why are MOSFETs available in both enhancement and depletion modes, while JFETs operate almost invariably in the depletion modes.  (4)      

  Ans :   In a JFET, if the gate is forward biased, excess carrier junction occurs and gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but device is never operated with gate forward biased because gate current is undesirable.  

Q.143 Sketch the output characteristics for N-channel JFET with gate-source voltage shorted (i.e. VGS=0). How Ohmic, Pinch-off and Breakdown regions are created?  

  Ans : FET-Drain characteristic with Short-gate   Initially when VDS is zero, there is no attracting potential at the drain, so no current flows in spite of the fact that the channel is fully open. Thus ID = 0.   For small supply applied voltage VDS, the N-type bar acts as a simple semiconductor resistor, and the drain current ID increases linearly with the increase in VDS, up to the knee point. This region of the curve is called the Channel Ohmic Region.   With the increase in drain current ID, the ohmic voltage drop between the source and channel region reverse biases the gate junction. The reverse biasing of the gate junction is not uniform throughout. The reverse bias is more at the drain end than that at the source end of the channel, the conducting portion of the channel begins to constrict more at the drain 

end. Eventually a voltage VDS is reached at which channel is pinched off, (ie. All the free charges from the channel gets removed), is called the Pinch-off voltage (VP)   If VDS is continuously increased, a stage comes when the gate channel junction breaks down. At this point the drain current increases very rapidly and the JFET may be destroyed.   This is known as Avalanche Effect.  

Q.144 Describe the structure, symbol and operation of SCR with the help of suitable diagrams.   (8)     Ans :  The SCR (Silicon controlled rectifier) is a controlled rectifier constructed of a silicon semiconductor material with a third terminal for control purposes. The basic operation of SCR is different from that of an ordinary two layer semiconductor diode in that, the third terminal gate determines when the rectifier switches from the open circuit to short circuit state. SCR deice is a switch .Ideally it remains off or appears to have infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal. The thyristor then switches ON and current flows and continues to conduct without further gate signals.    Q.145 Explain the following terms as referred to an operational amplifier (i)        Input offset Voltage (ii)       Input offset Current (iii)      Slew Rate (vii)  CMRR  (12)             Ans : (i) Input offset Voltage       

   (a) Output offset voltage   (b) Elimination of Output                 off-set voltage When the inputs of the op-amp are grounded, there is almost always an output offset voltage as shown in fig(a) because the input transistors have different VBE values.