Ec 2 Marks

Unit - I 1. Why bypass and coupling capacitor are used in amplifier circuits? i. Bypass capacitor CE :  The capacitor connected in parallel with the emitter resistor RE is called as the emitter bypass capacitor.  This capacitor offers a low reactance to the amplified ac signal. Therefore the emitter resistor RE gets bypassed through CE for only the ac signals.  This will increase the voltage gain of the amplifier. Moreover as C E acts as an open circuit for dc voltages, it does not bypass R E for dc conditions. Thus, presence of CE does not alter the dc biasing conditions. ii. Input coupling capacitor :  The input coupling capacitor is used for coupling the ac input voltage V i to the base of the transistor.  As capacitors block dc, this capacitor helps to block any dc component present in Vi and couples only the ac component of the input signal. iii. Out put coupling capacitor :  This capacitor couples the amplifier out put to the load resistance or to the next stage of the amplifier. It is used for blocking the dc part and passing the ac part of the amplified signal to the load. 2. Define load line. The straight line, which is drawn on the output characteristics of the transistor, is known as load line. The word load line is used because the slope of this line is – 1 / RC. Whre RC is the load resistance. 3. Define bandwidth. A range of frequency over which the amplifier performance is satisfactory is called its bandwidth. It is always placed in between the high frequency region and low frequency region. 4. What is Darlington pair? The Darlington connection of two BJTs is as shown in fig. the two transistors Q 1 and Q2 are directly connected in the Darlington connection. It is also known as Darlington pair. As seen, the collectors of the two transistors are connected together, emitter of Q1 is connected to the base of Q2 and emitter of Q2 acts as the emitter of Darlington connection. 5. Classify amplifier based on coupling.  RC coupled amplifier  Transformer coupled amplifier  DC (direct coupled) amplifier 6.

Compare the CE, CC and CB amplifiers. SL.No. 1. 2. 3. 4. 5.

Quantity / Parameter Input terminal Output terminal Common terminal Input resistance (Ri) Output Resistance (Ro)

CE Base Collector Emitter Medium Medium

CB Emitter Collector Base Low High

CC Base Emitter Collector High Low

6. 7. 8.

Current gain (AI) Voltage gain (AV) Applications

High High AF voltage amplifiers

Less than 1 High Low noise Pre-amplifiers

High Less than 1 Buffer amplifiers

7. What is phase shift distortion?  If the phase shift introduced by the amplifier for different input frequencies is not proportional to frequency then phase distortion will take place. The phase distortions are not detectable by the human ears as they are insensitive to the phase changes.  Therefore, phase shift distortion takes place due to unequal phase shifts of the input signal at different frequencies. 8. What is difference between voltage amplifier and power amplifier? Small signal amplifiers are also known as “ Voltage amplifiers”. This is because these amplifiers are used primarily for voltage amplification but they are not capable of supplying a large power to the loads such as loud speakers. The large signal amplifier (power amplifier) will increase the current sourcing and sinking capability. So at its output we get a high voltage, high current signal that means a high power signal. Thus the power amplifier is basically a current amplifier. 9. What is crossover distortion in class B push pull amplifier? In class B push pull amplifiers the output signal gets distorted near the zero crossings. Therefore this distortion is called as the “ crossover distortion”. Due to crossover distortion the conduction angle of each transistor in the class B amplifier becomes less than 180°. 10. What are the types of bias method? 1. Fixed bias circuit (single base resistor biasing) 2. Collector to base bias circuit 3. Voltage divider bias (self-bias) circuit. Unit II 1. What is the differential amplifier? An amplifier which amplifies the difference between the two input signals is called as differential amplifier. 2. What is a common mode signal? State its characteristics. The output of the practical differential amplifier not only depends on the difference voltage but also depends on the average common level of the two inputs. Such an average level of the two input signals is called common mode signal denoted as Vc.

3. What is CMRR? The ability of a differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR. It is defined as the ratio of the differential voltage gain Ad to common mode voltage gain AC.

4. What is differential input impedance? How it can be decreased? It is also called as differential input resistance(Rin) and it is defined as the equivalent resistance between one of the inputs to ground terminal when the other input terminal is connected to ground. Rin should ideally be and practically as high as possible.

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The various methods of realizing the high input resistance for the differential amplifier circuit are, 1) use of Darlington pair. 2) use of FET 3) use of swamping resistors. What are the applications of differential amplifiers? 1. As basic building block of OP-AMP. 2. As input stage of many bio-medical instruments. 3. As input stage of power oscilloscope. What are the different configurations of a differential amplifier? The four important configurations of a differential amplifier as follows: 1. Dual input, balanced output differential amplifier. 2. Dual input, unbalanced output differential amplifier. 3. Single input, balanced output differential amplifier. 4. Single input, unbalanced output differential amplifier. Define tuned amplifiers. Tuned amplifier is an amplifier having a tuned circuit as load. That means in place of a load resistance, now a tuned circuit is used. The tuned amplifiers are designed for amplification of input signal, over a narrow band of signal frequencies centered about fr (resonant frequency). What is input bias current? Input bias current can be defined as the current flowing into each of the two input terminals when they are biased to the same voltage level i.e., when the op-amp is balanced.

9. Define PSRR. The power supply rejection ratio is defined as the ratio of the change in input offset voltage due to the change in supply voltage producing it, keeping other power supply voltage constant. It is also called power supply sensitivity. So if VEE is constant and due to change in VCC, there is change in input offset voltage then PSRR is expressed as,

For fixed VCC, if there is change in VEE causing change in input offset voltage then,

10. What is slew rate? The slew rate is defined as the maximum rate if change of output voltage with time. The slew rate is specified in V / µ sec. Thus Slew rate = S = 11. What is a chopper? Chopper is a static switch to provide variable DC voltage from a source of constant DC voltage.

Unit III 1. What is the concept of feedback? Feedback is defined as the process in which a part of output signal (voltage or current) is returned back to the input. In the feed back process a part of output is sampled and fed back to the input . Thus at the input of an amplifier using feedback two signals will be simultaneously present. One of them is the original input signal itself and the other one is the fed back signal. The fed back signal can be in phase with or out of phase with the original input signal. 2. Define positive and negative feedback. If the original input signal and the feedback signal are in phase, the feed back is called as “positive feed back”. However if these two signals are out of phase then the feedback is called as “negative feedback” Positive feed back used in oscillators and negative feedback is used in amplifiers. 3. What are the classifications of feedback? 1. Voltage series feedback 2. Current series feedback 3. Current shunt feedback 4. Voltage shunt feedback 4. What is the condition of oscillation? 1. The loop gain of the circuit must be ≥ 1i.e., and 2. The phase shift around the circuit must be zero. 5. What is open loop gain? The amplifier gain is A i.e., it amplifies its input Vi, A times to produce Vo.

This is called open loop gain of the amplifier. 6. Mention two reasons. Why LC oscillator is prepared over RC at radio frequency? 1. It operates in high frequency range from 200 kHz upto few GHz. 2. We can vary the frequency range easily using tank circuit. 7. What is the basic principle of operating of a RC oscillator? For producing oscillations in an oscillator circuit we need positive feedback which means that the voltage signal feedback should be in phase with the input signal. For providing a positive feedback at one particular frequency, an inverting amplifier may be used with a feed back network that causes a phase shift of 180° at the desired frequency of oscillation. The 180° phase shift in the feedback signal can be obtained by a suitable network consisting of three R-C sections as shown in fig. 8. How oscillation occurs in crystal oscillator? Certain materials such as quartz exhibit a unique property called “piezo electric” property. It states that if mechanical force is applied to a quartz crystal then it generates electric potential. Also if electric field is applied to a crystal it vibrates mechanically. If we apply mechanical vibrations to a quartz crystal then under proper operating conditions we can obtain electrical oscillations from it. A typical equivalent circuit of a crystal is shown in fig. The resonating frequency is ,

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9. Compare the oscillator and amplifier. SL.No Oscillators Amplifiers 1. Oscillator produces the waveform of Amplifier amplifies the input signal desired frequency. magnitude to desired level. 2. In oscillators positive feedback is applied In amplifiers negative feedback is applied 3. Only we can vary the out put frequency Only we can vary the output signal not signal amplitude amplitude not frequency 4. Used as signal generators and RF sources Used as audio amplifier 10. Write the expression for frequency of oscillation for a Hartley oscillator. The frequency of oscillation is given by: Without mutual inductance With mutual inductance. Unit IV 1. Define pulse and wave shaping circuits. Pulse circuits: The circuits producing various types of pulses like square pulse and triangular pulse are known as Pulse circuits. Wave shaping circuits: The circuit which operates on the input signal to produce an output signal of the required shape is known as wave shaping circuit. 2. Draw the block diagram of linear wave shaping circuits.

3. What is clipper? Clippers: The clipping circuits using diodes have the ability to “clip” off or remove a portion of the input signal without distorting the remaining part of the waveform. 4. What is clamper? The circuits which are used to add a d.c level as per the requirements to the a.c output signal are called clamper circuits. 5. Mention types of clippers and clampers: Types of clippers: 1. Series clippers 2. Parallel clippers Types of clampers: 1. Positive clampers 2. Negative clampers 6. What is meant by Schmitt trigger? Schmitt trigger is type of bistable multi-vibrator using transistors.

7. What is meant by unsymmetrical triggering? In unsymmetrical triggering, two trigger inputs are used, one to set the circuit in one particular stable state and other to reset the circuit to the opposite state. It is also called setreset triggering. 8. What is meant by multi-vibrator? The electronic circuits which are used to generate non - sinusoidal wave forms are called multivibrators. 9. What is the advantage of UJT relaxation oscillator? The UJT relaxation oscillator maintains constant output frequency even though the supply voltage fluctuates. 10. What is the application of saw tooth oscillators? 1. Used in time – base signal generators 2. Used in power control circuits 3. Used in SCR triggering circuits Unit V 1. What is rectifier? What are its types? Rectifier is an electronic device which converts an alternating (ac) voltage or current into a unidirectional (dc) voltage or current. Types of rectifier: 1. Half wave rectifier 2. Full wave rectifier i. Full wave rectifier with center tapped transformer ii. Full wave bridge rectifier 2. What are the important points to be studied and analyzing the various rectifier circuits? 1. Average load current (IL dc) 2. RMS load current (IL rms) 3. Ripple factor 4. Rectification efficiency and TUF. 5. Average load voltage (VLdc) 6. RMS load voltage (VLrms) 7. voltage regulation 3. Define rectifying efficiency. Rectifying efficiency is defined as the ratio of DC output power into AC input power of a rectifier.

4. What are the disadvantages of half wave rectifier? 1. Due to the unidirectional current flow through the transformer, there is a possibility of core saturation. To avoid it, transformer size must be increased. 2. Ripple factor is high(1.21). 3. Low rectification efficiency (40%). 4. Low TUF (only 28%) which indicates that the transformer is not being used effectively. 5. Low DC output voltage and current. 6. Larger filter components are required.

5. What is the function of filters? Filter is used to reduce the ripple contents in the output of a rectifier to obtain a pure dc voltage. 6. Compare L filter and C filter. SL.No Parameter Capacitor(C) filter Inductor(L) filter 1. Place of filter Across the load In series with the load 2. Useful in Reducing ripple in load Reducing ripple in load voltage current 3. Lowest ripple in the load No load or light loads Heavy loads voltage at 4. Suitable for Light load applications Heavy load applications 5. Surge current through diodes Very high and must be Low and need not be controlled controlled 6. Expression for ripple factor RF = RF = 7. Size of filter Small and compact Bulky 7. List the merits of IC regulators. 1. IC voltage regulators are versatile and relatively inexpensive. 2. IC voltage regulators are available with features such as a programmable output, currentvoltage boosting, internal short circuit current limiting and thermal shutdown. 3. IC voltage regulators are making floating operation for high voltage applications. 8. List the advantages of Zener regulator. 1. Simple circuits 2. Only 2 or 3 components are required to be used 3. Low cost 9. Mention how over load protection is provided in series voltage regulators. Over load protection is provided voltage of transistor Q1. This results in in series voltage regulators by adding a reduction of the conduction level of current limiting circuit. The current transistor Q1. Thus further increase in limiting consists of a transistorQ3 and a load current is prevented. resistor R5 (approximately 1Ω) as shown in fig. connected between base and emitter terminals of transistorQ3. With normal load current, transistor Q3 remains off because the voltage drop across R5 is small(less than about 0.7 V necessary for making the transistor Q3 on ). With the excessive load current the voltage drop across R5 becomes large enough to turn transistor Q3 on. The collector current of transistor Q3 flows through R3, thereby decreasing the base 10. Write some applications of bridge rectifier. 1. Laboratory power supplies 2. High current power supplies 3. Battery charger 4. DC power supplies for various electronic circuits.