MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI-621213. QUESTION BANK DEPARTMENT: EEE SUBJECT CODE: EE2203 SEMESTER : III SUBJECT NAME: ELECTRONIC DEVICES &CIRCUITS UNIT 4-AMPLIFIERS AND OSCILLATORS PART A (2 Marks) 1. Name the types of feedback amplifiers. (AUC MAY 13, DEC06) Voltage Series feedback amplifier Voltage shunt feedback amplifier Current series feedback amplifier Current shunt feedback amplifier 2. State piezo electric effect. (AUC AMY 13) When a mechanical stress is given to a crystal, a potential difference is developed across a crystal or when a potential difference is applied across a crystal, the mechanical structure of the crystal is deformed. This effect is called as piezoelectric effect 7 such a crystal is called piezoelectric crystal. 3. State the condition for oscillators. (AUC MAY 12) The essential conditions for oscillations are, 1. ǀ A β ǀ = 1, the magnitude of open loop gain must be unity. 2. The total phase shift around the closed loop is zero or 360 o. 4. What are the advantages of negative feedback. (AUC MAY 12, MAY 10) The advantages of negative feedback are, Stabilization of gain Increased bandwidth Decreased distortion Decreased noise Increase in input impedance Decrease in output impedance 5. State Bar Khausen criterion for sustained oscillation(auc NOV 12, DEC 11, APR09) Same as Q3 6. List the advantages of crystal oscillator (AUC NOV 12) SSathya Priyadharshini. Asst. Prof./EEE 1
The advantage of crystal is its very high Q as a resonant circuit, which results in good frequency stability for the oscillator. However since the resonant frequencies of crystal are temperature dependent, it is necessary to enclose the crystal in a temperature controlled oven to achieve the frequency stabilization in order of one part in 10 10 7. What are the advantages of differential amplifier?(auc DEC 11) 8. Define CMRR and write its significance in differential amplifiers. (AUC MAY10, APR 09) The figure of merit called Common Mode Rejection Ratio(CMRR) defines the ability of the differential amplifier to reject common mode signal. 9. Mention the operating modes of Differential amplifier. (AUC NOV 10) Differential amplifier with resistive loading. Differential amplifier with active loading. The four differential amplifier configurations are following: Dual input, balanced output differential amplifier. Dual input, unbalanced output differential amplifier. Single input balanced output differential amplifier. Single input unbalanced output differential amplifier. 10. Mention any two high frequency LC oscillators.(auc NOV 10) 11. What is positive feedback? (AUC MAY 04) If the feedback signal φ f is inphase with the input signal φ s the net effect of feedback will increase the input signal given to the amplifier φi. Hence φ i = φ s + φ f. Hence the input voltage applied to the amplifier is increased thereby increasing the output φ o. This type of feedback is called positive or regenerative feedback. 12. What is negative feedback? (AUC MAY 06) If the feedback signal φ f is out of phase with the input signal φ s the net effect of feedback will decrease the input signal given to the amplifier φi. Hence φ i = φ s - φ f. Hence the input voltage applied to the amplifier is increased thereby decreasing the output φ o. This type of feedback is called negative or degenerative feedback. 13. Define feedback factor β. (AUC MAY 05) Feedback factor β is defined as the ratio of φ f to φ o. 14. What is a differential amplifier? SSathya Priyadharshini. Asst. Prof./EEE 2
An amplifier, which is designed to give the difference between two input signals, is called the differential amplifier. 15. What is the function of a differential amplifier? The function of a differential amplifier is to amplify the difference of two signal inputs, i.e., where Ad is the differential gain 16. When two signals V1 and V2 are connected to the two inputs of a difference amplifier, define a difference signal Vd and common-mode signal Vc The difference signal Vd is defined as the difference of the two signal inputs, i.e., Vd = V1 V2 The common-mode signal Vc is defined as the average of the two signals, I.e., Vc =1/2 (V1 + V2) 17. What is the differential-mode voltage gain of a differential amplifier? It is given by Ad = ½ (A1-A2) 18. What is the common-mode gain AC in terms of A1 and A2? It is given by Ac = A1 + A2 19. Define CMRR. The common-mode rejection ratio(cmrr) of a differential amplifier is defined as the ratio of the differential-mode gain to common-mode gain. CMRR = Ad Ac 20. What are the ideal values of Ad and Ac with reference to the differential amplifier? Ideally, Ac should be zero and Ad should be large, ideally infinite. 21. Express CMRR in d B. CMRR (db) = 20 log Ad 20log Ac. 22. What are advantages of differential amplifier? It has high gain and high CMRR. 23. List some applications of differential amplifiers? Used in IC applications, AGC circuits and phase inverters. 24. Define (i) feedback (ii) positive feedback and (iii) negative feedback. i. Feedback: The process of combining a fraction of the output (of a Device-amplifier) back to its input is called feedback. SSathya Priyadharshini. Asst. Prof./EEE 3
ii. Positive Feedback: If the feedback is in phase to the input, it is called positive feedback. Here iii. Negative Feedback: When the feedback is in opposition (out of phase) to the input, it is called negative feedback. Here 25. What loop gain of a feedback amplifier. In a feedback amplifier, when the signal passes through an amplifier 26. Mention the four connections in Feedback. 1. Voltage series feedback. 2. Voltage shunt feedback 3. Current series feedback. 4. Current shunt feedback. 27. Explain the voltage series feedback. In this case, the feedback voltage is derived from the output voltage and fed in series with input signal. The input of the amplifier and the feedback network are in series is also known as series parallel in parallel, hence this configuration is also known as series parallel feedback network. 28. Explain the voltage shunt feedback. The input of amplifier and the feedback network are in parallel and known as parallel parallel feedback network. This type of feedback to the ideal current to voltage converter, a circulating having very low input impedance and very low output impedance. 29. Explain the current series feedback. When the feedback voltage derived from the load current and is fed in series with the input signal, the feedback is said to be current series feedback, the inputs of the amplifier and the feedback network are in series and the output are also in series. This configuration is also called as series-series feedback configuration. 30. Explain the current shunt feedback. When the feedback voltage is derived from the load current and a fed in parallel with the input signal, the feedback is said to be current shunt feedback. Herein the inputs of the amplifier and the feedback network are in parallel and the outputs are in series. This configuration is also known as parallel series feedback 31. Write the effects of negative feedback. 1. The gain becomes stabilized with respect to changes in the amplifier active device parameters like hfe. 2. The non-linear distortion is reduced there by increasing the signal handling capacity or the dynamic range of the amplifier. 32. Write the conditions for a circuit to oscillate. SSathya Priyadharshini. Asst. Prof./EEE 4
1. The oscillator circuit should consist of an amplifier and a portion of the output should be feedback to the input. For sustained oscillations, the feedback voltage must be in phase with the input, i.e., total phase shift around the loop must be 360. 2. The amount of energy or power feedback to the input must be sufficient to the input circuit. 33. Mention the classification of oscillators. According to the frequency determining networks, 1. RC oscillators 2. LC oscillators 3. Crystal oscillators 34. List the advantages of phase shift oscillator. 1. The phase shift oscillator does not required conductance or transformers. 2. It is suitable for the low frequency range i.e., from a few hertz to several hundred KHz. The upper frequency is limited because the impedance of RC network may become so small that it loads the amplifier heavily. 35. Write the disadvantages of Phase shift oscillator. 1. It is necessary to change the C or R in all the three RC networks simultaneously for changing the frequency of oscillations. This is practically difficult. 2. It is not suitable for high frequencies. 36. Write the main drawback of LC oscillators. 1. The frequency stability is not very good. 2. They are too bulky and expensive and cannot be used to generate low frequencies. 37. Define Piezo electric effect. Certain crystal, when suitable wt, develop a potential difference between opposite faces, magnitude and polarity of which depends on pressure or tension applied to the other pair of perpendicular faces. This effect is called Piezo electric effect. 38. Define differential amplifier. When two or more input is applied to the amplifier and receive the single output is called differential amplifier. VO = V1 - V2 40. Define stability. The variation of input is applied to the system; to get the constant output is called stability. SSathya Priyadharshini. Asst. Prof./EEE 5
41. Define switch. A switch is a device which opens or closes the electrical circuit, i.e., can turn on or off current in an electrical circuit. An ideal switch has zero internal resistance when it is closed an infinite leakage resistance when it is open PART B(16 Marks) 1. What do you understand by differential amplifier? Draw the circuit diagram and explain the working of differential amplifier. Explain the circuit operation at CM & DM (AUC NOV 12, MAY 13, NOV10) Differential amplifier A differential amplifier is a type of electronic amplifier that multiplies the difference between two inputs by some constant factor (the differential gain). Differential amplifier symbol The inverting and non-inverting inputs are distinguished by " " and "+" symbols (respectively) placed in the amplifier triangle. Vs+ and Vs are the power supply voltages; they are often omitted from the diagram for simplicity, but of course must be present in the actual circuit. Differential Amplifiers: Differential amplifier is a basic building block of an op-amp. The function of a differential amplifier is to amplify the difference between two input signals. How the differential amplifier is developed? Let us consider two emitter-biased circuits as shown in fig. 1. SSathya Priyadharshini. Asst. Prof./EEE 6
Fig. 1 The two transistors Q1 and Q2 have identical characteristics. The resistances of the circuits are equal, i.e. RE1 = R E2, RC1 = R C2 and the magnitude of +VCC is equal to the magnitude of VEE. These voltages are measured with respect to ground. To make a differential amplifier, the two circuits are connected as shown in fig. 1. The two +VCC and VEE supply terminals are made common because they are same. The two emitters are also connected and the parallel combination of RE1 and RE2 is replaced by a resistance RE. The two input signals v1 & v2 are applied at the base of Q1 and at the base of Q2. The output voltage is taken between two collectors. The collector resistances are equal and therefore denoted by RC = RC1 = RC2. Ideally, the output voltage is zero when the two inputs are equal. When v1 is greater then v2 the output voltage with the polarity shown appears. When v1 is less than v2, the output voltage has the opposite polarity. The differential amplifiers are of different configurations. The four differential amplifier configurations are following: 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. SSathya Priyadharshini. Asst. Prof./EEE 7
2. Draw the circuit diagram & explain the principle of operation of following oscillators (i) RC Phase shift oscillator (ii) any one LC oscillator. (AUC NOV 12, APR09) The RC Phase Shift Oscillator: At low frequencies (around 100 KHz or less), resistors are usually employed to determine the frequency oscillation. Various circuits are used in the feedback circuit including ladder network. A block diagram of a ladder type RC phase shift oscillation is shown in fig. 4. It consists of three resistor R and C capacitors. If the phase shift through the amplifier is 180º, then oscillation may occur at the frequency where the RC network produces an additional 180 phase shift. SSathya Priyadharshini. Asst. Prof./EEE 8
A block diagram of a ladder type RC phase shift oscillation is shown in fig. 4. It consists of three resistor R and C capacitors. If the phase shift through the amplifier is 180º, then oscillation may occur at the frequency where the RC network produces an additional 180 phase shift. For phase shift equal to 180o between Vx and VO, imaginary term of Vx / VO must be zero. Therefore, SSathya Priyadharshini. Asst. Prof./EEE 9
This is the frequency of oscillation. Substituting this frequency in Vx / VO expression. In order to ensure the oscillation, initially Aβ >1 and under study state Aβ =1. This means the gain of the amplifier should be initially greater than 29 (so that Aβ >1) and under steady stat conditions it reduces to 29. The Colpitts Oscillator: Wein bridge oscillator is not suited to high frequencies (above 1MHz). The main problem is the phase shift through the amplifier. The alternative is an LC oscillator, a circuit that can be used for frequencies between 1MHz and 500MHz. The frequency range is beyond the frequency limit of most OPAMPs. With an amplifier and LC tank circuit, we can feedback a signal with the right amplitude and phase is feedback to sustain oscillations. Fig. 3, shows the circuit of colpitts oscillator. The voltage divider bias sets up a quiescent operating point. The circuit then has a low frequency voltage gain of rc / r'e where rc is the ac resistance seen by the selector. Because of the base and collector lag networks, the high frequency voltage gain is less then rc / r'e. Fig. 4, shows a simplified ac equivalent circuit. The circulating or loop current in the tank flows through C1 in series with C2. The voltage output equals the voltage across C1. The feedback voltage vf appears across C2. This feedback voltage drives the base and sustains the oscillations developed across the tank circuit provided there is enough voltage gain at the oscillation frequency. Since the emitter is at ac ground the circuit is a CE connection. SSathya Priyadharshini. Asst. Prof./EEE 10
Most LC oscillators use tank circuit with a Q greater than 10. The Q of the feedback circuit is given by Because of this, the approximate resonant frequency is This is accurate and better than 1% when Q is greater than 1%. The capacitance C is the equivalent capacitance the circulation current passes through. In the Colpitts tank the circulating current flows through C1 in series with C2. Therefore C = C1 C2 / (C1 +C2) 3. Describe the construction of Hartley oscillator and explain the working principle. Derive the expression for frequency of oscillation & condition for starting of oscillation. (AUC MAY 12, MAY 13,NOV10, APR09) Hartley Oscillator: The figure shows Hartley oscillator when the LC tank is resonant, the circulating current flows through L1 in series with L2. Thus, the equivalent inductance is L = L1 + L2. SSathya Priyadharshini. Asst. Prof./EEE 11
4. Explain the operation of voltage series feedback amplifier & current shunt feedback amplifier. (AUC MAY 12, MAY09) Voltage Series Feedback SSathya Priyadharshini. Asst. Prof./EEE 12
Current Series Feedback: SSathya Priyadharshini. Asst. Prof./EEE 13
5. Explain the concept of negative feedback in amplifier. Derive the expression for voltage gain, input impedance and output impedance. (AUC DEC 11) 6. (i)draw the block diagram of a voltage series feedback amplifier and derive the equation for input impedance, output impedance and the voltage gain. (AUC MAY 09) (ii) Calculate the voltage gain, input and output resistances of a voltage series feedback amplifier having AV = 300, Ri = 1.5 k, Ro = 50 k and β = 1/15. SSathya Priyadharshini. Asst. Prof./EEE 14
7. Derive the equation for differential mode gain and common mode gain of a differential amplifier. (AUC NOV 10) Discuss the various topologies of feedback amplifier. (AUC APR 09) Voltage Series Feedback SSathya Priyadharshini. Asst. Prof./EEE 15
Current Shunt Feedback: SSathya Priyadharshini. Asst. Prof./EEE 16
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Voltage Series Feedback: Voltage Shunt Feedback: 8. Explain the effects of negative feedback in amplifiers.(auc APR 09) 9. Draw the circuit of crystal oscillator & explain its operation. (AUC NOV 13) SSathya Priyadharshini. Asst. Prof./EEE 18