Integrated Circuits: Aktu Previous Solved Question Paper, Notes

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Section A: Integrated Circuits Short Questions

a. What is meant by the term matched transistors.

Ans. When two transistors are intended to be equivalent, i.e., the same size, orientation, and surroundings, as well as having identical values of saturation current, current gain, and early voltage, they are referred to as matched transistors.

b. What is a Current Mirror circuit ? Give its need.  

Ans. Current mirror circuit: A current mirror copy of input current circuit is one in which the output current is compelled to equal the input current.

Need:

Current mirror circuits are commonly employed in integrated circuits as well as a variety of other applications where current must be balanced between two legs.

c. Define and give significance of Slew Rate. 

Ans. Slew rate: Slew rate is defined as the maximum rate of change of output voltage with time. Its unit is V/μsec. 

Significance: The slew rate of an ideal Op-Amp is unlimited, implying that the output voltage may instantly follow changes in the input step voltage. The higher the value of slew rate, the better the performance of the Op-Amp.  

d. What do you mean by the quadrant operation of multiplier?  

Ans. 1. The quadrant specifies the circuit’s suitability for bipolar signals at its inputs.

2. Only positive input signals are accepted by the first quadrant device, one bipolar signal and one unipolar signal by the second quadrant device, and two bipolar signals by the fourth quadrant device.

 e. What do you mean by a frequency response of a filter circuit? 

Ans. 1. Frequency response is a quantitative measure of a filter circuit’s output spectrum in response to a stimulus that is used to characterize the system’s dynamic.

2. It is a measure of the magnitude and phase of the output in relation to the input as a function of frequency.

f. Differentiate wide band and narrow band pass filter. 

Ans. 

g. What role does PDN play in CMOS implementation?  

Ans. The function of PDN is to provide connection between GND and V_out to pull V_out to logic. 0.

h. Differentiate between a peak detector and sample and hold circuit. 

Ans.  

i. Describe the need of voltage limiter circuits. 

Ans. To limit the voltage level, a voltage limiter is utilised. When a constant, dependable voltage is required, a voltage limiter is the preferred device.

j. List the application of PLL. 

Ans.

• i. Frequency multiplication/division
• ii. Frequency translation 
• iii. AM detection 
• iv. FM demodulation

Section B: Integrated Circuits Long Questions

a. Find out the overall gain of an op-amp IC741 giving its cascaded equivalent circuit derived for its three stages. Also drive the relationship between f_T and Slew Rate for IC741.    

Ans. A. Overall gain of an Op-Amp IC-741:  

The overall voltage gain A_v of the Op-Amp is the product of voltage gains of each stage as given by, 

where,  A_d = Gain of differential amplifier stage

A₂ = Gain of the second stage

A₃ = Gain of the output stage 

B. Slew rate: The slew rate is defined as the maximum rate of change of output voltage per unit of time and is expressed in volts per microsecond, i.e., 

C. Relationship between f_t and SR:

1. Consider a voltage follower shown in Fig.(a). The input is large amplitude, high frequency sine wave.  

3. Fig.(b) shows the input-output waveform. Then, output 

4. The rate of change of the output is given by, 

5. The maximum rate of change of the output occurs when cos ωt = 1. That

6. Therefore, slew rate 

where,  f = Input frequency (Hz) 

V_m = Peak output amplitude.

b. Draw the generalized impedance converter and derive its impedance equation. Also simulate an Inductor.  

Ans. Generalized impedance converter: 

1. Generalized impedance converters (GICs) are Op-Amp circuits that employ RC networks for simulating frequency-dependent impedance elements such as inductors. Fig. shows the circuit of a GIC.

2. From Fig. at node 1, 

where V₁ = V_cc and V₂ = V₀₁. Now, between nodes 2 and 4, we obtain (using KCL) 

3 From the Fig. we observe that nodes 1 and 3 are virtually shorted, and hence using KCL 

where   V₀₂ = V₄ 

4. Rearranging eq.(2.5.3) yields 

5. Now by using KCL between nodes 4 and 5, we have 

6. Substitution for V₅ = V_cc and rearrangements give  

7. Substituting for V₀₂ from eq. (2.5.6) into eq. (2.5.4), we get

8. Rearranging eq. (2.5.7), we have

9. Substituting for V₀₁ from eq. (2.5.8) into (2.5.1) and simplifying, we obtain  

10. Rearrangement of eq. (2.5.9) yields the input impedance of the circuit 

11. Eq. (2.5.10) shows that the circuit shown in Fig. can be used as grounded impedance whose nature and value depends on the nature and values of impedance elements Z₁ to Z₅.

Simulate an Inductor: Fig. shows the Antoniou inductance simulation circuit.  

c. Describe temperature compensated Log amplifier using two op-amp and explain its operation. 

Ans. 1. The logarithmic amplifier is very sensitive to temperature. To minimize the temperature effects, the circuit called temperature-compensated logarithmic amplifier is used, in which two matched diodes are used to cancel the temperature-dependent offset term ‘log I_s’. 

2. Similarly, a thermistor, which is a temperature-sensitive resistor can be used to cancel the temperature-dependent scaling factor ‘ηV_T’

3 The output voltage of Op-Amp A₁ ia negative of the voltage across diode D₁ and is given by

4. Similarly, the voltage across diode D₂ is given by

where I is the forward current and I_S is the reverse saturation current of diode D₂
5. As both diodes are matched, their reverse saturation currents are same. The voltage at the non-inverting terminal of Op-Amp A₂ is given by 

6. Thus, the temperature-dependent offset term is eliminated from the output of A₁. The Op-Amp A₂ isa non-inverting amplifier and its output is given by 

7. Thus the circuit shown in Fig. cancels the temperature-dependent terms present at the output of the log amplifier and provides a temperature-independent logarithmic output.  

d. Sketch the logic gate symbolic representation of clocked SR flip-flop using NAND gate. Also sketch its CMOS circuit implementation and explain its operation. 

Ans. A. Clocked SR flip-flop using NAND gate: Fig. shows gate level schematic of clocked NAND based SR flip-flop circuit. 

B. CMOS Implementation: 

1 A simpler implementation of a clocked SR flip-flop is shown in Fig. Here, pass transistor logic is employed to implement the clocked set-reset function. 

2. The SR flip-flop comprising two cross-coupled inverters and two pass transistors Q₅ and Q₆ The pass transistors are turned ON when the clock (ɸ) is high, and they connect the flip-flop input S and R. The pass transistors act as transmission gates allowing the inputs S and R.

C. Operation: 

• 1. Consider the flip-flop output has the initial state Q = 1 and Q(bar) = 0, and the input R = 1 and S = 0 is applied to the input of flip-flop. 
• 2 When the clock ɸ is high, the transIstors Q₅ and Q₆ are turned ON 
• 3. For this input R= 1 and S =0, the transistor Q₃ is turned ON and pull down the out put Q = 0.
• 4. These output Q is applied to the input of Q₂ and Q₁ transistors, this will make the transistor Q₂ is turned ON and the output Q(bar) becomes high. 
• 5. Now consider the output has initial state Q = 0 and Q(bar) = 1, and the input R= 0 and S = 1. When the clock ɸ is high, the pass transistors Q₅ andQ₆ are turned ON. For this input R = 0 and S =1, the transistor Q₁ turned ON and Q₂ is turned OFF. 
• 6. This causes the output Q(bar) = 0 and Q(bar) is applied to the input of transistors Q₃ and Q₄. Now the transistor Q₄ turned ON and this make the output Q is high.  

e. Draw the block diagram of a PLL and explain its operation. Explain lock-in-range, capture range and pull-in time of a PLL.

Ans. A. Principle of operation of PLL:  

• 1. The two inputs of the phase detector or comparator are the input voltage V_s at frequency f_s and the feedback voltage from a voltage controlled oscillator (vcO) at frequency f_o.
• 2. The phase detector compares these two signals and produces a DC voltage V_e which is proportional to the phase difference between f_s and f_o.
• 3. The output voltage V_e of the phase detector is called as error voltage. This error voltage is then applied to low-pass filter.
• 4. Low-pass filter removes the high frequency noise present in the phase detector output and produces a ripple free DC level.  
• 5. This DC level is amplified to an adequate level by the amplifier and applied toa VCO. 
• 6. The DC amplifier output voltage is called as the control voltage V_C.

• 7. The control voltage V_C is applied at the input of a VCO. The output frequency of VCO is directly proportional to the DC control voltage V_C.
• 8. The VCO frequency f_o is compared with the input frequency f_s by the phase detector and it is adjusted continuously until it is equal to the input frequency f_s i.e.. f_o = f_s
• 9. Once, the action of shifting VCO frequency in a direction to reduce the frequency difference between f_s and f_o starts, we say the signal is in the capture range. 
• 10. When the output frequency is exactly the same as the input frequency, PLL is then said to be locked. 
• 11. Once locked the output frequency f_o is identical to f_s except for a finite phase difference.
• 12. This phase difference generates a control voltage V_C to shift VCOD frequency from f_o to f_s thereby maintaining the lock. Once locked, PLL tracks the frequency changes of the input signal. 

B. Lock-in range: Once locked, the PLL can track frequency variations in incoming signals. The frequency range across which the PLL can keep lock with the incoming signal is referred to as the lock-in range or tracking range.

C. Capture range: The range of frequencies over which the PLL can acquire lock with an input signal is called the capture range.

D. Pull-in time: The overall time required by the PLL to establish lock is referred to as pull-in time. The initial phase and frequency difference between the two signals, as well as the loop gain and loop filter parameters, all influence this.

E. Applications: 

• 1. Frequency divider
• 2. Frequency multiplier 
• 3. Frequency synthesizer 
• 4. AM detector 
• 5. FM detector  
• 6. FSK demodulator.

Section 3: BJT Complementary Push-Pull Output Stage

a. Describe the operation and characteristics of a BJT complementary push-pull output stage.

Ans. Fig.1 shows a class B output stage. It consists of a complementary pair of transistors (npn and pnp) connected in such a way that both cannot conduct simultaneously. 

Operation:

• 1. When the input voltage v_i is zero, both transistors are cut-off and the output voltage v_o is zero.
• 2. As v_i goes positive and exceeds about 0.5 V, Q, conducts and operates as an emitter follower. In this case v_o follows v_i (i.e., v_i = v_o – v_BEN) and Q_N supplies the load current. 
• 3. Meanwhile, the emitter-base junction of Q_P will be reverse-biased by the V_BE of Q_N, which is approximately 0.7 V. Thus Q_P will be cut-off.
• 4. If the input goes negative by more than about 0.5 V, Q_P turns ON and acts as an emitter follower.  
• 5. Again v_o follows v_i (.e., v_o = v_i + v_EBP), but in this case Q_p supplies the load current and Q_N will be cut-off.      
• 6. We conclude that the transistors in the class B stage of Fig. 2 are biased at zero current and conduct only when the input signal is present. 
• 7. The circuit operates in a push-pull fashion: 
  • i. Q_N pushes (sources) current into the load when v_i is positive, and
  • ii. Q_P pulls (sinks) current from the load when v_i is negative.  

Transfer characteristic:

• 1. In Fig. 2 there exists a range of v_i centered around zero where both transistors are cut-off and v_o is zero. This dead band results in the cross-over distortion. 
• 2. The effect of cross-over distortion will be most pronounced when the amplitude of the input signal is small. Cross-over distortion in audio power amplifiers gives rise to unpleasant sounds.

b. Give circuit description of IC741 with the help of its block diagram.  

Ans. A. Op-Amp circuit: 

• 1. Generally BJT based Op-Amp is focused on 741 Op-Amp circuit. If we use IC design, the circuits use large number of transistors but relatively few resistors and only one capacitor. 
• 2. The block diagram of typical Op-Amp is shown in Fig. 

• 3. The equivalent circuit of Op-Amp is shown in Fig.

B. Characteristics of ideal Op-Amp:  

• 1. Infinite voltage gain, A_v = ∞. 
• 2. Infinite input resistance, R_i = ∞. 
• 3. Zero output resistance, R_o = 0. 
• 4. Zero output voltage when input voltage is zero. 
• 5. Infinite bandwidth. 

C. Bias circuit: 

• 1. A bias circuit is utilized to provide proportionate current in a transistor’s collector.
• 2. Certain transistors form a current mirror, in which the transistor collector generates bias current for the Op-other Amp’s output stage.
• 3. Finally some transistors works to provide the V_BE drop between the bases of output transistors for proper functioning of the device.

Section 4: Narrow Band Reject Filter

a. Draw and explain Narrow Band Reject Filter. Also, find its transfer function. 

Ans. A. Band-reject filter:

The band-reject filter, also known as the band-elimination or band-stop filter, attenuates frequencies in the stopband and allows them to pass outside of it.

B. Types:

i. Narrow band-reject filter:

1. The narrow band-reject filter, often called the notch filter, is the twin T network cascaded with the voltage follower as shown in Fig.(a).

2. Applying Kirchhoff’s current law at node V_a, we get 

3. Applying Kirchhoff current law at node V_b we get

4. From the above three node voltage eq. (2.17.1), (2.17.2) and (2.17.3), the transfer function can be written as

5. In the steady-state, that is s = jω, 

6. At 3 dB cut-off frequency, 

8. Upon solving the above quadratic equation, we obtain the upper and lower half power frequencies as, 

9. The 3 dB bandwidth is 

ii. Wide band-reject filter: 

1. Fig.(a) shows a wide band-reject filter that is obtained by paralleling a high-pass filter with a cut-off frequency of f with a low-pass filter.

2. With cut-off frequency of f_H provided f_L > f_H and a summing amplifier connected in series to add the filtered individual passband components. 

3. The passband gains of both the high-pass and low-pass sections must be equal.

4. The frequency response characteristic of the wide band-reject filter is shown in Fig.(b). 

b. Compare and contrast active filters and passive filters. Design band pass filter with single op-amp for the given specifications : f_L – 1 KHz; f_H = 1.2 KHz, A_F =- 5. 

Ans. A. Comparison: 

B. Numerical:

1. The narrow band pass filter is shown in Fig.

Section 5: GILBERT Analog Multiplier

a. Draw the circuit diagram for monostable multivibrator with operational amplifier. Explain its operation. Derive the expression for its time period. 

Ans. A. Monostable multivibrator: 

• 1. Fig. shows the circuit diode diagram of monostable multivibrator. A D₁ clamps the capacitor voltage to 0.7 V when the output is at +V_sat.
• 2. The negative going pulse signal of magnitude V₁ (triggering signal) passing through the differentiator R₃C₁ and diode D₂ produces a negative going triggering pulse and is applied to the (+) input terminal. 

B. Operation:  

• 1. For monostable operation, the trigger pulse width T_p should be much less than T, the pulse width of the monostable multivibrator.  
• 2. The diode D₂ is used to avoid malfunctioning by blocking the positive spikes that may be present at the differentiated trigger input.
• 3. Fig. shows the trigger and output waveform. 
• 4. When V_o is +V_sat, voltage divider R₁ and R₂ feedback V_UT to the (+ve) input. The diode D₁ clamps the (-ve) input at approximately 0.7 V (because the diode is forward biased). 
• 5. The feedback voltage at (+ve) terminal is higher than (-ve) terminal therefore Op-Amp holds V_o at +V_sat This output state is called as stable state. 
• 6. If the negative spike (trigger signal) is applied to (+ ve) of Op-Amp which is higher than the voltage at (-ve) terminal. The combination o feedback voltage and negative trigger voltage will be pulled below the voltage at (-ve) input.
• 7. Once the (+ ve) input becomes negative with respect to the (- ve) input, V_o switches to -V_sat With this change, the one-shot is now in its timing state. This state is an unstable state.
• 8. Due to V_o = – V_sat, the diode D₁ is reverse biased and the capacitor C charges, the -ve) input. The (- ve) input becomes more and more negative with respect to ground. When the capacitor voltage is more than (+ ve) terminal, V_o switches to + V_sat.

C. Expression for time period : 

1. The general solution for a signal time constant low pass RC circuit with V_i and V_f as initial and final value is 

The output V_C is 

If the time constant T = RC and when t = T  

4. After simplification, the pulse widths is obtained as

b. What do you mean by the quadrant operation of multiplier? Draw and explain a GILBERT analog multiplier. 

Ans. A. Quadrant operation of multiplier:

• 1. The quadrant specifies the circuit’s suitability for bipolar signals at its inputs.
• 2. Only positive input signals are accepted by the first quadrant device, one bipolar signal and one unipolar signal by the second quadrant device, and two bipolar signals by the fourth quadrant device.

B. GILBERT analog multiplier: 

• 1. The GILBERT multiplier cell is an emitter coupled cell modification that allows for four-quadrant multiplication. As a result, it serves as the foundation for the majority of integrated circuit balanced multipliers.

• 2. The GILBERT multiplier cell is made up of two cross-coupled emitter-coupled pairs connected in series with an emitter coupled pair. 
• 3. The collector currents of Q₃ and Q₄ are given by

• 4. Similarly the collector currents of Q₅ and Q₆ are given by  

• 5. The collector currents I_C1 and I_C2 of transistors Q₁ and Q₂ can be expressed as 

• 6. Substituting eq. (3.28.5) in eq. (3.28.1) and (3.28.2), we get  

• 7. Similarly, substituting eq. (3.28.6) in eq. (3.28.3) and (3.28.4), we get 

• 8. The differential output current 𝚫l is given by

• 9. Substituting eq. (3.28.7) to (3.28.10) in eq. (3.28.11) and employing exponential formulae tor hyperbolic functions, we get 

eq. (3.28.12) shows that when V₁ and V₂ are small, the GILBERT cell shown in Fig. can be used as a four-quadrant analog multiplier with the use of current-to-voltage converters. 

Section 6: Features of CMOS Circuit

a. Explain the structure and operation of CMOS inverter. Realize the circuit of 2 input NOR gate and 2 input NAND gate using CMOS and explain the operation. 

Ans. A. CMOS inverter:

CMOS inverter circuit: 

• 1. Fig. shows the CMOS inverter. In the CMOS inverter, the PMOS and NMOS devices Q_P and Q_N are driven simultaneously by an input 

Working operation: 

1. When input is high (≃ V_DD)Q_N is made to conduct, while Q_P is forced to cut-off. This causes the output becomes low (V_o = 0) as shown in Fig. 

2. When input is low (≃ V_DD)Q_N is OFF and Q_P becomes ON, therefore the output becomes logic high (V_o = V_DD) as shown in Fig.

3. Table shows the operation of CMOS inverter circuit.

B. Realize of 2 input NOR gate and 2 input NAND gate: 

2 input NOR gate: 

Realization of 2 input NOR gate circuit is shown in Fig. 

Operation: The logic operation of NOR gate is such that the output is HIGH only when all inputs are LOW for remaining all other conditions, the output is LOW. 

2 input NAND gate: 

Realization of 2 input NAND gate circuit is shown in Fig. 

Operation: A NAND gate produces a LOW output only when all the inputs are HIGH. When any of the inputs is LOW, the output will be HIGH. 

b. Discuss the features of CMOS circuit. Describe D-F/F circuit using NAND CMOS gates.  

Ans. A. Features of CMOS circuit: 

• 1. The output is always connected to V_DD or GND and in steady state; it gives full logic swing (between 0 V and V_DD) voltage transfer characteristics and large noise margins.
• 2. Logic levels are not dependent upon the relative sizes of the devices.
• 3. There is no direct path between V_DD and GND in steady state. Thus, static power dissipation of CMOS circuit is negligible.  
• 4. It has high input impedance and fast switching speed. 

B. D-FF circuit using NAND CMOS gates:

• 1. Fig. shows the circuit realization of D flip-flop with CMOS NAND gate. This circuit consists of two stages implemented by SR NAND latches.  

• 2. The input stage (the two latches on the left) processes the clock and data inputs to guarantee that the output stage receives the correct input signals (the single latch on the right).
• 3. If the clock is low, independent of the data input, both output signals of the input stage are high; the output latch is unaffected and stores the prior state.
• 4. When the clock signal changes from low to high, only one of the output voltages (depending on the data signal) goes low and set/resets the output latch. If D =0, the lower output becomes low; if D= 1, the upper output becomes low.
• 5. If the clock signal is high, the outputs retain their states regardless of the data input, and the output latch is forced to remain in the matching state since the input logical zero remains active while the clock is high.
• 6. Hence the role of the output latch is to store the data only while the clock is low.
• 7. The D latch is normally, implemented with transmission gate (TG switches as shown in the Fig.

Section 7: Block Diagram of IC 555

a. Explain the block diagram of IC 555. Design a 555 timer as astable multivibrator with an output signal with frequency 2 KHz and 75 % duty cycle.  

Ans. A. Block diagram of IC 555: 

• 1. In the stable state, the output Q(bar) of the flip-flop (FF) is high. This makes the output low because of power amplifier which is basically an inverter.  
• 2. If negative going trigger pulse is applied to pin 2 and should have its DC level greater than the threshold level of the lower comparator (i.e.. V_CC/3), now the trigger passes through (Vcc/3), the output of the lower comparator goes high and sets the FF (Q = 1,Q(bar) = 0). Therefore the output of IC 555 becomes high. 
• 3. When the threshold voltage at pin 6 passes through (23) V_CC the output of the upper comparator goes high and resets the PF (Q = 0,Q(bar) = 1).
• 4. The reset input (pin 4) is used to reset the FP and the flip fop output Q(bar) becomes high and the output of IC 666 becomes low because the output of FF is 1.  

B. Numerical: 

2. Duty cycle is more than 50 % hence modified circuit must be used. 

3 The modified circuit is shown in Fig.

4. Let  C = 0.01 𝛍F 

The charging of C takes place through R₁ and diode D while discharging takes place through R₂ only

5. Using values of T_ON, T_OFF and C, 

R₁ = 8.658 k𝛺

and R₂ = 20.202 k𝛺

b. Describe the working of an VCO with the help of functional block diagram of VCO IC 566.  

Ans.

• 1. The pin configuration and the basic block diagram of IC 566 VCO are shown in the Fig. (a) and (b) respectively. The frequency of oscillation is determined by an externally connected resistor R₁ and a capacitor C₁. 
• 2. The control voltage or the modulating input v_c is applied at the control terminal (pin 5). 
• 3. The triangular voltage obtained at pin 4 is shown in Fig. (c). It is generated by alternately charging the capacitor C₁ by one current source, and discharging it linearly through another current source. The amount of charge and discharge voltage swing is determined by the Schmitt trigger.  
• 4. The Schmitt trigger also provides the square-wave output at pin 3 through the power amplifier A₃ and the triangular output is available at pin 4 through the buffer amplifier A₁.

Operation of VCO: 

• 1. The output voltage swing of the Schmitt trigger is set to the levels V_cc and 0.5 V_cc. In Fig. if R_a = R_b in the positive feedback path, the voltage at the non-inverting terminal of Op-Amp A₂ swings from 0.5 V_cc to 0.25 V_cc.
• 2. During charging of C₁ when the voltage across C₁ just exceeds 0.5 V_cc the Schmitt trigger switches to LOW (0.5 V_cc) and the capacitor starts discharging.
• 3. When the voltage across C₁ reduces to 0.25 V_cc the Schmitt trigger switches to HIGH (V_cc).
• 4. By maintaining the source current and sink current of the two current sources equal, a uniform triangular voltage with equal positive and negative slopes is obtained at pin 4.

• 5. The square-wave output of Schmitt trigger, inverted and buffered is available at pin 3. 
• 6. The waveforms at the output pins 3 and 4 are shown in Fig. (c).