GLOBAL INSTITUTE OF TECHNOLOGY, JAIPUR

RTU Paper Solution Dec 2019

Analog Electronics,Code: 3EX4-06

GLOBAL INSTITUTE OF TECHNOLOGY

RTU EXAMINATION (Electrical Engg. III Sem.) 2019-20 3EX4-06 Analog Electronics (EE, EX)

Solution

Max. Time: 3 Hours Max. Marks: 120

Part-A

Ans 1 PIV of Diode

The peak inverse voltage is the specified maximum voltage that a diode rectifier can block.

Ans 2 Zener Diode and VI Characteristics

It is mainly a special property of the diode rather than any special type of equipment. The special

property of the diode is that there will be a breakdown in the circuit if the voltage applied across a

reversely biased circuit. This does not allow the current to flow across it. When the voltage across

the diode is increased, temperature also increases and the crystal ions vibrate with greater

amplitude and all these leads to the breakdown of the depletion layer. The layer at the junction of ‘P’

type and ‘N’ type. When the applied voltage exceeds an specific amount Zener breakdown takes

place and diode starts conducting.

Ans 3 Multistage Amplifier

In Multi-stage amplifiers, the output of first stage is coupled to the input of next stage using a

coupling device. These coupling devices can usually be a capacitor or a transformer. This process of

joining two amplifier stages using a coupling device can be called as Cascading.

Ans 4 Barkhausen’s Criteria for Oscillation

The Barkhausen stability criterion is a mathematical condition to determine when a linear electronic

circuit will oscillate. It is widely used in the design of electronic oscillators, and also in the design of

general negative feedback circuits such as op amps, to prevent them from oscillating

Here in feedback network, A*β = -1 at ω= ωo, i.e. the magnitude of loop gain should be one and

phase of loop gain should be unity ( the feedback network introduces 180 deg phase shift, the other

180 deg phase shift is provided by mixer. This condition is called Barkhausen criterion.

Ans 5 difference between Amplifier and Oscillator.

Amplifier Oscillator

Amplifier is an electronic circuit which

gives output as amplified form of input.

Oscillator is an electronic circuit which

gives output without application of input.

The amplifier does not generate any

periodic signal.

The oscillator is generating of the

periodic electronic signal.

Amplifier uses negative feedback. Oscillator uses positive feedback.

Amplifier operates as a multiplier. Oscillator operates as a source

The Amplifier provides amplified

signal. Oscillator is gives oscillatory signal.

Amplifiers do nothing till input signal is

fed to the input.

Oscillators produce signals from the

moment of powered.

Ans 6. What is Miller effect.

The miller effect name is due to scientist John Milton miller. With the help of miller theorem, the

capacitance of the equivalent circuit of the inverting voltage amplifier can be increased by placing

extra impedance between input and output terminals of the circuit. Miller theorem states that a circuit

having an impedance (Z), connecting between two nodes where the voltage levels are V1 and V2.

When this impedance is replaced by two different impedance values and connected to the same

input & output terminals to the ground for analyzing the frequency response of the amplifier as well

as to increase the input capacitance. Such an effect is called a Miller effect. This effect occurs only

in inverting amplifiers.

Ans 7 Use of Bleeder in Zener Voltafe Regulator.

A bleeder resistor is a resistor connected in parallel with the output of a high-voltage power supply

circuit using Zener diode for the purpose of discharging the electric charge stored in the power

supply's filter capacitors when the equipment is turned off, for safety reasons.

Ans 8 Wein bridge Oscillator

Wein Bridge Oscillator is used in audio and sub-audio frequency ranges (20 – 20 kHz). This is also,

in fact, a phase-shift oscillator. It employs two transistors, each producing a phase shift of 180°, and

thus producing a total phase-shift of 360° or 0°.

Ans 9 Advantage of Stagger Tuned Amplifier.

Staggered tuning is a technique used in the design of multi-stage tuned amplifiers where each

stage is tuned to a slightly different frequency. In comparison to synchronous tuning (where each

stage is tuned identically). It produces a wider bandwidth at the expense of reduced gain. It also

produces a sharper transition from the passband to the stopband.

Ans 10 Define ‘T’ Model of Bipolar Transistor

This model represents that transistor as a voltage controlled current source with control voltage

vbe and include the input resistance looking into the emitter.

Part B

Ans 1 Half wave Rectifier and PIV of Diode

Half Wave Rectifier

A rectifier is a device that converts alternating current (AC) to direct current (DC). It is done by using a diode or a group of diodes. Half wave rectifiers use one diode, while a full wave rectifier uses multiple diodes. The working of a half wave rectifier takes advantage of the fact that diodes only allow current to flow in one direction.

Peak Inverse Voltage of Half Wave Rectifier

Peak Inverse Voltage (PIV) is the maximum voltage that the diode can withstand during reverse bias condition. If a voltage is applied more than the PIV, the diode will be destroyed.

PIV across diode is twice when Capacitor connected is due to the voltage at its terminals due to storage of the charge. Hence double voltage appears across diode at reverse biasing.

Ans 2 Input Static Characteristic PNP Transistor in CE Configuration The characteristic curve for the typical NPN transistor in CE configuration is shown in the figure below. In the active region, the collector current increases slightly as collector-emitter VCE current increases. The slope of the curve is quite more than the output characteristic of CB configuration.

Ans 3 Drain and Transfer Characteristics of an N-Channel depletion MOSFET The transfer characteristics of n-channel depletion MOSFET shown in following Figure which indicate that the device has a current flowing through it even when VGS is 0 V. This indicates that these devices conduct even when the gate terminal is left unbiased, which is further emphasized by the VGS0 curve

Ans 4 RC Coupled Amplifier When an AC input signal is applied to the base of first transistor, it gets amplified and appears at the collector load RL which is then passed through the coupling capacitor CC to the next stage. This becomes the input of the next stage, whose amplified output again appears across its collector load. Thus the signal is amplified in stage by stage action. The important point that has to be noted here is that the total gain is less than the product of the gains of individual stages. This is because when a second stage is made to follow the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. Hence, in a multistage amplifier, only the gain of the last stage remains unchanged.

As we consider a two stage amplifier here, the output phase is same as input. Because the phase reversal is done two times by the two stage CE configured amplifier circuit. Frequency response curve is a graph that indicates the relationship between voltage gain and function of frequency. The frequency response of a RC coupled amplifier is as shown in the following graph.

From the above graph, it is understood that the frequency rolls off or decreases for the frequencies below 50Hz and for the frequencies above 20 KHz. whereas the voltage gain for the range of frequencies between 50Hz and 20 KHz is constant. We know that,

G = 20 log ( Vout / Vin )

Ans 5 Effect of Negative Feedback on the bandwidth and distortion in amplifier

If 40 dB of feedback has been applied to an amplifier means that the amplifier gain has been reduced by 40 dB, (that is, by a factor of 100). Thus,

Without negative feedback, the amplifier open-loop (Av) gain falls off to its lower 3 dB frequency (f1(OL)), as illustrated. This is usually due to the impedance of bypass capacitors increasing as the frequency decreases. Similarly, the open-loop upper cutoff frequency (f2(OL)) is produced by transistor cutoff, by shunting capacitance, or by a combination of both. As discussed earlier, the circuit open-loop bandwidth is,

Now from frequency response for the same amplifier when Effects of Negative Feedback in Amplifiers is used. The closed-loop gain (ACL) is much smaller than the open-loop gain, and ACL does not begin to fall off (at high or low frequencies) until Av (open-loop) falls substantially. Consequently, f1(CL) is much lower than f1(OL), and f2(OL) is much higher than f2(OL). So, the circuit bandwidth with negative feedback (the closed-loop bandwidth) is much greater than the bandwidth without negative feedback.

It can be shown that there is a 90° phase shift associated with the open-loop gain at frequencies below f1(OL) and above f2(OL).

When Av = 1/B,

Thus, for a Effects of Negative Feedback in Amplifiers designed to have the widest possible bandwidth, the cutoff frequencies would occur when the open-loop gain falls to the equivalent of 1/B. Thus, f2(CL) occurs when,

So, for example, the cutoff frequencies for a negative feedback amplifier designed for a closed-loop gain of 100 would occur when the open-loop gain falls to 100. It is seen that, negative feedback increases amplifier bandwidth. The upper cutoff frequency for an amplifier is usually greater than 20 kHz, and the lower cutoff frequency is around 100 Hz, or lower. So, f2, f1, and consequently,

This means that the amplifier bandwidth is essentially equal to the upper cutoff frequency. The amplifier gain multiplied by the upper cutoff frequency is a constant quantity. This is known as the gain-bandwidth product. Therefore,

So, the closed-loop upper cut-off frequency for a negative feedback amplifier can be calculated from the open-loop upper cutoff frequency, the open-loop gain, and the closed-loop gain E

ffect on Distortion: Harmonic Distortion occurs when a transistor or other device is driven beyond the linear range of its characteristics. When the base-emitter voltage changes (±vbe) are very small, the collector current changes by equal positive and negative amounts (±ic). When ±vbe is large, the change in ±ic is greater than the change in -ic. This is because of the non-linearity in the IC/VBE characteristics. The result is harmonic distortion(or nonlinear distortion) in the waveform of ic and in the amplifier output. The distorted waveform can be shown to consist of a fundamental frequency waveform and a number of smaller amplitude harmonic components. The fundamental waveform is the amplified signal, or amplifier output voltage (vo), and the harmonics are unwanted voltage components (VH). The harmonic distortion is the rms value of VH expressed as a percentage of the rms value of vo. The harmonics (generated within the feedback loop) are reduced by Effects of Negative Feedback in Amplifiers by a factor of (I + AvB). So that,

So, negative feedback reduces harmonic distortion. Ans 6. Leg and Lead Compensator using OPAMP. Op amp Lag compensator A lag compensator is one which adds a pole the system and is used to improve the steady state response of the system. A lag compensator provides phase lag (negative phase) in the frequency response of system. A simple lag compensator realized using Op amp is shown below

Using voltage divider principle the transfer function of the system is Vo/Vs =(1+s*C*R1)/(1+(R1+R2)*s*C), hence the phase added to system is given as Ω = tan-1(ω*C*R1) – tan-1(ω*C*(R1+R2)) Since R1<R1+R2 and tan-1(x) is increasing function (if a <b, tan-1(a)< tan1(b)) Ω is negative (phase lag). [/ezcol_2third_end]

Op amp Lead compensator A lead compensator is one which adds a zero the system and is used to improve the transient response of the system. A lead compensator provides phase lead (positive phase) in the frequency response of system. A simple lead compensator realized using Op amps is shown below

Using the concept of virtual ground and voltage divider principle the transfer function of the system is Vo/Vs=(1+s*C*R1)/(1+(R1*R2/(R1+R2))*s*C), hence the phase added to system is given as Ω =tan-1(ω*C*R1) -tan-1(ω*C*R1*R2/(R1+R2)). Since R1 > R1*R2/(R1+R2) Ω is positive(phase lead). Ans. 7 Precision Half Wave Rectifier The modified version of half wave rectifier using opamp is shown in the figure

Consider a sine wave signal applied to half wave rectifier circuit. When Vi > 0 volts the half wave rectifier can be simplified and redrawn as follows

redrawn HWR for Vi >0 V

When Vi>0 V Then the diodes D1 and D2 are forward biased, The diode D1 shorts the output of the opamp to the inverting terminal. Using the concept of virtual ground the inverting terminal will be at ground potential (Non-inverting terminal will be at 0 v ). Hence the output of opamp will be zero volts.

When vi>0 simplified HWR When Vi<0 V The diodes D1, D2 will be reverse biased and are open. The circuit of HWR is redrawn and simplified as shown below. The combination shown in the figure resembles a inverting amplifier with gain -Rf/R1. Hence the output of opamp

V0 = -Rf/R1 (Vi) The output will be positive as Vi is negative.

HWR redrawn for Vi >0 V

Simplified HWR for Vi < 0 The characteristics can be summarized as follows Vi > 0 v ; D1, D2 ON ;Vo = 0 Vi < 0 v ; D1, D2 OFF ;Vo = –(Rf/R1)*Vi

Part C

Ans 1 Full Wave Rectifier with Capacitor Filter

The main function of full wave rectifier is to convert an AC into DC. As the name implies, this rectifier rectifies both the half cycles of the i/p AC signal, but the DC signal acquired at the o/p still have some waves. To decrease these waves at the o/p, filter is used. In the full wave rectifier circuit using a capacitor filter, the capacitor C is located across the RL load resistor. The working of this rectifier is almost the same as a half wave rectifier. The only dissimilarity is half wave rectifier has just one-half cycles (positive or negative) whereas in full wave rectifier has two cycles (positive and negative).

Once the i/p AC voltage is applied throughout the positive half cycle, then the D1 diode gets forward biased and permits flow of current while the D2 diode gets reverse biased & blocks the flow of current.

Throughout the above half cycle, the current in the D1 diode gets the filter and energizes the capacitor. But, the capacitor charging will occur just when the voltage which is applied is superior to the capacitor voltage. Firstly, the capacitor will not charge, as no voltage will stay among the capacitor plates. So when the voltage is switched on, then the capacitor will get charged immediately.

Throughout this transmission time, the capacitor gets charged to the highest value of the i/p voltage supply. The capacitor includes a highest charge at the quarter waveform in the positive half cycle. At this end, the voltage supply is equivalent to the voltage of the capacitor. Once the AC voltage begins falling & turns into less than the voltage of the capacitor, after that the capacitor begins discharging gradually.

As the i/p AC voltage supply gets the negative half-cycle, then the D1 diode gets reverse biased but the D2 diode is forward biased. Throughout the negative half cycle, the flow of current in the second diode gets the filter to charge the capacitor. But, the capacitor charging occurs simply while the applied AC voltage is superior to the voltage of the capacitor.

The capacitor in the circuit is not charged fully, so the charging of this does not occur instantly. Once the voltage supply becomes superior to the voltage of the capacitor, the capacitor gets charging. In both the half cycles, the flow of current will be in the similar direction across the RL load resistor.

Thus we acquire either whole positive half cycle otherwise negative half cycle. In this case, we can get the total positive half cycle.

Full-wave Rectifier with Capacitor Filter Outputs

In FWR PIV rating of diode 2Vm.

Ans. Hybrid Pi Model of CE Transistor Amplifier

Ans. 3. RC Phase Shift Oscillator RC phase-shift oscillator is formed by cascading three RC phase-shift networks, each offering a phase-shift of 60o, as shown below.

Here the collector resistor RC limits the collector current of the transistor, resistors R1 and R (nearest to the transistor) form the voltage divider network while the emitter resistor RE improves the stability. Next, the capacitors CE and Co are the emitter by-pass capacitor and the output DC

decoupling capacitor, respectively. Further, the circuit also shows three RC networks employed in the feedback path. This arrangement causes the output waveform to shift by 180 degree during its course of travel from output terminal to the base of the transistor. Next, this signal will be shifted again by 180 degree by the transistor in the circuit due to the fact that the phase-difference between the input and the output will be 180o in the case of common emitter configuration. This makes the net phase-difference to be 360 degree, satisfying the phase-difference condition. One more way of satisfying the phase-difference condition is to use four RC networks, each offering a phase-shift of 45o. Hence it can be concluded that the RC phase-shift oscillators can be designed in many ways as the number of RC networks in them is not fixed. However it is to be noted that, although an increase in the number of stages increases the frequency stability of the circuit, it also adversely affects the output frequency of the oscillator due to the loading effect. The generalized expression for the frequency of oscillations produced by a RC phase-shift oscillator is given by

Where, N is the number of RC stages formed by the resistors R and the capacitors C. Ans 4 Zero Crossing Detector A zero-crossing detector or ZCD is one type of voltage comparator, used to detect a sine waveform transition from positive and negative, that coincides when the i/p crosses the zero voltage condition. The applications of the Zero Crossing Detector are phase meter and time marker generator. Zero crossing detector is a voltage comparator that changes the o/p between +Vsat & –Vsat when the i/p crosses zero reference voltage. In simple words, the comparator is a basic operational amplifier used to compare two voltages simultaneously and changes the o/p according to the comparison. In the same way, we can say ZCD is a comparator.

Zero crossing detector circuit is used to produce an o/p stage switch whenever the i/p crosses the reference i/p and it is connected to the GND terminal. The o/p of the comparator can drive various outputs such as an LED indicator, a relay, and a control gate. Analog to Detector Circuit Ans 5 An Analog-Digital Converter (ADC) ADC is a widely used electronic component that converts an analog electric signal (usually a voltage) into a digital representation. The ADCs are at the front-end of any digital circuit that needs to process signals coming from the exterior world. Its schematic symbol is:

The output of a microphone, the voltage at a photodiode or the signal of an accelerometer are examples of analog values that need to be converted so that a microprocessor can work with them. Many ways have been developed to convert an analog signal, each with its strengths and weaknesses. The choice of the ADC for a given application is usually defined by the requirements : if speed, use a fast ADC; if need precision, use an accurate ADC; All ADCs work under the same principle: they need to convert a signal to a certain number of bits N. The sequence of bits represents the number and each bit has the double of the weight of the next, starting from the Most Significant Bit (MSB) up to the Least Significant Bit (LSB). In a nutshell, we want to find the sequence of bits bN−1, bN−2, ..., b0 that represents the analog value Vin as

The MSB has weight Vref/2, the next Vref/4, etc., and the LSB has weight Vref/2N. Therefore, more bits leads to more precision in the digital representation. Here simplify the range to be between 0 and Vref, although the range may be between any two values. Pipelined ADCs Pipelined converters convert the input in a number of steps proportional to the number of bits. At each step, the input signal is compared to half the reference value. If it is higher, half the reference value is subtracted to the input and the bit corresponding to that step is 1. Otherwise, it is 0. In either cases, the remaining value is doubled and passed to the next stage. Note that each stage is taking care of one bit, so a new value can be applied to the input every cycle.

The number of stages increases only with the number of bits.