UNIT- IV ELECTRONICS

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Cassandra Potter
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1 UNIT- IV ELECTRONICS INTRODUCTION An operational amplifier or OP-AMP is a DC-coupled voltage amplifier with a very high voltage gain. Op-amp is basically a multistage amplifier in which a number of amplifier stages are interconnected to each other in a very complicated manner. Its internal circuit consists of many transistors, FETs and resistors. All this occupies a very little space. So, it is packed in a small package and is available in the Integrated Circuit (IC) form. The term Op-Amp is used to denote an amplifier which can be configured to perform various operations like amplification, subtraction, differentiation, addition, integration etc. OP-AMP An op-amp has two input terminals and one output terminal. The op-amp also has two voltage supply terminal. It has a differential input and a single ended output. The terminal marked as negative (-) is called as an inverting terminal And the terminal marked as positive (+) is called as a non-inverting terminal of the operational amplifier. If we connect an input signal at the inverting terminal (-) of the op-amp than the amplified output signal is π radians (180 ) out of phase with respect to the applied input signal. If an input is connected to the non-inverting terminal (+) than the output signal obtained will be in phase i.e. it will have no phase shift with respect to the input signal.

2 BLOCK DIAGRAM OF OP-AMP Block Diagram This contains four stages, INPUT STAGE The first stage of an op-amp is almost a differential amplifier and the last stage is usually a class-b Push- Pull emitter follower. The input stage should have the following characteristics (a) High input resistance (typically 10Mohm) (b) Low input bias current (typically 0.5microamps) (c) Small input offset voltage (typically 10mV) (d) High CMRR (typically 70db) (e) High open- loop voltage gain (typically10 4 ) INTERMEDIATE STAGE In most of the amplifier, an intermediate stage (dual input, unbalanced output differential amplifier) is provided which increases the overall gain of the op-amp. Because of direct coupling between the first two stage, the dc level at the output of the intermediate stage, is well above the ground potential. This require a level translator as the successing stage in order to bring the DC level back to the ground potential.

3 LEVEL SHIFTING STAGE The level shifter (translator) circuit is used after the intermediate stage to shift the DC level at the output of the intermediate stage downward to zero volts with respect to ground. Usually, the third stage is an emitter follower using constant current source. It step up the input impedance of the stage consisting of Q 6 by a factor of T A. Note that Q 6 is the drives for the output stage. Incidentally, the plus sign 0. The Q 5 collector means it is connected to the V cc supply. Similarly, the minus sign at the bottom of R 2 and R 3 means these are connected to the V EF supply. OUTPUT STAGE The last stage is a complementary Push amplifier (Q 9 and Q 10). Q 11 is a part of current mirror sources, current through the compensating diodes (Q 7 and Q 8). Q n is the input volt of the mirror and biasing resistor R 3 sets up the desired mirror current. CC is called the compensating capacitor (typically 30pico Farad), has a prounced on the frequency response. It is needed to prevent oscillations and wanted signals within the amplifier. The output stage should have the following desirable properties i. Large output voltage shift capability ii. Large output current shift capability iii. Low output resistance iv. Short circuit protaloic An emitter follower as the output stage can provide low output resistance and class B and class AB stage can provide large amount of output power.

4 ADVANTAGES AND DISADVANTAGES OF OP- AMP `Advantages of Op-Amp Good thermal stability Low offset voltage Low offset current High reliability Disadvantages of Op-Amp It is difficult to realize the large values of resistance and capacitance. No methods to fabricate transformer using linear IC s. Applications The integrated op-amp s offers all the advantages of IC s such as high reliability, small size, cheap, less power consumption. They are used in variety of applications such as Inverting & Non-inverting amplifiers, Unity gain buffer, Summing amplifier, Differentiator, Integrator, Adder, Instrumentation amplifier, Wien bridge oscillator, Filters etc. Equivalent Circuit of Op-Amp: Equivalent Circuit IDEAL OP-AMP CHARACTERISTICS 1. Infinite voltage gain (So that maximum output is obtained) 2.Infinite input resistance (Due to this almost any source can drive it) 3.Zero output resistance (So that there is no change in output due to change in load current) 4.Infinite bandwidth 5.Zero noise

5 6.Zero power supply rejection ratio (PSSR=0) 7.Infinite common mode rejection ratio (CMMR= ) CHARACTERISTICS OF OP- AMP: Characteristics of op-amp is classified into two types, They are i. Ac characteristics ii. Dc characteristics AC Characteristics: AC characteristics can be further classified into two forms Slew rate Frequency response. SLEW RATE: It is the maximum rate of change of output voltage passed by a stepped input voltage. It is denoted by SR. The value is 0.5v/us. FREQUENCY RESPONSE Closed Loop Gain: 0In closed loop gain there will be some feedback. Feedback is nothing but the elements like capacitors or resistors is connected across the input and output terminals. Open Loop Gain: In open loop gain, there will not be any feed back. For 741c in open loop gain the value is infinity.. DC Characteristics: DC characteristics can be further classified into many forms. Input bias current Input offset current Input offset voltage Input resistance Input capacitance Output offset voltage Thermal drift Thermal drift is further classified into two types

6 i. Input offset current drift ii. Input offset voltage drift INPUT BIAS CURRENT: The average of the currents entering in to the (-)input and(+)input terminals of an op-amp is called as an input bias current. Its value is 500nA for IC741. INPUT OFFSET CURRENT: The algebraic difference between the currents in the (-) input and (+) is referred to as input offset current. It is 200nA for IC741. input INPUT OFFSET VOLTAGE: It is the voltage difference that must be applied between the input terminals of an op-amp. Since the voltage could be positive or negative. For IC741, the maximum value is 6mv. INPUT RESISTANCE: This is the differential input resistance of input terminals with the other terminal connected to the ground. For the IC741, the input resistance is 2Mohms. INPUT CAPACITANCE: It is the equivalent capacitance that can be measured at either of the input terminal with the other terminal connected to the ground. A typical value of Ci is 1.4pF. INPUT VOLTAGE: This is the common mode voltage that can be applied to both input terminals without disturbing the performance of an op-amp. For the 741c, their range is(+ or -)13V. COMMON MODE REJECTION RATIOCMRR) It is the ability of op-amp to reject the signal common to both the inputs. Generally, it is high. For 741c, the value is 90db. SUPPLY VOLTAGE REJECTION RATIO: The change in an op amp s input offset voltage due to variations in supply voltage is called supply voltage rejection ratio. For 741c, the value is 150uV\V. It is also called as power supply rejection ratio.

7 OUTPUT OFFSET VOLTAGE: It is the voltage presented at the output terminal when both the input terminals are grounded. For 741c, the value is (+ or -) 15mV. INPUT OFFSET CURRENT DRIFT: It is the rate of change of input offset current with respect to the temperature of balanced op amp. The value is 0.2nA/degree celcius. INPUT OFFSET VOLTAGE DRIFT: The rate of change of input offset voltage with respect to the temperature of balanced op amp. Its value is 0.15mV/degree celcius. INPUT COMMON MODE RANGE: It is the maximum range of signal that can be applied as a common mode. For 741c, the value is (+ or -)13v. INPUT DIFFERENTIAL RANGE: It is the maximum differential signal that can be applied to the input terminals of the op amp without affecting the normal operations. COMMON MODE GAIN: It is the ratio of common mode output voltage to common mode input voltage. The value is less than 1v. It is denoted by Avcm. DIFFERENTIAL MODE GAIN: It is the ratio of differential mode output voltage to the differential mode input voltage. It is denoted as Avdm. OUTPUT RESISTANCE: It is the resistance measured at the output terminal of op amp with respect to ground for 741c, the value is 75 ohms. POWER CONSUMPTION: It is the quiescent power consumed by the op amp for its operation. For 741c, the value is 85mv.

8 OUTPUT SHORT CIRCUIT CURRENT: This is the current that may flow if an op amp get shorted accidentally. For 741c, the value is 85mv. FULL POWER BAND WIDTH: It is the maximum frequency up to which the output voltage range can obtain without distortion. INVERTING AMPLIFIER An inverting amplifier not only amplifies the input signal but also produce a phase shift in voltage between the input and the output. + _ v IN R 2 v OUT 2 R vin 1 Inverting Amplifier The op amp circuit consists of a resistor R1 and a feedback resistor Rf. R1 is connected between the input and the inverting terminal of the op amp. The Rf is connected between the input inverting(2) terminal and the output(6) of the op amp. The non inverting terminal (3) is grounded. The input and the output of the inverting amplifier are out of phase with each other. Since the input impedance of the op-amp is large, current cannot enter into the op-amp. So output current is same as the input current i.e. I1 = I0. The input is given to the inverting terminal and the non-inverting terminal is virtually grounded therefore the node voltage is zero. So voltage developed across Rf is equal to the output voltage Vn of the circuit. W.K.T,

9 V i = I 1 I 1 = V i/ V o = I o * R f V o = -I 1* R f (I o = -I 1) Here the ve sign indicates that the input and the output are in the opposite direction V o = -V i * R f / NON-INVERTING AMPLIFIER: Av = -R f / A non-inverting amplifier amplifies the signal and the output is same as that of the input _ R 2 v IN + v OUT v IN R R 2 1 Non Inverting Amplifier In the non-inverting amplifier the input is applied to the non-inverting terminal(3) and the resistor R1 is grounded. The voltage at the inverting terinal (Vi) must be same as that at the noninverting terminal. The input impedance of the op-amp is very high so that the entire voltage given is obtained at the node a. If I 1 is the current through the resistor, V i = I 1. Since the voltage drop across R1 is equal ot the difference between Vi and Vo, I 0 = V 0 V i R f I o* R f = V o-v i

10 V o = V i +I o.* R f We know that, I 1 = I o, V o = V i + I 1.R f From (1) & (2)= A V = V 0 V i VOLTAGE FOLLOWER A V = V i + I 1 R f I 1 = I 1 + I 1 R f I 1 = + R f Voltage follower circuit is obtained from the non-inverting amplifier by removing the resistors R1 and Rf. Voltage follower circuit as unity gain. The input voltage is applied at the non-inverting terminal of the op-amp, and the inverting input is connected to the op-amp output. Because of the direct connection of the inverting input and the output terminals a 100% voltage series feedback is applied, i.e. R1 = Rf =0. From the voltage gain for the non inverting amplifier. A V = V 0 V i Av = 1+(R f / ) Av = 1 Thus the voltage follower has unity gain. It is called as source follower in FET. DIFFERENTIATOR The circuit performs the mathematical operation of differentiation. i.e. the output wave form is the derivative of the input waveform. The differentiator provides a constant output above a cut-off frequency and passes no signal below this frequency. So the differentiator is also called as high pass filter (HPF). The non inverting terminal (3) is grounded. Therefore node a voltage is zero. i.e. Va = 0

11 Differentiator (i.e.) V o dvi At node a, C f dv i dt + V 0 R f = 0 The node a is virtually grounded. Therefore Vn = Va = 0 V 0 dv i = C f dt Here the ve sign is introduced because the input is given to the inverting terminal V o dv i / dt V o = -C 1R f(jωvi) V 0 = -R fc 1jωV i A v = V 0 V i = R f C 1 jωv i A v = V 0 Vi = R f C 1 ω Av = ωr fc 1 A v = Vo/V i = f/f a [where f a = 1/(2ПR fc 1)]

12 Input to Differentiator Output of Differentiator INTEGRATOR The integrator provides a constant output below a cut-off frequency and passes no signal above this frequency. So the integrator is also called as low pass filter (LPF). If we inter change the resistor and the capacitor of the differentiator we get the circuit of an integrator.

13 Integrator (i.e) Vo Vi At node a V i + C f dv 0 dt = 0 C f dv 0 dt = V i dv 0 dt = V i C f dv 0 = V idt C f Integrating on both sides dv 0 = V i dt C f dv 0 t = 1 C f V i (t) A v = V 0(jω) V i (jω) = 1 jω C f A v = 1 ω C f

14 Input to Integrator INSTRUMENTATION AMPLIFIER Output of Integrator In many industrial and consumer applications the measurement and control of physical conditions are very important. (for eg) measurement of temperature & humidity inside a dairy or a meat plant permit the operators make necessary adjustments to maintain product quality. Similarly precise temperature control of plastic furnace is needed to produce a particular type of plastic. Generally, a transducer is used at the measuring site to obtain the required information easily & safely. The transducer is a device that converts one form of energy into another. A resistive transducer whose resistance changes as a function of some physical energy is connected in one arm of bridge with a small circle around it & is denoted by (R ± R), where R T is the resistance of transducer and R the change in resistance of RT. The bridge in the circuit is dc exited but could be ac exited as well. For the balanced bridge at some reference condition,

15 Instrument Amplifier V b = V a R B (V dc ) = R A(V dc ) R B + R c R A + R T R c R B = R T R A Generally resistors RA, RB, RC are selected so that they are equal in value to the transducer resistance RT at some reference condition the reference condition is the specific value of the physical quantity under measurement at which the bridge is balanced this value is normally established by the designer and depends on the transducer s characteristics, the type of physical quantity to be measured, and the desired application. The bridge is balanced initially at a desired reference condition. However, as the physical quantity to be measured changes, the resistance of the transducer also changes, which causes the bridge to unbalance (Va Vb). The output voltage of the bridge can be expressed as a function of the change in resistance of the transducer. Let the change in resistance of the transducer be R. Since Rb & Rc Are fixed resistors, the voltage Vb is constant. However, the voltage Va varies as a function of the change in transducer resistance. Therefore, according to the voltage divider rule,

16 R A (V dc ) V a = R A + (R T + R) V a = R BA(V dc ) (R B + R c ) Consequently, the voltage Vab across the output terminals of the bridge is, V ab = V ab = V a V b However, if R A = R B = R C = R T = R, then R A V dc R A + R T + R R BV dc R B + R c V ab = R(V dc ) 2(2R + R) the (-)ve sign indicates that V a < V b(since RT increases) The basic gain differential amplifier is (-R F/) V 0 = V ab R f = R(V dc ) 2(2R + R) Generally, R is very small. Therefore we can approximate (2R + R) = 2R. V 0 = R f R 4R V dc This equation indicates that V is directly proportional to the change in resistance R of the transducer since the change in resistance is caused by a change in physical energy, a meter connected at the output can be calibrated in terms of the units of that physical energy. Features of Instrumentation Amplifier R f High gain High CMRR Low Power consumption High slew rate Applications Of Instrumentation Amplifier as temperature indicator as light-intensity meter as thermal conductivity meter in analog weight scale

17 SUMMER (or) ADDER A circuit whose output is the sum of all the inputs given is called summer or summing amplifier. There are two types of summer (i) inverting summer & (ii) non-inverting summer. Inverting Summer A summer amplifier with two input voltages V1 and V2, two input resistors R1 and R2, feedback resistor Rf is shown in the fig. below _ R 3 v 1 v 2 R 2 + v OUT R3 v1 R1 v 2 R 3 R2 Inverting Summing The voltage at the node a is zero as the non inverting input terminal is virtually grounded. The nodal equation by KCL at node a is V 1 + V 2 R 2 + V 0 R 3 = 0 V 0 = R f R1 V 1 + V 2 V 0 (V 1 + V 2 ) Non Inverting Summer Non Inverting Summing

18 For non inverting, V 0 = 1 + R f R V in V in = V 1 R1 + V 2 R2 + V 3 R3 1 R1 + 1 R R 3 V 0 = 1 + R f R V 1 R1 + V 2 R2 + V 3 R3 1 R1 + 1 R R 3 If = R 2 =R 3 = R V 0 = 1 + R f R V 1 + V 2 + V 3 3 DIFFERENTIAL AMPLIFIER (or) SUBTRACTOR A circuit that amplifier the difference between the two signals. _ R 2 v 1 v 2 R 3 R 4 + v OUT v R R3 R 4 R 2 R2 v1 R 1 R1 Subtractor The differential amplifier is very useful in instrumentation circuits. The voltage V1 and V2 are applied at op-amp input terminals. The different voltage at the input terminal of the op-amp is zero. Node a and b are at the same potential designated as V3. Consider node a, V 3 V 2 + V 3 V 0 R 2 = 0 Consider node b V 3 V 1 + V 3 R 2 = 0

19 Rewriting eqn (1) & (2) Subtracting equating (3) from (4) R 2 V 3 V 2 = V 0 R R 2 V 3 V 1 = 0 V 2 + V 1 = V 0 R 2 V 1 V 2 = V 0 R 2 FILTERS Types of Filters V 0 = R 2 V 1 V 2 (a) (b) (a) Low Pass Filter (b) High Pass Filter (a) (b) (a) Band Pass Filter (b) Band Stop Filter

20 Low Pass Filter First Order Low Pass Butterworth Filter RC filter is connecting non inverting input V 2 = 1 jωc R jωc V i ω = 2πf V 2 = jωr 3 C V i General non inverting output V 0 = 1 + R f R V in We can write, V 0 = 1 + R f R jωr 3 C V i V 0 = 1 + R f V i R jωr 3 C V 0 = 1 + R f V i R j2πfr 3 C

21 V 0 V i = A f 1 + j f f C A f = 1 + R f R f- Frequency of input signal f C- Cut of frequency of filter f C = 1 2πR 3 C High Pass Filter High Pass Filter V 2 = R 3 R jωc V 2 = jωr 3C 1 + jωr 3 C V i For non inverting output V 0 = 1 + R 2 V 2

22 V 0 V i = 1 + R 2 V 0 V i = 1 + R 2 jωr 3 C 1 + jωr 3 C j2πfr 3 C 1 + j2πfr 3 C V 0 V i = A f j f f C 1 + j f f C Where A f = R f C = 1 2πR 3 C VOLTAGE TO CURRENT CONVERTER [TRANSCONDUCTANCE AMPERE] In many application it is necessary to convert voltage signal to a proportional output current. V- I converter with floating load V- I converter with grounded load Since the voltage at node a is V i V-I Converter With Load Z L V i = i L i L = V i (i.e.) the input voltage V i is converted into an output current of V i

23 NOTE: Same current flows through signal source and load V-I Converter With Grounded Load Z L Applying KCL, i 1 + i 2 = i L V i V 1 R + V 0 V 1 R = i L V i + V 0 2V 1 = Ri L V 1 = Ri L V 0 V i 2 V 1 = Ri L + V 0 + V i 2 Since op-amp is used in non-inverting mode Gain = 1 + R R Gain = 2 Output Voltage, V 0 = 2V 1 V i = I L I L = V i Input impedance of non-inverting amplifier is very high. Advantages It is used in low voltage DC and AC voltmeter

24 It is used in led and Zener diode CURRENT TO VOLTAGE CONVERTER [TRANSRESISTANCE AMPLIFIER] Photocell, photodiode, photovoltaic cell give an output current, (i.e.) proportional to an incident radiant energy or light. The circuit through this devices can be converted to voltage by using a current voltage converter and there by the amount of light or radiant energy incident on the photo device can be measured. -ve terminal is at virtual ground, no current flows through R s and current Is flows through the feedback resistor R f, thus the output voltage V 0 = i s R f I-V Converter Resistor R f is sometime shunted with a capacitor C f to reduce high frequency noise and the possibility of oscillation.

EE LINEAR INTEGRATED CIRCUITS & APPLICATIONS

EE LINEAR INTEGRATED CIRCUITS & APPLICATIONS UNITII CHARACTERISTICS OF OPAMP 1. What is an opamp? List its functions. The opamp is a multi terminal device, which internally is quite complex. It is a direct coupled high gain amplifier consisting of