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Engineered for Tomorrow

Engineered for Tomorrow. Subject Name: LINEAR INTEGRATED CIRCUITS Subject Code: 10EC46 Prepared By: M.P.PRIYADARSHINI Department: ECE Date: 2/3/2015. Engineered for Tomorrow. TABLE OF CONTENTS. OP AMP INTRODUCTION SYMBOL AND TERMINALS BLOCK DIAGRAM OF OP AMP OP AMP PARAMETERS

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Engineered for Tomorrow

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  1. Engineered for Tomorrow Subject Name: LINEAR INTEGRATED CIRCUITS Subject Code: 10EC46 Prepared By: M.P.PRIYADARSHINI Department: ECE Date: 2/3/2015

  2. Engineered for Tomorrow TABLE OF CONTENTS • OP AMP INTRODUCTION • SYMBOL AND TERMINALS • BLOCK DIAGRAM OF OP AMP • OP AMP PARAMETERS I. COMMON MODE SIGNAL II. CMRR III. INPUT OFFSET VOLTAGE IV. INPUT BIAS CURRENT VI. INPUT IMPEDANCE VII.SLEW RATE VIII.OUTPUT IMPEDANCE • NON INVERTING AMPLIFIER • INVERTING AMPLIFIER • VOLTAGE FOLLOWER • SUMMING AMPLIFIER • DIFFERENTIAL AMPLIFIER • OFFSET NULLING

  3. Engineered for Tomorrow Op-Amp – The Surface An Operational Amplifier (Op-Amp) is an integrated circuit that uses external voltage to amplify the input through a very high gain. We recognize an Op-Amp as a mass-produced component found in countless electronics. Fig (1) OPAMP IC and Pin Configuration

  4. What is an Op-Amp? – The Inside • The actual count varies, but an Op-Amp contains several Transistors, Resistors, and a few Capacitors and Diodes. • For simplicity, an Op-Amp is often depicted as follows which has mimimum of five terminals such as inverting input, non inverting input ,power supply and an output. Positive Power Supply Inverting Input Output Non-Inverting Input Negative Power Supply

  5. Basic Block Diagram of Op-Amp V1 Out put Stage (Buffer) Level Shifter Gain Stage (C E Amp.) Input Stage (Diff. Amp.) I/P VO An Op-Amp can be conveniently divided in to four main blocks An Input Stage or Input Diff. Amp. The Gain Stage The Level Translator An Out put Stage V2 Op-Amp IC

  6. Basic Block Diagram of Op-Amp(contn.,) • A Differential Amplifier i.e, it is the input stage for the op-amp. It provides the amplification of the difference voltage b/w the two inputs. • A Voltage Amplifier(s) i.e, it is the second stage of op-amp and is usually a classA amplifier that provides additional gain. • A Push-Pull Class B Amplifier i.e, it is typically used for output stage.

  7. The Ideal Op-Amp • It has infinite gain and infinite bandwidth. • It has infinite input impedance (open) so that it does not load the driving source. • It has zero output impedance. • These characteristics are illustrated in figure. The input voltage ,Vin , appears b/w the two terminals, and the output voltage is AvVin , as indicated by the internal voltage source symbol.

  8. OPAMP PARAMETERS Common Mode • Two signal voltages of the same phase, frequency and amplitude are applied to the two inputs as shown is figure 12-6. • This results in a zero output voltage (as difference is 0V). • This action is called common-mode rejection.

  9. OPAMP PARAMETERS CMRR(COMMON MODE REJECTION RATIO): • Desired signals can appear on only one input or with opposite polarities on both input lines. These desired signals are amplified and appear on the output. • Unwanted signals (noise) appearing with the same polarity on both input lines are essentially cancelled by the op-amp and don’t appear on the output. The measure of an amplifier’s ability to reject common-mode signals is a parameter called the CMRR (Common-Mode Rejection Ratio). • The CMRR is often expresses in decibels (dB) as

  10. OPAMP PARAMETERS(CONTN) • INPUT OFFSETVOLTAGE • Ideal op-amp produces zero volts out for zero volts in. • However, in op-amp a small dc voltage,VOUT(error) , appears at the output when no differential input voltage is applied. Its primary cause is a slight mismatch of the baseemitter voltages of the differential amplifier input stage of an op-amp. • Input offset voltage,VOS ,is the differential dc voltage required between the inputs to force the output to zero volts. • Typical values of input offset voltage are in the range of 2mV or less. It is 0V in ideal case. • Input offset voltage drift is a parameter related to,VOS ,that specifies how much change occurs in the input offset voltage for each degree change in temperature.

  11. OPAMP PARAMETERS(CONTN) • INPUT BIAS CURRENT • Input terminals of a differential amplifier are the transistor bases and therefore , the input currents are the base currents as shown in the figure.

  12. OPAMP PARAMETERS(CONTN) INPUT IMPEDANCE • The Differential input impedance is the total resistance between the inverting and non-inverting terminals as illustrated in figure (a) next slide. Differential impedance is measured by determining the change in bias current for a given change in differential input voltage. • The common-mode input impedance is the resistance between each input and ground and is measured by determining the change in bias current for a given change in common-mode input voltage shown in figure next slide .

  13. OPAMP PARAMETERS(CONTN) INPUT OFFSET CURRENT • Ideally, the two input bias currents are equal and thus their difference is zero. • However, in a practical op-amp, the bias currents are not exactly equal. • The input offset current , IOS is the difference of the input bias currents expressed as an absolute value.

  14. OPAMP PARAMETERS(CONTN) INPUT OFFSET VOLTAGE

  15. OPAMP PARAMETERS(CONTN) • OUTPUT IMPEDANCE • The output impedance is the resistance viewed from the output terminal of the op-amp as shown in figure below

  16. OPAMP PARAMETERS(CONTN) • Slew Rate • The slew rate (SR) of an op-amp is the maximum rate at which the output voltage can change in response to input voltage. • When the SR is too slow for the input, distortion results. • For example when a input sine wave is applied to a voltage follower it produces a triangular output waveform. • The triangular waveform results because the op-amp simply cannot move fast enough to follow the sine wave input. • This happens because voltage change in the second stage ( Voltage Amplifier(s) ) is limited by the charging and discharging of capacitors. • The slew rate is expressed as : SR = ∆VO/∆t .The unit of SR is volts per microseconds.

  17. Non inverting amplifier configuration • An op-amp in a closed-loop configuration as a non-inverting amplifier with a controlled amount of voltage gain is shown in figure 12-16 next slide. • The input signal is applied to the non-inverting (+) input. • The output is applied back to the inverting(-) input through the feedback circuit (closed loop) formed by the input resistor Ri and the feedback resistor Rf. • Resistors Ri and Rf form a voltage-divider circuit which reduces Vout and connects the reduced voltage Vf to the inverting input. • The feedback voltage is expressed as

  18. VOLTAGE FOLLOWER CONFIGURATION • Voltage Follower • It is a special case of the non-inverting amplifier where all of the output voltage is fed back to the inverting (-) input by a straight connection as shown in figure 12-19. • The straight feedback connection has a voltage gain of 1. • The closed-loop voltage gain of a non-inverting amplifier is 1/B. • Since B= 1 for a voltage follower case, the closed-loop voltage gain of the voltage follower is Acl(VF) = 1/1= 1

  19. INVERTING AMPLIFIER CONFIGURATION • InvertingAmplifier • An op-amp connected as an inverting amplifier with a controlled amount of voltage gain as shown in figure. • The input signal is applied through a series input resistor Ri to the inverting (-) input. • The output is fed back through Rf to the same input. The noninverting input is grounded. • At this point, the ideal op-amp parameters mentioned earlier are useful in simplifying the analysis of this circuit. • Particularly the concept of infinite input impedance is of great value. • An infinite input impedance implies zero current at the inverting input. • If there is zero current through the input impedance, then there must be no voltage drop between the inverting and noninverting terminals. • This means that the voltage at the inverting (-) input is zero. This zero voltage at the inverting input terminal is referred to as virtual ground.

  20. INVERTING AMPLIFIER CONFIGURATION • Since there is no current at the inverting input, the current through Riand the current through Rfare equal as shown in figure i.e, Iin= If. • Also the voltage across Rf equals -Vout because of virtual ground and therefore IF = -VOUT/RF since If = IIN+ -VOUT/RF = VIN/Ri Rearranging the terms -VOUT/VIN = RF/Ri Therefore the overall gain is given by ACL= -RF/Ri. The –ve sign indicates out of phase relation between the input and output. =

  21. Op-Amp Summing Amplifier • The summing amplifier is a circuit in which the ouput is the summation of all the inputs amplified by the gain. • If equal value resistors are used output equals the input. • If input resistor equals twice the feedback resistor then the circuit can be used as a average circuit. • It can be configured either to be a inverting summer /non invertring summer circuit. • The output is given as VOUT= -RF/RIN (V1+V2+…VN)

  22. Differential Amplifier Using Op Amp The differential amplifier is a circuit in which the output is the difference of the inputs. Its also called as subtractor circuit.From the figure since the inverting terminal is at virtual ground therefore

  23. OFFSET NULLING • The op amp offset null capability is one that is available on many op-amp chips. • The offset null capability is used to reduce small DC offsets that can be amplified. These can be important in DC amplifiers where these small voltages can then become significant where large gains are required. • Op amp input offsets : • An op amp is a differential amplifier. This means that when there is no difference between the two inputs, e.g. when the inputs are shorted together, there should be no voltage on the output. • Unfortunately under these circumstances there is always a small offset because no op amp is never perfect and completely balanced. There is always a small input offset voltage. • This input offset voltage is small and arises from mismatches in the differential input stage of the op amp chip. These small offsets are caused by a variety of unavoidable issues within the manufacture of the op amp. They include aspects including mismatched transistor pairs, collector currents, current-gain betas (β), collector or emitter resistors, etc..

  24. OFFSET NULLING(CONTN.,) • The op amp offset null connections enable the input circuit balance to be obtained by applying external circuitry. • For circuits where it is necessary to remove or null the offset, many op-amp chips provide two pins that enable this to be done. Using the offset null adjustment requires a potentiometer with its wiper connected to the negative supply with some op amps or to 0 V with others so it is necessary to check the data sheet. The value for the potentiometer may typically be around 10 KΩ to 100 KΩ but again check the data sheet for the most suitable value. op amp offset null adjustment

  25. OFFSET NULLING (CONTN.,) • On op amps with an offset null capability two pins are provided as shown in the diagram. • The offset null capability for op amps is often used in instrumentation applications. For example where the small DC voltages produced by thermocouples or other sensors need to be amplified. The offset null may be used in other applications where DC amplification is required and the offsets need to be removed. • Although the most common method is to use a preset potentiometer, many instruments may use self-calibration where the DC offset is measured under zero input conditions and then a digital value used to counter the offset that is measured. This could be achieved by applying the required voltage to the offset null, applying an equal and opposite DC offset, or applying the measured offset in any digital processing that may be undertaken.

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