Operational Amplifiers - Concordia College

1 1 Operational AmplifiersA Linear IC circuitOperational Amplifier (op-amp) An op-amp is a high-gain amplifier that has highinput impedance and low output impedance. An ideal op-amp has infinite gain and inputimpedance and zero output impedance. An integrated circuit (IC) contains a number ofcomponents on a single piece of semiconductor. Most op-amps are IC 741 Operational AmplifierOp-Amp Input/Output We consider the op-amp as a singlecomponent with inputand outputcharacteristics. Two signal inputs: Inverting Non-inverting Two dc power supplyleads (+ and ) One output lead3Op-Amp PackagesThe Operation of Op-amps The input stage of an op-amp is adifferential amplifier.

2 The op-amp amplifies the differencebetween the two input terminal voltages. +V1V2 Vdiff =V2 V14Op-Amp Output The output of the amplifier is determined by The gain of the amplifier. The polarity relationship between V1 and V2. The values of the supply voltages, +V and -V. The load resistanceOp-Amp Gain The maximum possible gain of an op-ampis called the open-loop gain AOL. Generally AOL is greater than 10,000. Typical values are on the order of 200,000. An ideal op-amp would have infinite Polarity The output polarity follows the sign of Vdiff.

3 If V2 V1 > 0 the output polarity will be positive. If V2 V1 < 0 the output polarity will be negative. +V1V2 Supply Voltages The supply voltages determine the limits of outputvoltage swing. No matter what the gain and inputvoltages the output value can not exceed +V or V. In practice the maximum output voltage is slightlyless than the supply voltages. For resistive loads > 10k the output voltages areabout 1V less than the supply voltages. For resistive loads > 2k the output voltages are about2V less than the supply Loop Op-amp Use As the open loop gain of most op-amps isextremely large the output of an open-loop circuitis either the maximum positive or negativevoltage.

4 +V1V2+15 V-15 V1212V14V14 VVVVVout<>!"#$+=Feedback Circuits7 Feedback Most op-amp circuits are designed to usefeedback. Feedback is defined as taking a portion of theoutput of a circuit and coupling or feeding it backinto the input. If the output fed back is in phase with the inputthen the circuit has positive feedback. If the output fed back is out of phase with theinput then the circuit has negative Feedback Most Amplifiers use negative feedback. Disadvantages: decreased gain. Advantages: increased circuit stability, increased input impedance, decreased output impedance, increased frequency bandwidth at Voltage Feedback A fraction B < 1 of the output voltage is subtracted from theinput vinvoutv outinBvvv!

5 ="Negative Voltage Feedback The closed loop gain, Av, is defined as The closed loop gain can be calculated from two equations-BAOL vinvoutv 9 Negative Voltage Feedback Solving for Av gives Usually the open-loop gain is so large that we can approximate:-BAOL vinvoutv Negative Feedback The gain of the amplifier circuitdepends only on B, the fractionof output voltage fed back. B can be made very constant sothat the amplifier has great gainstabilization. Example: B could be determinedby two resistors in a voltagedivider voutBvout10 Negative Feedback Impedance The input and output impedance is alsochanged by the Circuits With NegativeFeedback11 Non-Inverting Amplifier Using Kirchoff s rule, Ohm s Law, and our knowledge ofop-amps we can derive a closed loop-voltage gain for thenon-inverting amplifier circuit shown i2v1v2 Non-Inverting Amplifier As the input resistance of the op-amp is very large we canneglect i.

6 The output voltage is given by the voltage difference andthe open-loop i2v1v212 Non-Inverting Amplifier Combining the previous equations we find: If the open-loop gain is very large:vinvoutR2R1i1i i2v1v2! Av=voutvin=AOL(R1+R2)(AOL+1)R1+R2 Inverting Amplifier Using Kirchoff s rule, Ohm s Law, and our knowledge ofop-amps we can derive a closed loop-voltage gain for theinverting amplifier circuit shown belowvinvoutR2R1i1i i2v1v213 Inverting Amplifier The output voltage is related to the voltage difference.

7 Neglecting i and combining the equations givesvinvoutR2R1i1i i2v1v2 Inverting Amplifier For a very large open-loop gainbecomesvinvoutR2R1i1i i2v1v214 The Two Golden Rules ofOp-Amp Circuits Notice in both derivations two approximations were made: (1)the input current i flowing into the op-amp was neglectedcompared to other currents; and (2) the open-loop op amp gainAOL was assumed to be very large compared to the gain withfeedback. These two approximations can be extended to form two goldenrules for analyzing an op-amp circuits with negative feedback.

8 Op-Amp Current Rule (OACR): The current into or out of eachop-amp input terminal is approximately zero. Op-Amp Voltage Rule (OAVR): The voltage difference betweenthe two op-amp input terminals is approximately Current Rule The OACR basically says that the inputimpedance of the op-amp is much higherthan the external input impedance from theinput terminal to ground. For BJT op-amps input impedance is on theorder of 10M . For FET op-amps input impedance is on theorder of 1012 .15Op-Amp Voltage Rule The OAVR is the equivalent of saying that the open-loopgain is infinite.

9 The output of the op-amp can never be greater than thesupply voltage (~15V) which means that (v2-v1) must beless that 150 V for a typical AOL or the output will besaturated. Therefore if the op-amp is not saturated then thedifference between the input terminals must be nearly zero. The rule says that in an actual op amp circuit the negativefeedback plus the high gain of the op-amp effectively zerosthe difference between the two Amp OACR: i1 = i2 OAVR: v1 = v2 = vinvinvoutR2R1i1i2v1v216 Inverting Amp OACR: i1 = i2 OAVR.

10 V1 = v2 = 0vinvoutR2R1i1i2v1v2 Instrumentation Amplifier17 Peak Detection AmplifierPositive Feedback18 Positive Feedback Circuits Rather than placing a portion of the outputback into the inverting input a portion of theoutput is sent back to the non-invertingterminal to produce positive Feedback Circuits Oscillators19 Positive Feedback Circuits Oscillators