NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT # 1 OPERATIONAL AMPLIFIERPagara, Sheila Marie P. July 26, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
OPERATIONAL AMPLIFIERSOperational amplifiers are linear devices that have all the properties required for nearly idealDC amplification and are therefore used extensively in signal conditioning, filtering or toperform mathematical operations such as add, subtract, integration and differentiationOp-amp Idealized Characteristics Infinite Open Loop Gain, (Avo) Infinite Input impedance, (Zin) Zero Output impedance, (Zout) Infinite Bandwidth, (BW) Zero Offset Voltage, (Vio) From these "idealized" characteristics above, we can see that the input resistance isinfinite, so no current flows into either input terminal (the "current rule") and thatthe differential input offset voltage is zero (the "voltage rule").
Open-loop Frequency Response Curve of Op-amps An Operational Amplifiers Bandwidth The operational amplifiers bandwidth is the frequency range over which the voltagegain of the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximumoutput value as shown below.
THE INVERTING AMPLIFIER The Closed-Loop Voltage Gain of an Inverting Amplifier is given as. and this can be transposed to give Vout as: The negative sign in the equation indicates an inversion of the output signal withrespect to the input as it is 180o out of phase. This is due to the feedback being negative invalue.
Transresistance Amplifier Circuit . A Transresistance Amplifier also known as a "transimpedance amplifier", is basically acurrent-to-voltage converter They can be used in low-power applications to convert a verysmall current generated by a photo-diode or photo-detecting device etc, into a usable outputvoltage which is proportional to the input current as shown. The simple light-activated circuit above, converts a current generated by the photo-diode into a voltage. The feedback resistor Rf sets the operating voltage point at the invertinginput and controls the amount of output. The output voltage is given as Vout = Is x Rf.Therefore, the output voltage is proportional to the amount of input current generated by thephoto-diode.
THE NON-INVERTING AMPLIFIER In the Inverting Amplifier , "no current flows into the input" of the amplifier and that"V1 equals V2". This was because the junction of the input and feedback signal (V1) are at thesame potential in other words the junction is a "virtual earth" summing point. Because of thisvirtual earth node the resistors, Rf and R2 form a simple potential divider network across thenon-inverting amplifier with the voltage gain of the circuit being determined by the ratiosof R2 and Rf.Non-inverting Amplifier Configuration The closed loop voltage gain of a Non-inverting Amplifier is given as: We can see from the equation above, that the overall closed-loop gain of a non-inverting amplifier will always be greater but never less than one (unity), it is positive innature and is determined by the ratio of the values of Rf and R2. If the value of the feedbackresistor Rf is zero, the gain of the amplifier will be exactly equal to one (unity). If resistor R2 iszero the gain will approach infinity, but in practice it will be limited to the operationalamplifiers open-loop differential gain, (Ao).
Voltage Follower (Unity Gain Buffer) In this non-inverting circuit configuration, the input impedance Rin has increased toinfinity and the feedback impedance Rf reduced to zero. The output is connected directly backto the negative inverting input so the feedback is 100% and Vin is exactly equal to Vout givingit a fixed gain of 1 or unity. As the input voltage Vin is applied to the non-inverting input thegain of the amplifier is given as:
THE SUMMING AMPLIFIERSumming Amplifier Configuration Circuit Summing Amplifier Equation A Scaling Summing Amplifier can be made if the individual input resistors are "NOT"equal. Then the equation would have to be modified to: This allows the output voltage to be easily calculated if more input resistors areconnected to the amplifiers inverting input terminal. The input impedance of each individualchannel is the value of their respective input resistors, ie, R1, R2, R3 ... etc.Summing Amplifier Applications Summing Amplifier Audio Mixer – If the input resistances of a summing amplifier areconnected to potentiometers the individual input signals can be mixed together by varyingamounts. Digital to Analogue Converter – Another useful application of a Summing Amplifier is asa weighted sum digital-to-analogue converter. If the input resistors, Rin of the summingamplifier double in value for each input.
DIFFERENTIAL AMPLIFIERDifferential Amplifier The amplified output signal of an Operational Amplifier is the difference between thetwo signals being applied to the two inputs. In other words the output signal isa differential signal between the two inputs and the input stage of an Operational Amplifier isin fact a differential amplifier Thus far we have used only one of the operational amplifiers inputs to connect to theamplifier, using either the "inverting" or the "non-inverting" input terminal to amplify a singleinput signal with the other input being connected to ground. But we can also connect signals toboth of the inputs at the same time producing another common type of operational amplifiercircuit called a Differential Amplifier.Differential Amplifier Configuration Differential Amplifier Equation
THE INTEGRATOR AMPLIFIERIntegrator Amplifier Configuration Circuit As its name implies, the Integrator Amplifier is an operational amplifier circuit thatperforms the mathematical operation of Integration, that is we can cause the output torespond to changes in the input voltage over time. The integrator amplifier acts like a storageelement that "produces a voltage output which is proportional to the integral of its input voltagewith respect to time". In other words the magnitude of the output signal is determined by thelength of time a voltage is present at its input as the current through the feedback loop chargesor discharges the capacitor as the required negative feedback occurs through the capacitor.
THE DIFFERENTIATOR AMPLIFIER The basic Differentiator Amplifier circuit is the exact opposite to that ofthe Integrator operational amplifier circuit that we saw in the previous tutorial. Here, theposition of the capacitor and resistor have been reversed and now the reactance, Xc isconnected to the input terminal of the inverting amplifier while the resistor, Rf forms thenegative feedback element across the operational amplifier as normal. This circuit performs the mathematical operation of Differentiation, that is it "producesa voltage output which is directly proportional to the input voltages rate-of-change with respectto time". In other words the faster or larger the change to the input voltage signal, the greaterthe input current, the greater will be the output voltage change in response, becoming more ofa "spike" in shape.
Improved Differentiator Amplifier Adding the input resistor Rin limits the differentiators increase in gain at a ratioof Rf/Rin. The circuit now acts like a differentiator amplifier at low frequencies and anamplifier with resistive feedback at high frequencies giving much better noise rejection.Additional attenuation of higher frequencies is accomplished by connecting a capacitor C1 inparallel with the differentiator feedback resistor, Rf. This then forms the basis of a Active HighPass Filter.