NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT # 1 OPERATIONAL AMPLIFIERAgdon, Berverlyn B. July 26, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
OPERATIONAL AMPLIFIERS It is an amplifier which is designed to be used with other circuit components to performeither computing functions (addition, subtraction) or some type of transfer operation, such asfiltering. Operational amplifiers are usually high-gain amplifiers with the amount of gaindetermined by feedback. Schematic symbols of an op ampCharacterstics of Op-Amp 1. Very high gain 2. Very high input impedance 3. Very low output impedanceSince no single amplifier stage can provide all these characteristics well enough to be consideredan operational amplifier, various amplifier stages are connected together. The total circuit madeup of these individual stages is called an operational amplifier. This circuit (the operationalamplifier) can be made up of individual components (transistors, resistors, capacitors, etc.), butthe most common form of the operational amplifier is an integrated circuit. The integrated circuit(chip) will contain the various stages of the operational amplifier and can be treated and used asif it were a single stage.Block diagram of Op-Amp
Types Of Op-AmpsComparator- Compares two voltages and switches its output to indicate which voltage is larger.Inverting amplifier - uses negative feedback to invert and amplify a voltage. The Rin,Rf resistornetwork allows some of the output signal to be returned to the input. Since the output is 180° outof phase, this amount is effectively subtracted from the input, thereby reducing the input into theoperational amplifier. This reduces the overall gain of the amplifier and is dubbed negativefeedback. Zin = Rin (because V − is a virtual ground)Theory of operation: An Ideal Operational Amplifier has 2 characteristics that imply theoperation of the inverting amplifier: Infinite input impedance, and infinite differential gain.Infinite input impedance implies there is no current in either of the input pins because currentcannot flow through an infinite impedance. Infinite differential gain implies that both the (+) and(-) input pins are at the same voltage because the output is equal to infinity times (V+ - V-). Asthe output approaches any arbitrary finite voltage, then the term (V+ - V-) approaches 0, thus thetwo input pins are at the same voltage for any finite output.Non-inverting amplifier - amplifies a voltage (multiplies by a constant greater than 1)Although this circuit has a large input impedance, it suffers from error of input bias current.The non-inverting ( + ) and inverting ( − ) inputs draw small leakage currents into the operationalamplifier. These input currents generate voltages that act like unmodeled input offsets. Theseunmodeled effects can lead to noise on the output (e.g., offsets or drift). Assuming that the twoleaking currents are matched, their effect can be mitigated by ensuring the DC impedancelooking out of each input is the same. The voltage produced by each bias current is equal to theproduct of the bias current with the equivalent DC impedance looking out of each input. Makingthose impedances equal makes the offset voltage at each input equal, and so the non-zero bias
currents will have no impact on the difference between the two inputs.Differential amplifier - the difference of two voltages each multiplied by some constant(determined by the resistors).Voltage follower (Unity Buffer Amplifier) - Used as a buffer amplifier to eliminate loadingeffects (e.g., connecting a device with a high source impedance to a device with a low inputimpedance).Due to the strong (i.e., unity gain) feedback and certain non-ideal characteristics of realoperational amplifiers, this feedback system is prone to have poor stability margins.Consequently, the system may be unstable when connected to sufficiently capacitive loads. Inthese cases, alag compensation network (e.g., connecting the load to the voltage follower througha resistor) can be used to restore stability. The manufacturer data sheet for the operationalamplifier may provide guidance for the selection of components in external compensationnetworks. Alternatively, another operational amplifier can be chosen that has more appropriateinternal compensation.Summing amplifier When , and Rf independent When Zn = Rn
Inverting integrator - There are several potential problems with this circuit. It is usuallyassumed that the input Vin has zero DC component (i.e., has a zero average value). Otherwise,unless the capacitor is periodically discharged, the output will drift outside of the operationalamplifiers operating range. Even when Vin has no offset, the leakage or bias currents into theoperational amplifier inputs can add an unexpected offset voltage to Vin that causes the output todrift. Balancing input currents and replacing the non-inverting ( + ) short-circuit to ground with aresistor with resistance R can reduce the severity of this problem. Integrates the (inverted) signal over timeInverting differentiator - Differentiates the (inverted) signal over time. It is a filter with a single zero at DC (i.e., where angular frequencyω = 0 radians) and gain. Thehigh-pass characteristics of a differentiating amplifier (i.e., the low-frequency zero) can lead tostability challenges when the circuit is used in an analog servo loop (e.g., in a PID controller witha significant derivative gain). In particular, as aroot locus analysis would show, increasingfeedback gain will drive a closed-loop pole toward marginal stability at the DC zero introducedby the differentiator.Instrumentation amplifier - Combines very high input impedance, high common-moderejection, low DC offset, and other properties used in making very accurate, low-noisemeasurementsIs made by adding a non-inverting buffer to each input of the differential amplifier to increasethe input impedance.
Schmitt trigger - A bistable multivibrator implemented as a comparator with hysteresis.In this configuration, the input voltage is applied through the resistor R1 (which may be thesource internal resistance) to the non-inverting input and the inverting input is grounded orreferenced. The hysteresis curve is non-inverting and the switching thresholds arewhere Vsat is the greatest output magnitude of the operational amplifier.Alternatively, the input source and the ground may be swapped. Now the input voltage is applieddirectly to the inverting input and the non-inverting input is grounded or referenced. Thehysteresis curve is inverting and the switching thresholds are . Such a configurationis used in the relaxation oscillator shown below.Relaxation oscillatorBy using an RC network to add slow negative feedback to the inverting Schmitt trigger,a relaxation oscillator is formed. The feedback through the RC network causes the Schmitttrigger output to oscillate in an endless symmetric square wave (i.e., the Schmitt trigger in thisconfiguration is an astable multivibrator).
Inductance gyratorSimulates an inductor (i.e., provides inductance without the use of a possibly costly inductor).The circuit exploits the fact that the current flowing through a capacitor behaves through time asthe voltage across an inductor. The capacitor used in this circuit is smaller than the inductor itsimulates and its capacitance is less subject to changes in value due to environmental changes.This circuit is unsuitable for applications relying on the back EMF property of an inductor as thiswill be limited in a gyrator circuit to the voltage supplies of the op-amp.Negative impedance converter (NIC) - Creates a resistor having a negative value for any signalgeneratorIn general, the components R1, R2, and R3 need not be resistors; they can be any component thatcan be described with an impedance.
Wien bridge oscillator - Produces a very low distortion sine wave. Uses negative temperaturecompensation in the form of a light bulb or diode.Precision rectifier - The voltage drop VF across the forward biased diode in the circuit of apassive rectifier is undesired. In this active version, the problem is solved by connecting thediode in the negative feedback loop. The op-amp compares the output voltage across the load with the input voltage and increases its own output voltage with the value of VF. As a result, the voltage drop VF is compensated and the circuit behaves very nearly as an ideal (super) diode with VF = 0 V. The circuit has speed limitations at high frequency because of the slow negative feedback and due to the low slew rate of many non-ideal op-amps.Logarithmic outputPutting these two formulae together and considering that the output voltage is the negative of thevoltage across the diode (Vout = − VD), the relationship is proven.Exponential output