NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Building, Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT # 1 OPERATIONAL AMPLIFIERReyes, Ron Henreb July 26, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
OPERATIONAL AMPLIFIERS: Operational amplifiers (op amps) were originally used for mathematical operationsin analog computers. They typically have 2 inputs, a positive (non_inverting) input and anegative (inverting) input. A signal fed into the positive (non_inverting) input will producean output signal which is in phase with the input. If the signal is fed into the negative(inverting) input, the output will be 180 degrees out of phase when compared to the input. They are designed to be used with other circuit components to perform eithercomputing 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. There are a bazillion (technical term) applications for op amps. The followingsection is an attempt to give you a basic understanding of just a few applications. None ofthe power supply connections are shown. Most op amp circuits used in audio use a ±15 voltpower supply (especially when the audio equipment has a switching power supply). Theycan also be used with a single ended supply (no negative voltage) in head units and othersuch equipment that have no switching power supply. The diagram below shows the schematic symbol for an op amp.
OP AMP OPERATION: The circuit below shows a simple buffer circuit. The input impedance of an op ampis extremely high (on the order of 1012 ohms). It might be used if the input signal to the opamp was coming from a source which could supply almost no current. The output of the opamp can easily drive 1000 ohms or more. The output, when used as a buffer, willtheoretically be identical to the input signal. I cant say it is identical because there is asmall amount of distortion in all amplifier circuits. The distortion in this circuit would beEXTREMELY low and would most likely be inaudible. OP AMPs as Amplifiers: An op amp can also easily amplify a signal such as audio. The diagram below showsthe circuit for an op amp that would give an output signal twice as large as the input. Opamps dont like errors. To get amplification, you induce an error in the signal going back tothe negative input of the op amp. An op amp will do everything in its power to get thesignal on the negative input to match the signal on its positive input. To get an output thatstwice as large as the input, you use 2 equal value resistors as a voltage divider to reduce thereturn (feedback) signal at the negative input by half. If the return signal doesnt match theinput signal, the op amp will increase the output until the signal returned to the negativeinput is the same as the input to the positive input. Since the voltage divider cuts the signalin half, the signal at the output must be doubled. You can create any amount of gain neededby changing the value of ONE of the resistors in the feedback path. The actual limit of gainwill be determined by the op amp design. When using an op amp as a non-invertingamplifier, the gain will always be greater than or equal to 1. To get a gain of less than 1, youneed to use a voltage divider on the input signal.
OP AMP Inverters: An op amp can produce a signal which is 180 degrees out of phase (inverted) withrespect to the input signal. The diagram below shows an op amp used as an inverter. To usean op amp as an inverting amplifier, you must send the signal into the negative inputinstead of the positive input. As I said before, the op amp will do everything it possibly canto make the voltage (signal) on the the negative input match the positive input. In thefollowing diagram, you can see that the positive input is connected to ground. Its shown asbeing connected through a resistor but the resistance to ground in unimportant. What isimportant is that the positive input has no signal (its connected to the reference, ground).This means that the op amps negative input will have no visible (voltage) signal on it.When youre driving the negative input it will act as a virtual ground. The input isconverted from a voltage drive to a current drive. The change in current is what drives theop amp. This is important to know because when you look at the negative input with anoscilloscope, you will see no signal (when the circuit is an inverting amplifier). I said earlierthat the op amp inputs had a very high impedance. While this is true, when using theinverting input with feedback (which is necessary for audio reproduction), the inputimpedance becomes the value of the input resistor. OP AMP Error Correction: An op amp is commonly used in a circuit where error correction is required. Opamps cant (generally) supply a large amount of current at its output. If a signal is fed to thepositive input of an op amp and the op amp is driving a circuit which CAN supply a largeamount of current (like the regulator that we will work with later on this page), the outputof the whole system can be fed back into the negative input of the op amp. This will allowthe op amp to compare the output (of the whole system) to the input signal and correct asneeded. If the op amp is used in a circuit which needs little current at its output, the op ampcan still monitor the output and correct as needed. If you read the servo page before this page, youll remember that there was areference point indicated by a green arrow. The positive input on the op amp is analogousto the green arrow. The height of the green arrow would be analogous to the voltage on thepositive input. The sensor would be analogous to the valves which would also be analogousto the negative input of the op amp. The error correction would come at the output of theop amp (instead of the hydraulic actuator).
The next diagram has a resistor in series with the output of the op amp and the loadwhich is to be driven by the op amp. The resistor represents anything that may be betweenthe op amp and the load. The resistor could actually be a long run of wire, resistance in thecopper of a printed circuit board or anything else that may cause the signal to be distorted.If the op amp didnt monitor the signal at the load, the signal would be distorted (in thiscase, the simple series resistance would only reduce the signal level). If the resistor wouldinstead be an external circuit designed to increase the output current (such as thetransistors, resistors, capacitors... of a power amplifier), the output of the op amp may noteven resemble the final output signal. The op amp would do everything possible to get thefinal output to match the input signal. A look back at the amplifier page shows a circuitwhere an op amp is used for error correction in a power amplifier circui . The output resistance simulates any series resistance or anything else that maydistort the signal. The op amp tries to make theoutput voltage match the input voltage. Ifyou make the output resistance 0 ohms the opamp voltage and output voltage are equalbecause there is no output resistance to cause a voltage drop (distortion). Op Amps as Comparators: An op amp can be used to compare 2 different voltages. If you apply a referencevoltage to one of the inputs and then use the other input to monitor a voltage from somepoint in a circuit, the output of the op amp will go from high to low (or vice-versa) as themonitored voltage crosses the reference voltage.
The voltage follows a curve because the capacitor is charging. The capacitor chargesfaster at first then it slows as it approaches full charge. The blue line indicates the voltageon the positive input. Since the 2 voltage divider resistors are of equal value, the voltage onthe positive input is exactly half of the power supply voltage. You can also see that thevoltage on the output of the op amp is high (close to the power supply voltage). If you pushthe button, the capacitor will start to charge. The little lines in the window are voltageindicators. Think of them as a portion of the trace on an oscilloscope. As the capacitorcharges (and the voltage starts to rise), the line goes up (it follows the voltage). The voltageon the output does not change until the voltage on the negative input is higher than thevoltage on the positive input. Remember that the previous circuits had a feedback signalreturn path between the output of the op amp to the negative input. Since there is nofeedback, the gain is essentially (ideally) infinite. This will make the output swing from itsmaximum positive output voltage to its maximum negative output voltage. If there were afeedback resistor, the output voltage would not swing as far. With a feedback resistor, youcould get the op amps output voltage to be an inverted version of the voltage on thenegative input. Remember that the circuit is a comparator. Its comparing the voltage onthe 2 inputs. When the voltage on the negative input is below the reference voltage (on thepositive input), the output is high. As soon as the voltage on the negative input goes abovethe voltage on the positive input, the output goes low. If you look at the white line sweepingfrom left to right you can see that the green line instantly transitions from high to low atthe point where the blue and yellow lines intersect Note: This is the same basic principle that the fan controller on the cooling fans pageemploys. The fan controller uses the negative input for the reference and the positive input(whos voltage is controlled by the thermistor) as the input to the comparator. Maximum and Minimum Input/Output Values: Some op amps can not accept inputs that equal the power supply voltage (or groundin the case of a single ended† supply). If the input is beyond the safe input values, the inputmay lead to unexpected output values. For instance, if the negative input in the previouscircuit were grounded, some op amps would erroneously give a low output instead of ahigh output. As soon as the input voltage moved slightly above ground, the op amp wouldagain operate as youd expect. If you need an op amp to accept inputs close to ground, youneed to get an op amp suited for the task. The op amp on the cooling fans page is such adevice. Its an LM358 which can be found on the TI site. A single ended supply is one that uses only ground and EITHER a positive ORnegative voltage. A split supply has a positive voltage AND a negative voltage (belowground). If you see a power supply described as a 12 volt supply, it likely means that it issimply a single ended supply. If the supply voltage is expressed as ±15 volts, its a splitsupply. In audio, split supplies are most common. For digital equipment, the comparatorswould likely be powered by a single ended 5 volt supply.
Op Amps as Regulators: If you need a high quality linear regulator, an op amp can save a lot of effort. In thefollowing demo, you can see that there is a simple zener shunt regulator connected to thepositive input of the op amp. This becomes the reference voltage. If the zener is a 6.2 voltdevice, the reference will be 6.2 volts. Actually the reference voltage will likely be a littlemore or less than 6.2 volts (due to tolerances and the actual current flowing through thediode). If the voltage is precisely 6.2 volts on the positive input, the output of the regulator(the emitter of the current boost NPN bipolar transistor) will be precisely 6.2 volts. Thefeedback line from the emitter to the negative input of the op amp allows the op amp tomonitor the output and compensate for changing load current. If the load resistordecreases in resistance, the output current increases (because we have a regulated voltagesource). Without the feedback, the output from the regulator would likely drop a little. Inmost cases that would be fine. In some circuits, however, the change in voltage would beunacceptable. When you push the button in the following demo, the resistance will decrease. Youwill notice that the regulator output current through the resistor increases in proportion tothe fall in resistance. You will also notice that the output voltage is rock solid. If you lookcarefully, you can see that the output voltage from the op amp increases slightly to increasethe current through the base of the transistor (which is needed to maintain the properoutput voltage) Op-amps can be connected into two basic configurations, Inverting and Non-inverting. The Open-loop gain called the Gain Bandwidth Product, or (GBP) can be veryhigh and is a measure of how good an amplifier is. Very high GBP makes an operationalamplifier circuit unstable as a micro volt input signal causes the output voltage to swinginto saturation. By the use of a suitable feedback resistor, (Rf) the overall gain of theamplifier can be accurately controlled.
The Differential Amplifier produces an output that is proportional to thedifference between the 2 input voltages. Adding more input resistor to either the invertingor non-inverting inputs Voltage Adders or Summers can be made. Voltage follower op-amps can be added to the inputs of Differential amplifiers to produce high impedanceInstrumentation amplifiers. The Integrator Amplifier produces an output that is the mathematical operationof integration. The Differentiator Amplifier produces an output that is the mathematicaloperation of differentiation. Both the Integrator and Differentiator Amplifiers have aresistor and capacitor connected across the op-amp and are affected by its RC timeconstant. In their basic form, Differentiator Amplifiers suffer from instability and noise butadditional components can be added to reduce the overall closed-loop gain.