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Physics 623The Operational AmplifierJuly 26, 20061 Purpose• To understand the basic concepts and characteristics of an ideal operational amplifier.• To compare the characteristics of a high performance low cost IC op amp with thoseof the ideal.• To investigate the properties of the IC op amp in several circuits which demonstrateits great versatility and range of operation.• Note that this is a two-week lab. You should be able to complete through part 4 orpart 5 during the first week.2 DiscussionModern integrated circuit operational amplifiers (op amps) have characteristics so closelyapproaching the ideal op amp that they are components of almost all signal processing andinstrumentation equipment. The properties of a commonly used 741 op amp are comparedto the ideal values listed in the table below:Parameter Ideal Value µA741 Value 2006 best avail. value(not all in same device)Input current Iin= 0 ∼ 80 nA 0.01 pAVoltage gain Av= ∞ 200, 000 2 × 106Bandwidth BW= ∞ 1 MHz at Av= 1 1000 MHzBalance Vo= 0 for Vi= 0 Vo= 0 at Vi= 1 − 2 mV 5 µVTemperature none (dV/dT )offsettypically (dV/dT )offset0.1 µV/Cdependence 15 µV/◦COp amps can be obtained commercially for $0.50 to $3.00 each. This and their versatilityare the reasons for their great popularity. As you perform the suggested investigations, bealert to observe and note those cases in which the real op amp fails to match the idealproperties. Try to find the relevant properties in the “spec sheets”, and take note of the“typical applications” section – this is a prime spot for getting circuit ideas or finding outhow new devices might be used.Almost all applications of op amps involve negative feedback as indicated in the diagrambelow:1Z'ZAViVoAssume that point A is a virtual ground (i.e. V−= V+), and that Ii= 0, and prove thegeneral result thatAvf=VoVi= −Z0Z.3 Procedure• For each circuit you will test, first draw a schematic diagram in your lab notebook, thatincludes IC pin numbers for the integrated circuit (obtainable from the specificationsheets).• Near the supply pins (±15 V ), connect filter capacitors (0.01 µF ) to ground. As youcan see from the circuits in this handout, the power supply connections and associatedfilter capacitors are not normally shown on the main circuit diagram. However, placea schematic in your lab notebook that shows the power supply connections, includingthe bypass capacitors as shown in the figure below.V+V-Csi_cap.01C2si_cap.01-15+15• Where appropriate in the exercises listed below, compare the measured gain A andfrequency response to the values listed on the specification sheets.1. Connect the op amp as an ordinary amplifier as shown below.• Find the “output voltage swing” and the “slew rate” specifications on the datasheet and determine what they mean.2• Can you measure them?• Check the magnitude and phase of the gain.• Measure the bandwidth and the input impedance at the Vinterminals. (Theinsertion of the Rtestresistor is useful in determining the input impedance.)100k10kVoutVinRtest2. Connect the op amp as a non-inverting amplifier. R1= 10k,R2= 100k.• Confirm the magnitude and phase of the gain.• What is the input impedance in this configuration?VinVoutR2R13. Construct an adder. Verify, using three or four pairs of input values, that:Vo= − (v1+ v2)It is most convenient to use the square 10k potentiometers with pins on the back sideas adjustable voltage sources for v1and v2.3VoutV1V2+15V-15V-15V+15v10K10K10K4. As shown below, connect the op amp as an integrator. Use positive pulses 1 msecwide at a frequency of 40 Hz as the input.• Observe the input and output using scope inputs with DC coupling. Becauseof the 100k resistor, this is not a true integrator, but is referred to as a leakyintegrator.• What is the effect of the resistor?VoutVin10k100k0.1 F5. Now modify the integrator to be a charge sensitive amplifier. This is the electronicanalog of a ballistic galvanometer.• Charge a 0.1 µ F capacitor on the +15 V supply.• Connect it to the inverting input.• Derive the equation:V◦= −1CfZiindt = −QinCf• Experimentally confirm it with your measurement.• Try another value of input capacitance and note whether the peak value of Vouttracks the input charge.4Vout0.1 F1.0 F4.7M+15V6. Construct a logarithmic amplifier as shown below.• Vary Vinand plot Vo= A + B ln Viusing a semi-log scale. Five or six values aresufficient – be sure they cover a wide range in V0. (You can use the Log functionson your calculator and the ruled paper in your notebook for the plot.)• Does it make any sense to use an AC input voltage for this measurement?• What are your values of A and B?500k1N91410kVinVout7. Note that, while an Op-Amp has differential inputs, the gain stabilized amplifier cir-cuits in parts 1 and 2 above do not.• To gain the advantages of a differential input with an accurate gain and goodcommon mode rejection, construct the Instrumentation Amplifier as shownbelow.• Understand out how it works.• Try a few combinations of v+inand v−into find the differential and common modegains.5V+V-10k10kVout100k100k(This particular circuit has several drawbacks. Check Horowitz & Hill for a nifty “3Op-Amp Instrumentation Amplifier” that has:• a CMRR over 100,000• an essentially infinite input impedance on both inputs• gain that is adjustable over a wide range by changing one resistor.)8. The following circuit will be used in the next lab, so build it on the circuit board thatis marked with your name and save it. This is a high gain circuit and some care in thelayout is needed; the instructor should have an example to show you before you start.The circuit consists of three identical stages, which should be arranged from left toright across the upper breadboard block, leaving about 1/3 of the block free on theleft. Clip the component leads short and bend them with your pliers so that thecomponent bodies lie flat against the breadboard – this minimizes stray capacitances.Use a single bus on the socket for all the grounds – for example, the innerstrip just below the op amps. It is a good idea to put 0.1 µF ceramic capacitorsfrom the ± 15 V supply pins to the ground bus as close as possible to each op-amp.The instructor will show you a sample layout before you start. Follow this layout veryclosely. A good layout is critical for the next two labs.The circuit for a single tuned amplifier stage is:VinVout0.0033


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UW-Madison PHYSICS 623 - The Operational Amplifier

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