EXPERIMENT #1 Operational-Amplifier Basics and Beyond Objective The primary objective of this experiment is to provide familiarization with the basic properties and applications of the integrated-circuit (IC) operational amplifier, the op amp, one of the most versatile building blocks currently available to electronic-circuit designers. The emphasis will be primarily on the nearly ideal op amp and on what is easily and conveniently implemented. Introduction Read section 2.1 to 2.6 in “Microelectronic circuits” by Sedra and Smith. The IC op amp is a versatile circuit building block. It has found many applications in practical circuits and systems. The ideal op amp responds only to the difference input signal, that is, (v2 – v1), and provides at the output terminal a signal A(v2 – v1), where A, the open-loop gain, is very large (104 to 106) and ideally infinite. The ideal op amp has an infinite input resistance and a zero output resistance. In this experiment, your work will be concentrated on the 741-type op amp provided, two per IC, in an 8-pin dual-in-line package (DIP) whose schematic connection diagram and packaging are shown in Fig 1.1. For power supplies, you will use two DC sources, +10 V and -10 V, or ±10 V for short. Fig 1.1 The MC 1458 --- Two 741-Type Op Amps in an 8-pin DIP (a) Block schematic (b) Top view of the dual-in-line package (DIP) with internal connections shown Prelaboratory 1. (a) For the inverting amplifier circuit to the right of node B in Fig 1.2, what is the expected closed-loop gain (as measured from node B to node D)? What is the input resistance viewed from the right of node B? (b) Consider the basic inverting op amp shown in Fig 1.2, with a 91 mV peak-to-peak signal applied at node B. For an ideal op amp, what signals would you measure at nodes C and D? 2. (a) Using an ideal op amp, design a non-inverting amplifier with a gain of +11 V/V, low currents in the associated resistor network, but no resistor larger than 10 kΩ. (b) What is the gain of the circuit in Fig 1.3, from node B to node D, with R1 shunted by a resistor of equal value? With R2 shunted likewise? With R2 shorted? 3. Calculate the expected gains for individual inputs A, D, F to output C, of the circuit shown in Fig 1.4. Laboratory Procedure 1. In this section, we will explore the basic operation of an inverting-amplifier, and the occurrence of a virtual ground. Assemble the circuit as shown in Fig 1.2. Adjust the supplies to ±10 V using the DVM. Fig 1.2 A Basic inverting Amplifier with Input Attenuator (for Testing) (a) With node A open (or grounded), measure B, C, and D. With node A connected to +10 V, measure B, C, and D. With node A connected to -10 V, measure B, C, and D. Tabulate VB, VC, VD, with VA=0, 10 V, -10 V. By making considerations of the location of the virtual ground, calculate two estimates of the voltage gain for the last two cases. (b) Connect node A to +10 V. Shunt R2 by a 10 kΩ resistor to reduce the gain by a factor 2. Measure the voltages at D and B. Shunt R1 by a 1 kΩ resistor to raise the gain by a factor 2. Measure D and B. Open the connection of R1 to node B, and add a resistor in series with R1, of equal value, joined to R1 at a new node to be called X. Measure nodes D, B, X, and C. Tabulate R1, R2, VD with VA, VB, VX. Calculate the input resistance Rin at node B. 2. In this section, we will explore the basic operation of a non-inverting amplifier and the behavior of a virtual short circuit. Assemble the circuit shown in Fig 1.3. Adjust the supplies to ±10 V using the DVM. Fig 1.3 A Basic Non-Inverting Amplifier with Input Attenuator (for Testing) (a) With node A connected to +10 V, measure B, C, and D. With node A connected to -10 V, measure B, C, and D. Tabulate VA, VB, VC, VD, for VA = 10 V, -10 V. Obtain an estimate of the voltage gain VD/VA by considering a virtual short circuit at the input of the op amp. (b) With node A connected to +10 V. shunt R2 with a resistor of equal value. Measure D, B, and C. Shunt R1 with a resistor of equal value. Measure D, B, and C. Short-circuit R2. Measure D and B. Tabulate VB, VC, VD, R2, R1, for various combinations of R1 and R2. Calculate the gain in each case. (c) With node A connected to a sine wave at 1 kHz having 5 V peak-to-peak amplitude, use an oscilloscope to measure the peak amplitude of signals at B, C, and D. Then shunt the op-amp input terminals with a resistor, RX = 1 kΩ, measure B, C, and D. Tabulate VA, VB, VC, VD, for RX = ∞ or 1 kΩ. Consider the gain in each case. 3. This section is devoted to investigating the properties and potential applications of a special three-input amplifier which facilitates difference measurement. Assemble the circuit shown in Fig 1.4, initially with A, D, and F grounded. Adjust the supplies to ±10 V using DVM. Fig 1.4 A Multi-Purpose Amplifier Topology (a) Now using a sine wave generator to which a 1 kΩ – 10 Ω voltage divider is connected, generate a 100 mV peak-to-peak voltage at 1 kHz. Connect the 100 mVpp signal in turn to one of A, D, F, separately (the other two remaining grounded), and measure the output signal at node C on the oscilloscope screen. Note the relative phase of the signals (in phase or out of phase). Tabulate VC for three cases: VA = 100 mVpp with VD = VF = 0, VD = 100 mVpp with VA = VF = 0 and VF = 100 mVpp with VA = VD = 0. Consider all three values of voltage gain from the inputs (A, D, F), to output (C), namely VC/VA, VC/VD, VC/VF. (b) To illustrate that common-mode signals can be amplified quite differently, and, in one case, hardly at all, we perform the following experiment. Connect the generator to provide a 5 V peak-to-peak signal to both A and D with F at ground. Measure the voltage at C. Connect the 5 Vpp signal to all of A, D, and F. Measure the voltages at C, B, and E. Tabulate VA, VB, VC, VD, VE, VF, for two cases: VA = VD = VAD with VF = 0 and VA=VD=VF=VADF. Consider the common-mode voltage gains from input to output. Find VC/VAD, and VC/VADF. To which of these cases does the idea of “common-mode rejection” apply? Laboratory Report In the laboratory report, for each of the measurements made on the circuits, you should tabulate all the measured data and complete the calculation and analysis as required by the Laboratory Procedure. You should compare the measured data with the analytical results and suggest reasons for any discrepancy.
