ELEC 350L Electronics I Laboratory Fall 2011IntroductionTheoretical BackgroundExperimental ProcedureELEC 350L Electronics I Laboratory Fall 2011Lab #1: Bar Graph Voltmeter Using Op-AmpsIntroductionThe operational amplifier, or “op-amp,” is one of the most important building blocks in modern analog circuit design. Its predictable performance and unique properties make the op-amp a very popular choice for many important applications. One such property is its ability to compare two voltage levels and produce an output voltage that indicates which of the two is greater. In this labexercise you will take advantage of this capability to construct a simple voltmeter that indicates the measured voltage using a bar graph.Theoretical BackgroundThe circuit symbol for the op-amp is shown below in Figure 1a. It has five key terminals. The two labeled VPOS and VNEG are connection points for DC voltage sources (power supplies) that activate the device and allow it to operate properly. The two voltage sources are usually equal in magnitude and opposite in sign; for example, VPOS might be +12 V and VNEG might be −12 V. Note that one terminal each of VPOS and VNEG is assumed to be connected to ground (i.e., to the reference node). For example, if VPOS = +12 V, then the positive side of a 12-V source is connected to the op-amp, and the negative side is connected to ground. Likewise, if VNEG = −12 V, then the negative side of a 12-V source is connected to the op-amp, and the positive side is connected to ground. The most significant constraint on the two power supplies is that VPOS must be greater than VNEG. Usually, the two values must be separated by at least several volts. Finally, note that significant current can flow into and out of the VPOS and VNEG nodes. In fact, the output current of the op-amp that flows into or out of the vo node comes from one power supply or the other, or both.Figure 1. (a) Circuit symbol for operational amplifier. (b) Equivalent circuit model applicable under conditions of linear operation. The open-loop gain A of the voltage-controlled voltage source is very large; typical values for commonly available op-amps are 105 to 108. The small triangle indicates the ground (reference) node.1 of 5+vo_sd−v+Av =A(vp – vn)+vn−+vp−+−sdVNEGVPOSvovnvp(a) (b)The terminals labeled vn and vp are called the inverting and non-inverting inputs, respectively, and the terminal labeled vo is called the output. Under conditions of linear operation, the functionof the op-amp can be modeled by the equivalent circuit shown in Figure 1b. Node voltages vn, vp,and vo are measured with respect to ground. Op-amps have very complex internal circuitry, but the purpose of that complexity is to produce a device that behaves in a manner that is modeled accurately by the very simple representation shown in Figure 1b. The voltage sources representing the power supplies are not included in the model because their contributions are modeled by the voltage dependent voltage source. Nevertheless, their presence is essential.Several important properties of op-amps become evident after a bit of study of the equivalent circuit model. First, because there is an open circuit between the inverting (vn) and non-inverting (vp) inputs, no current flows into either terminal. (Actually, some current does flow, but it can be assumed to be negligible in most practical cases.) Also, the output voltage (node vo) is directly proportional to the difference between the two input voltages if clipping is not occurring; that is, npovvAAvv .where the constant A, called the open-loop gain, has a very large value (105 to 108). Although the circuit model shown in Figure 1b does not explicitly indicate it, there is a limit on the range of output voltages that an op-amp can produce. Specifically, the output voltage can be no greater than VPOS and no less than VNEG, and for most real op-amps the upper and lower limits are one to two volts tighter than those two values. For example, if the power supply voltages are ±12 V, then vo might be able to vary between −11 V and +11 V or maybe −10.5 V and +10.5 V. Consequently, many practical op-amps circuits employ negative feedback, which is designed to keep the differential input voltage (v) down to very small values, typically only a few microvolts.When the output voltage of an op-amp reaches the upper or lower limit of its permissible range, the op-amp is said to be saturated. This property is put to use in a wide variety of practical circuits. If the non-inverting input voltage vp is just a bit larger than vn, then the output of the op-amp will saturate at VPOS. Likewise, if vp is a little less than vn, the output will saturate at VNEG. Thus, an op-amp can be used as a comparator, a device that indicates whether one voltage is greater than or less than another. An ideal comparator produces an output voltage at one of the power supply values if vp > vn or vp < vn. The condition vp = vn produces an undefined output in anideal comparator. The basic operation of a comparator is depicted in Figure 2.Figure 2. An ideal op-amp used as a comparator.2 of 5+−sdVNEGVPOSvo = VPOS, if vp > vn VNEG, if vp < vnvnvp+−+−Experimental ProcedureAn application of the comparator is shown in Figure 3. The circuit is a simple voltmeter that indicates the measured voltage, represented by Vtest, using a set of light-emitting diodes (LEDs). The voltage to be measured is applied across the non-inverting inputs and ground. The inverting inputs are connected to reference voltages Vref1 through Vref4, which are established by circuitry you will design. If the measured voltage Vtest is greater than a given reference voltage, then vp > vn for the corresponding op-map, and the output voltage of that op-amp saturates near the value VPOS (+10 V in this case). If Vtest is less than a given reference voltage, then the corresponding op-amp remains saturated near VNEG (−10 V).Figure 3. A simple bar graph voltmeter constructed from op-amps and light-emitting diodes. If no LEDs light up, then Vtest is less than 1 V (or is negative). If LED1 lights up, then Vtest is between 0 and 1 V, and so on. Connections to the ground node are indicated by small triangles. Wires that cross at intersections not covered by dots are not connected together.3 of 5+−sdLED4−10 V+10 VVtest+sd−470 +−sdLED3−10 V+10 V470 +−sdLED2−10 V+10 V470 +−sdLED1−10 V+10 V470 3-4 V1-2 V2-3 V> 4