Chapter 5 Operational Amplifiers (original) (raw)
Operational Amplifier UNIT 1 Electronic Circuit BTECH 2ND YR BY Mr. Amit Kumar Kesarwani
As well as resistors and capacitors, Operational Amplifiers, or Op-amps as they are more commonly called, are one of the basic building blocks of Analogue Electronic Circuits.Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation. An Operational Amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. These feedback components determine the resulting function or " operation " of the amplifier and by virtue of the different feedback configurations whether resistive, capacitive or both, the amplifier can perform a variety of different operations, giving rise to its name of " Operational Amplifier ". An Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the Inverting Input, marked with a negative or " minus " sign, (and nd the other one called the Non-inverting Input, marked with a positive or " plus " sign (+). The third terminal represents the Operational Amplifiers output port which can both sink and source either a voltage or a current. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain (A) multiplied by the value of the input signal and depending on the nature of these input and output signals, there can be four different classifications of operational amplifier gain. • Voltage – Voltage " in " and Voltage " out " • Current – Current " in " and Current " out " • Transconductance – Voltage " in " and Current " out " • Transresistance – Current " in " and Voltage " out " Since most of the circuits dealing with operational amplifiers are voltage amplifiers, we will limit the tutorials in this section to voltage amplifiers only, (Vin and Vout). The output voltage signal from an Operational Amplifier is the difference between the signals being applied to its two individual inputs. In other words, an op-amps output signal is the difference between the two input signals as the input stage of an Operational Amplifier is in fact a differential amplifier as shown below. Differential Amplifier The circuit below shows a generalized form of a differential amplifier with two inputs marked V1and V2. The two identical transistors TR1 and TR2 are both biased at the same operating point with their emitters connected together and returned to the common rail,-Vee by way of resistor Re.
An operational amplifier (abbreviated op-amp) is an integrated circuit (IC) that amplifies the signal across its input terminals. Op-amps are analog, not digital, devices, but they are also used in digital instruments. Op-amps are widely used in the electronics industry, and are thus rather inexpensive -the ones used in the lab are about $0.25 each! In this learning module, no details are given about the internal structure of the op-amp. Rather, we summarize many useful applications of op-amps.
After completing this chapter, you should be able to: • Convert between ordinary and decibel based power and voltage gains. • Utilize decibel-based voltage and power measurements during circuit analysis. • Define and graph a general Bode plot. • Detail the differences between lead and lag networks, and graph Bode plots for each. • Combine the effects of several lead and lag networks together in order to determine a system Bode plot. • Describe the use of digital computers in the area of circuit simulation. • Analyze differential amplifiers for a variety of AC characteristics, including single ended and differential voltage gains. • Define common-mode gain and common-mode rejection. • Describe a current mirror and note typical uses for it. G '= −10 dB −10dB −10dB −10 dB G '= −40dB Remember, if an increase in signal is produced, the result will be a positive dB value. A decrease in signal will always result in a negative dB value. A signal that is unchanged indicates a gain of unity, or 0 dB. To convert from dB to ordinary form, just invert the steps; that is, divide by ten and then take the antilog. G=log 10 −1 G' 10 G ' total =G ' 1 +G ' 2 +G ' 3 G ' total =10dB+16 dB+14dB G ' total =40 dB A computer hard drive read/write amplifier exhibits a gain of 35 dB. If the input signal is −42 dBV, what is the output signal? V ' out =V ' i n +A' v V ' out =−42 dBV+35dB V ' out =−7 dBV Note that the final units are dBV and not dB, thus indicating a voltage and not merely a gain. Example 1.12 A guitar power amp needs an input of 20 dBm to achieve an output of 25 dBW. What is the gain of the amplifier in dB? First, it is necessary to convert the power readings so that they share the same reference unit. Because dBm represents a reference 30 dB smaller than the dBW reference, just subtract 30 dB to compensate. 20 dBm=−10 dBW G '=P ' out −P ' i n G '=25dBW−(−10 dBW) G '=35 dB Note that the units are dB and not dBW. This is very important! Saying that the gain is "so many" dBW is the same as saying the gain is "so many" watts. Obviously, gains are "pure" numbers and do not carry units such as watts or volts. Note how the plot is relatively flat in the middle, or midband, region. The gain value in this region is known as the midband gain. At either extreme of the midband region, the gain begins to decrease. The gain plot shows two important frequencies, f 1 and f 2. f 1 is the lower break frequency while f 2 is the upper break frequency. The gain at the break frequencies is 3 dB less than the midband gain. These frequencies are also known as the half-power points, or corner frequencies. Normally, amplifiers are only used for signals between f 1 and f 2. The exact shape of the rolloff regions will depend on the design of the circuit. It is possible to design amplifiers with no lower break frequency (i.e., a DC amplifier), however, all amplifiers will exhibit an upper break. The break points are caused by the presence of circuit reactances, typically coupling and stray capacitances. The gain plot is a summation of the midband response with the upper and lower frequency limiting networks. Let's take a look at the lower break, f 1. Lead Network Gain Response Lead Network Gain Response Reduction in low frequency gain is caused by lead networks. A generic lead network is shown in Figure 1.3. It gets its name from the fact that the output voltage developed across R leads the input. At very high frequencies the circuit will be essentially resistive. Conceptually, think of this as a simple voltage divider. The divider ratio depends on the reactance of C. As the input frequency drops, X c increases. This makes V out decrease. At very high frequencies, where X c <<R, V out is approximately equal to V in. This can be seen graphically in Figure 1.4. The break frequency (i.e., the frequency at which the signal has decreased by 3 dB) is found via the standard equation f c = 1 2 π R C c c 45°0°F igure 1.6 Lead phase (exact). f θ 90°. 1f c f 10f c c 45°0°F igure 1.7 Lead phase (approximate). Example 1.14 A telephone amplifier has a lower break frequency of 120 Hz. What is the phase response one decade below and one decade above? One decade below 120 Hz is 12 Hz, while one decade above is 1.2 kHz. θ=arctan f c f θ=arctan 120 Hz 12Hz θ=84.3degrees one decade below f c (i.e, approaching 90 degrees) θ=arctan 120 Hz 1.2 kHz θ=5.71degrees one decade above f c (i.e., approaching 0 degrees) Remember, if an amplifier is direct-coupled, and has no lead networks, the phase will remain at 0 degrees right back to 0 Hz (DC). Lag Network Response Lag Network Response Unlike its lead network counterpart, all amplifiers will contain lag networks. In essence, it's little more than an inverted lead network. As you can see from Figure 1.8, it simply transposes the R and C locations. Because of this, the response tends to be inverted as well. In terms of gain, X c is very large at low frequencies, and thus V out equals V in. At high frequencies, X c decreases, and V out falls. The break point occurs when X c equals R. The general gain plot is shown in Figure 1.9. Like the lead network response, the slope of this curve is −6 dB per octave (or −20 dB per decade.) Note that the slope is negative instead of positive. A straight-line approximation is shown in Figure 1.10. We can derive a general gain equation for this circuit in virtually the same manner as we did for the lead network. The derivation is left as an exercise. A ' v =−10 log 10 (1+ f 2 f c 2) (1.10) Where f c is the critical frequency, f is the frequency of interest, A' v is the decibel gain at the frequency of interest. Note that accurate models for specific op amps or other devices that are not included in the simulator's libraries may be obtained from their manufacturer. This is often as simple as downloading them from the manufacturer's Web site. A listing of manufacturer's Web sites may be found in the Appendix. Simulators are by no means small or trivial programs. They have many features and options, not to mention the variations produced by the many commercial versions. This text does not attempt to teach all of the intricacies of SPICE-based simulators. For that, you should consult your simulator user's manual, or one of the books available on the subject. The examples in this book assume that you already have some familiarity with computer circuit simulators. We will be using simulations in the following chapters for various purposes. One thing that you should always bear in mind is that simulation tools should not be used in place of a normal "human" analysis. Doing so can cause no end of grief. Simulations are only as good as the models used with them. It is easy to see that if the description of the circuit or the components within the circuit is not accurate, the simulation will not be accurate. Simulation tools are best used as a form of doublechecking a design, not as a substitute for proper analysis. 42 Figure 1.16b Multisim gain and phase output graphs. 2 2 Operational Amplifier Internals Operational Amplifier Internals Chapter Learning Objectives Chapter Learning Objectives After completing this chapter, you should be able to: • Describe the internal layout of a typical op amp. • Describe a simple op amp computer simulation model. • Determine fundamental parameters from an op amp data sheet. • Describe an op amp based comparator, and note where it might be used. • Describe how integrated circuits are constructed. • Define monolithic planar construction. • Define hybrid construction. Reprinted courtesy of Philips Semiconductors Reprinted courtesy of Texas Instrutments Reprinted courtesy of Texas Instrutments 3 Negative Feedback Negative Feedback Chapter Learning Objectives Chapter Learning Objectives After completing this chapter, you should be able to: • Give examples of how negative feedback is used in everyday life. • Discuss the four basic feedback connections, detailing their similarities and differences. • Detail which circuit parameters negative feedback will alter, and how. • Discuss which circuit parameters are not altered by negative feedback. • Define the terms sacrifice factor, gain margin, and phase margin, and relate them to a Bode plot. • Discuss in general, the limits of negative feedback in practical amplifiers.
A Differential Op-Amp Circuit Collection
All op-amps are differential input devices. Designers are accustomed to working with these inputs and connecting each to the proper potential. What happens when there are two outputs? How does a designer connect the second output? How are gain stages and filters developed? This application note will answer these questions and give a jumpstart to apprehensive designers.