University of California at Berkeley Physics 111 Laboratory Basic Semiconductor Circuits (BSC) Lab 6 Op Amps I ©2007 by the Regents of the University of California. All rights reserved. References: Hayes & Horowitz Chapter 4 Horowitz & Hill Chapter 4 In this week’s lab you will study op amps and feedback. You will construct a comparator, follower, current source, and inverting, non-inverting, differential, and summing amplifiers. Before coming to class complete this list of tasks: • Completely read the Lab Write-up • Answer the pre-lab questions utilizing the references and the write-up • Perform any circuit calculations or anything that can be done outside of lab. • Plan out how to perform Lab tasks. Pre-lab questions: 1. Explain why the circuit in 6.2 exhibits hysteresis. 2. Design an inverting amplifier with a gain of ten and an input impedance of 1k. 3. What is the gain of the non-inverting amplifier used in 6.5? 4. Explain why the circuit below is a perfect current to voltage converter. VoutRIin What is the conversion factor? The Laboratory Staff will not help debug any circuit whose power supplies have not been properly decoupled! Last Revision: August 2007 Page 1 of 10 ©2007 Copyrighted by the Regents of the University of California. All rights reserved.Physics 111 BSC Laboratory Lab 6 Op Amps I Background Integrated Circuit Amplifiers As we have seen in the JFET labs, amplifiers constructed from discrete transistors have many unde-sirable features: • High gain amplifiers are difficult to design. • The amplifier’s gain is difficult to predict because it depends on the transistor’s transconduc-tance, which varies between transistors. • The gain depends on temperature. • The amplifier’s output impedance is not low. • Amplifiers made with bipolar transistors have low input impedances. • Discrete transistors are easy to burn out.1 Many of these problems can be eliminated by carefully designing complicated circuits using match-ing transistors. Such complete amplifier circuits, fabricated on a single piece of silicon, come pre-packaged as “integrated” circuits (ICs). Many types of integrated amplifiers are available, but the most useful type is the Operation Amplifier (op amp). Feedback Op amps circuits almost always use negative feedback: feedback is the most important principle of modern analog circuit design. To apply feedback to an amplifier is to feed some of its output back into its input. Positive feedback, where the output is used to enhance the input signal and increase the gain,2 has some obvious uses, but negative feedback, where the output is used to di-minish the input signal and decrease the gain, seems foolish at first. After all, why deliberately di-minish the gain of an amplifier? Indeed, negative feedback was greeted with incredulity when first invented by Harold Black and others in the 1920s. We shall see, however, that negative feedback dramatically improves the properties of amplifiers. Op Amps Op amps are differential amplifiers. Like the discrete differential amplifiers that you built last week, op amps have two inputs and one output. Unlike the discrete differential amplifiers, op amps have essentially infinite differential gain,3 negli-gible common mode gain, extremely high input impedance, very low output impedance, small temperature drifts and inconsequential piece-to-piece variation. Furthermore, they are insensitive to power supply fluctuations, difficult to burn out, cheap, and available in countless varieties.4 VoutV+V- As op amps are almost always superior to discrete amplifiers, modern analog circuit designs use them almost exclusively. Real op amps are so close to being perfect amplifiers (i.e. infinite gain and input impedance, zero output impedance, etc.) that they are often thought of as being truly perfect. This pretense greatly simplifies circuit design. In this lab, we will generally assume that our op amps are truly perfect; the Op Amps III lab will investigate some of their limitations. 1 Much more of a problem in our lab than in a production environment. 2 Historically, positive feedback was used in oscillator circuits. Nowadays positive feedback is used in comparators and a few other circuits. 3 True infinite gain is not possible, of course, but gains of over 105 are readily available. Last Revision: August 2007 Page 2 of 10 4 To give you some idea of the number of different types of op amps, Spice models are available on our computer for over 200 different types. ©2007 Copyrighted by the Regents of the University of California. All rights reserved.Physics 111 BSC Laboratory Lab 6 Op Amps I When used with negative feedback, ideal-op amp circuits can be designed following two simple rules: The Op Amp Golden Rules 1. The inputs draw no current. 2. The output attempts to do whatever is necessary to make the voltage difference between the two inputs zero. Op Amps Followers The simplest op amp circuit is the follower. The first golden rule implies that the input impedance is in-finite. The second rule is more interesting; the only way that the two inputs can be at the same voltage is for Vout = V− to equal V+ = Vin. Consequently the op amp behaves like a per-fect voltage follower. Last Revision: August 2007 Page 3 of 10 ©2007 Copyrighted by the Regents of the University of California. All rights reserved. Understanding how the second rule is actually satisfied is particularly easy for the follower. If the output Vout were to deviate low, V+ − V− = Vin – Vout > 0 and the amplifier would drive its output Vout higher, reestablishing Vin = Vout. If the output Vout were to deviate high, V+ − V− < 0 and the amplifier would drive its output lower, once again reestablishing Vin = Vout. If the input voltage Vin were to change, Vin = Vout would be similarly reestablished.5 VoutVin Op Amp Inverting Amplifiers A second simple op amp circuit is the inverting amplifier. Since the non-inverting input V+ is tied to ground, and the second golden rule insists that V+ = V−, the negative input V− must also be at ground. As this ground is enforced by the op amp itself rather than by any physical connection to ground, it is often called a virtual ground. The current flowing into the V− junction is then easy to calculate: I = Vin/R1. Since, according to the first golden rule, no current flows into the input itself, all of the current I must flow