Exp_4_fa09 - Physics 3330 Experiment#4 Fall 2009...

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Physics 3330 Experiment #4 Fall 2009 Operational Amplifiers and Negative Feedback Purpose This experiment shows how an operational amplifier (op-amp) with negative feedback can be used to make an amplifier with many desirable properties, such as stable gain, high linearity, and low output impedance. You will build both non-inverting and inverting voltage amplifiers using an LF356 op-amp. Introduction The purpose of an amplifier is to increase the voltage level of a signal while preserving as accurately as possible the original waveform. In the physical sciences, transducers are used to convert basic physical quantities into electric signals, as shown in Figure 4.1. An amplifier is usually needed to raise the small transducer voltage to a useful level. Transducer Oscilloscope Data Analysis System Amplifier Temperature, Magnetic Field, Radiation, Acceleration, etc. DC Power µ Volts to mVolts Figure 4.1 Typical Laboratory Measurement Volts A small voltage or current change at the input of the amplifier controls a much larger signal at the input of whatever circuit or instrument the amplifier’s output is connected to. Measuring and recording equipment typically requires input signals of 1 to 10 V. To meet such needs, a typical laboratory amplifier might have the following characteristics: 1. Predictable and stable gain. The magnitude of the gain is equal to the ratio of the output signal amplitude to the input signal amplitude. 2. Linear amplitude response, so that the output signal is directly proportional to the input signal. Experiment #4 4.1 Fall 2009
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3. According to the application, the frequency dependence of the gain might be a constant independent of frequency up to the highest frequency component in the input signal (wideband amplifier), or a sharply tuned resonance response if a particular frequency must be picked out. 4. High input impedance and low output impedance are usually desirable. These characteristics minimize changes of gain when the amplifier is connected to the input transducer and to other instruments at the output. 5. Low noise is usually important. Every amplifier adds some random noise to the signals it processes, and this noise often limits the sensitivity of an experiment. We will learn about amplifier noise in Experiment 8. Commercial laboratory amplifiers are readily available, but a general-purpose amplifier is expensive (>$1000), and most of its features might be unneeded in a given application. Often, it is preferable to design your own circuit using a cheap (<$1) op-amp chip. Op-amps have many other circuit applications. They can be used to make filters, limiters, rectifiers, oscillators, integrators, and other devices (see H&H Sections 4.14–4.20 for some examples). To get an idea of the variety of op-amps available, have a look at the National Semiconductor web site ( www.national.com ). They currently make over 300 types, offering trade-offs between speed, cost, power consumption, precision, etc. The op-amp is the most important building block of analog electronics.
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This note was uploaded on 11/10/2009 for the course PHYS 33 at Colorado.

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Exp_4_fa09 - Physics 3330 Experiment#4 Fall 2009...

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