Operational Amplifiers

Transcription

1 perational Amplifiers. perational Amplifiers perational amplifiers (commonly known as opamps) are integrated circuits designed to amplify small voltages (or currents) to usable levels. The physical packaging of a common type of opamp (the 74) is also shown in Figure A below. This package is referred to as an 8 pin DIP (dual inline package). Note carefully the small hole in the upper left hand corner (or notch in the upper center); this is used to indicate the location of pin on the package. Positive power supply (V) Inverting input () Noninverting input () utput (o) Negative power supply (V) Inverting input Noninverting input Negative power supply (V) 4 74 Figure A. 74 pamp schematic and pinout Positive power supply (V) utput pamps are composed of carefully matched sets of transistors and resistors. The opamp is shown on circuit diagram schematics as a triangle, given in Figure A above. Note that most bipolar opamps have at least five connections:. inverting input (),. noninverting input (),. output (o), 4. positive voltage supply (V), and 5. negative voltage supply (V). The output voltage o is determined by multiplying the difference between the voltages at the noninverting and inverting inputs ( and ) by an openloop gain, K: K *( ) o Characteristics of opamps are summarized here:. very high input impedance (0 6 ohms or more),. high openloop gain (K > 0 5 or more),. low output impedance (able to deliver o into small resistances), 4. fast response (slew rates of up to several volts per microsecond), and 5. able to reject common mode inputs (discussed below). The first two characteristics of opamps given above are commonly used to develop ideal circuit responses. Since the input impedance is very large, we often assume that the currents i and i are zero. If the openloop gain K is very large, and the output voltage o is

2 perational Amplifiers. relatively small, then the voltage difference * = assumed to be zero. must be very small, and is usually Single input, inverting amplifier circuit The most common opamp circuit is given below in Figure A. This arrangement is known as the single input, inverting amplifier and is similar to Figure. of your text. i V V o Figure A. Single input, inverting opamp circuit Based on the circuit drawn above, the following equation can be written by summing currents at the first node, i i i 0 Note that we have also applied our assumption that the current i = 0. From the circuit diagram, the voltage at the node is V * ; therefore, relationships for the currents can be written as i i * i * The voltage * is very small and is normally assumed to equal zero; therefore, we have i i i 0 This equation can be solved for the output voltage as a function of the input voltage i, the input resistance, and the feedback resistance as The gain of the opamp circuit is the ratio / and can be greater than, equal to, or less than. The equation given above is an ideal relationship and is subject to at least two restrictions:. the input resistance should be fairly large ( > 0 k minimum) to minimize the current i, and i

3 perational Amplifiers.. the output voltage remains in the range (V volt) (V volt), i.e. the output voltage cannot fall outside the opamp supply voltages. The restriction on the output voltage is termed saturation. The third resistor is selected to match the impedance seen at both the and terminals of the opamp. The nominal value for resistor is the parallel combination of and, Having the exact value for is not as critical to the operation of the circuit as the other resistance values. Therefore, a tolerance of ±0% is allowed for this resistance. In many simple opamp circuits the easiest way to create the correct resistance is to place an and an valued resistor in parallel between the negative positive terminal ( ) and the common bus. Multiple input, summing amplifier circuit pamp circuits are often used to add two or more signals. Figure A below shows a circuit which adds the two voltages and, and multiplies each by the same or different gains. 4 V V o Figure A. Multiple input, summing opamp circuit The theoretical relationship for the summing amp circuit above is given by Note that the input voltages and are connected to the terminal of the opamp and thus have a negative relationship with the output o. The input resistances and, are frequently equal in value, which leads to a slightly simpler equation,

4 perational Amplifiers.4 As with the singleinput inverting amp circuit, the resistance 4 is selected to match impedance at the positive and negative terminals of the opamp, 4 Single input, noninverting amplifier circuit In Figure A4 (and Figure. of your text) the single input, noninverting amplifier circuit is given. The ideal inputoutput relationship for this design is given by o i Note that the minimum gain for this configuration is, when is much greater than. Also note that the input voltage i is connected to the terminal of the opamp and thus has a positive relationship with the output o. V i V o Figure A4. Single input, noninverting opamp circuit As with the singleinput inverting amp, the resistance is selected to match impedance at the positive and negative terminals of the opamp,

5 perational Amplifiers.5 Dual input, difference amplifier circuit The circuit shown below in Figure A5 is known as a difference amplifier. The output voltage,, is related to the difference in the two input voltages. The ideal relationship for this circuit is o The signs on the two input terms are easily remembered if you note that and have a positive relationship while and have a negative one. This should be expected since is attached to the positive ( ) terminal of the opamp while is attached to the negative ( ) terminal. V V o Figure A5. Dual input, difference opamp circuit Instrumentation Amplifier Circuit The circuit shown in Figure A6 uses three opamps to create a single instrumentation amplifier. This circuit is ideal for amplifying the small signals found in many instrumentation applications, such as Wheatstone bridges. The two opamps used in the first stage provide a very large impedance (> M) to the input voltage i. A very large gain can be created by the proper selection of the single resistor G. G The instrumentation amplifier is often used with precision strain gage circuits to provide large, singlestage amplification. i i G Figure A6. Instrumentation Amp Circuit

6 perational Amplifiers.6 Voltage Follower Circuit The voltage follower circuit shown in Figure A7 can be used to isolate or buffer a highimpedance voltage source (that cannot supply much current) from the following circuit components. The theoretical relationship is very simple, i A typical situation is shown on the right where the voltage follower is used to buffer a potentiometer so that it acts like an ideal voltage divider. V V V i V Figure A7. Voltage Follower Circuit Integrator The circuit shown below in Figure A8 can be used to integrate electrical signals with respect to time. For example, integrating an acceleration signal (from an accelerometer for example) would ideally give an output signal proportional to velocity. Note that the feedback resistor has been replaced by a capacitor, which accumulates current to generate a voltage. The ideal relationship for the integrator circuit is t idt(0) C 0 Note the output voltage depends on the initial voltage stored on the capacitor, (0). Integrator circuits are also very sensitive to leakage or null currents. C i V V o Figure A8. Integrating pamp Circuit Note that a differentiating circuit can be easily formed by swapping and C in the circuit above. The input/output equation becomes d i o C dt The use of differentiators is generally discouraged, since they tend to amplify any noise present in the input signal. Nevertheless, they are found in some specialized applications.