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Refresh! Operational Amplifiers

By Andrew Leone, Contributing Editor

Op-amps are among the most widely used electronic devices, and are designed into many consumer, industrial, and scientific devices. General purpose, standard op amps sell for well under one dollar. But some integrated amplifiers with specialized features can cost in the $50 to $100 range. With only a handful of external components, the op amp can perform a wide variety of analog signal processing tasks.

What is an OP AMP?
The operational amplifier is a primary building block for analog systems that require amplification, active filtering, signal manipulation and other tasks.

Op amps were originally designed for use in analog computers to perform mathematical functions. Operations such as addition, subtraction, integration, and differentiation are examples of the mathematical operations possible with op amps.

The op amp symbol shown in Figure 1 is almost a "black box" for the designer. The designer doesn't necessarily care what is inside the op amp, but cares more about how the op amp behaves in certain configurations.

Op amps employ transistors (or long ago, tubes) as the amplification component. Resistors and capacitors are also used in conjunction with the amplification unit. Op amps can be implemented as discrete or integrated circuits. For ease in making calculations, op amps are often referred to as "ideal." Fortunately, the ideal op amp comes fairly close to the non-ideal or "real world" performance of an op amp. It is important, though, to understand the differences between ideal and non-ideal op amp conditions in order to make intelligent decisions about circuit design.

Today's op amps can reside in a single integrated circuit with transistors, resistors and capacitors in a monolithic package. In addition, more and more op amps incorporate specialized functions for better performance and application specific requirements.

Ideal OP AMPS
Figure 1 shows a generic symbol for the op amp. It has two input terminals (Vin+ and Vin-) and one output terminal (Vout). Op amps require DC power sources to operate. Most require two dc supplies on terminals labeled VCC+ and VCC-, but a single DC supply can be used in certain applications and topologies. This is known as single-supply operation.

The ideal op amp senses the difference between the voltage signals applied at the two input terminals. The difference can then be multiplied a number "A" also known as the "open loop gain," which causes the resulting voltage, A(Vin+ - Vin-) to appear at the output signal, Vout.

From the analysis of the ideal op amp, certain assumptions can be drawn for the purpose of simplifying calculations.

The ideal op amp should not draw any input current. The signal current into Vin+ and Vin- should essentially be zero, and in turn the input impedances of the two signal lines (Vin+ and Vin-) should be infinite.

The output voltage, Vout, is supposed to act as the output terminal of an ideal voltage source. In other-words, the voltage between Vout and ground will always be equal to A(Vin+ and Vin-). For an ideal op amp, the output impedance of Vout should be zero.

Obviously, the above parameters are not true in the real world. They are used only for the purpose of simplifying equations for op amp calculations.

Inverting and Non-Inverting INPUTS AND FEEDBACK
The op amp's inputs are made up of an inverting input (Vin-) and a non-inverting input (Vin+). In the ideal case, the op amp amplifies the difference in voltage between the two inputs. The difference in voltages is referred to as the " differential input voltage." In the op amp's most common use as an amplifier, its output voltage is manipulated by using a feedback control loop as shown in Figure 2. This is accomplished by feeding a fraction of the output signal (Vout) back to the inverting input (Vin-). This topology is known as negative feedback.

There are occasions where positive feedback is used. This is where a fraction of the output is fed back to the non-inverting input (Vin+). This topology is not used in amplifier circuits. It is for use in comparators and other non-amplifying circuits.

When no negative feedback exists, the op amp circuit is considered to be running "open-loop" and its output is the differential input voltage (Vin+ - Vin-) multiplied by the total gain of the amplifier (A).

An ideal op amp has infinite open-loop gain, infinite bandwidth, and infinite input impedance resulting in zero input current, infinite slew rate, zero output impedance and zero noise.

Real op amps can be very close, but never exactly like the ideal case. In fact, the actual parameters can drift over time and with changes in temperature and input conditions. With advances in transistor technologies, op amps approximate more closely to the ideal case, but cannot reach the ideal.

Non-Ideal Op Amps
THE NON-IDEAL OP AMP HAS THE FOLLOWING DEVIATIONS FROM THE IDEAL:

  • Finite gain
  • Finite input resistance
  • Non-zero output resistance
  • Input bias current
  • Input offset voltage
  • Finite bandwidth

Although there are deviations, the actual measurements are fairly close to the ideal. Also, the importance of a particular parameter depends on the application.

Op amps are designed to have very high voltage gain. The voltage difference between the inverting and non-inverting inputs is multiplied by the voltage gain to determine the output. Some op amps have the ability to show voltage gains of more than 100,000. However, Vout cannot exceed the positive dc power supply voltage or go below the negative dc supply voltage. As a rule of operation, the gain is limited by the difference of the incoming signal.

As mentioned, the inputs of an op amp have high input impedance. Because input impedance is defined as the effective resistance between the input terminals and because there is no current flow between the two input terminals, there is high input impedance. As for output impedance, the op amp will have low output impedance. The output impedance is defined as the effective internal resistance in series with the output. When external resistors and capacitors are added, the output resistance of the op amp is reduced almost to zero.

One very important parameter of op amps that makes them effective in sensor inputs and other noisy aspects of the analog world is common-mode rejection. Since op amps amplify the difference between the two inputs, any signal that is superimposed equally on the inputs will be ignored. This is a handy function. The canceling out of noise on both inputs makes the op amp very useful in communication as well as sensor input applications. Noise becomes amplified just as easily as the signal, but the op amp has the ability to remove the noise.

Application Circuit: Instrumentation Amplifier
Instrumentation amplifiers are an example of multiple op amps in a configuration with resistors to create a useful application circuit. An instrumentation amplifier is a type of differential amplifier that is specifically designed to meet characteristics for use in measurement and test equipment. These characteristics include very low DC offset, low drift, low noise, very high openloop gain and very high input impedances.

The instrumentation amp is needed where accuracy and stability necessary. The "common" topology for an instrumentation amp circuit is shown in Figure 3.

Manipulating Rgain varies the gain. In order to be most effective, the other resistors should be close to the same resistance, and the two input op amps need to share close to identical characteristics in order to be as stable and useful as possible.

Manufacturers of Op Amps
Many semiconductor companies manufacture op amps. These companies make sole sourced as well as commodity, multi-sourced devices. For a listing, check out www.eepn.com/sitecentral.

Company: EEPN

Product URL: Click here for more information

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