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Ideal Op Amp Attributes

An ideal op amp has infinite gain for differential input signals. In practice, real devices will have quite high gain (also called open-loop gain) but this gain won’t necessarily be precisely known. In terms of specifications, gain is measured in terms of VOUT/VIN, and is given in V/V, the dimensionless numeric gain. More often however, gain is expressed in decibel terms (dB), which is mathematically dB = 20 • log (numeric gain). For example, a numeric gain of 1 million (106 V/V) is equivalent to a 120 dB gain. Gains of 100-130 dB are common for precision op amps, while high speed devices may have gains in the 60-70 dB range.
Also, an ideal op amp has zero gain for signals common to both inputs, that is, common mode (CM) signals. Or, stated in terms of the rejection for these common mode signals, an ideal op amp has infinite CM rejection (CMR). In practice, real op amps can have CMR specifications of up to 130 dB for precision devices, or as low as 60-70 dB for some high speed devices.
The ideal op amp also has zero offset voltage (VOS=0), and draws zero bias current (IB=0) at both inputs. Within real devices, actual offset voltages can be as low as a μV or less, or as high as several mV. Bias currents can be as low as a few fA, or as high as several μA. This extremely wide range of specifications reflects the different input structures used within various devices, and is covered in more detail later in this chapter.
The attribute headings within Figure 1-1 for INPUTS and OUTPUT summarize the above concepts in more succinct terms. In practical terms, another important attribute is the concept of low source impedance, at the output. As will be seen later, low source impedance enables higher useful gain levels within circuits.
To summarize these idealized attributes for a signal-processing amplifier, some of the traits might at first seem strange. However, it is critically important to reiterate that op amps simply are never intended for use without overall feedback! In fact, as noted, the connection of a suitable external feedback loop defines the closed-loop amplifier’s gain and frequency response characteristics.
Note also that all real op amps have a positive and negative power supply terminal, but rarely (if ever) will they have a separate ground connection. In practice, the op amp output voltage becomes referred to a power supply common point. Note: This key point is further clarified with the consideration of typically used op amp feedback networks.
The basic op amp hookup of Figure 1-2 below applies a signal to the (+) input, and a (generalized) network delivers a fraction of the output voltage to the (−) input terminal. This constitutes feedback, with the op amp operating in closed-loop fashion. The feedback network (shown here in general form) can be resistive or reactive, linear or nonlinear, or any combination of these. More detailed analysis will show that the circuit gain characteristic as a whole follows the inverse of the feedback network transfer function.
A generalized op amp circuit with feedback applied
Figure 1-2: A generalized op amp circuit with feedback applied
The concept of feedback is both an essential and salient point concerning op amp use. With feedback, the net closed-loop gain characteristics of a stage such as Fig. 1-2 become primarily dependent upon a set of external components (usually passive). Thus behavior is less dependent upon the relatively unstable amplifier open-loop characteristics.
Note that within Figure 1-2, the input signal is applied between the op amp (+) input and a common or reference point, as denoted by the ground symbol. It is important to note that this reference point is also common to the output and feedback network. By definition, the op amp stage’s output signal appears between the output terminal/feedback network input, and this common ground. This single relevant fact answers the "Where is the op amp grounded?" question so often asked by those new to the craft. The answer is simply that it is grounded indirectly, by virtue of the commonality of its input, the feedback network, and the power supply, as is shown within Fig. 1-2.
To emphasize how the input/output signals are referenced to the power supply, dual supply connections are shown dotted, with the ± power supply midpoint common to the input/output signal ground. But do note, while all op amp application circuits may not show full details of the power supply connections, every real circuit will always use power supplies!
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