Ideal Op Amp

Sunday, April 29th, 2018 - Analog Electronics

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 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.

Defining the Ideal Op Amp

  • Gain: The primary function of an amplifier is to amplify, so the more gain the better. It can always be reduced with external circuitry, so we assume gain to be infinite.
  • Input Impedance: Input impedance is assumed to be infinite. This is so the driving source won’t be affected by power being drawn by the ideal operational amplifier.
  • Output Impedance: The output impedance of the ideal operational amplifier is assumed to be zero. It then can supply as much current as necessary to the load being driven.
  • Response Time: The output must occur at the same time as the inverting input so the response time is assumed to be zero. Phase shift will be 180°. Frequency response will be flat and bandwidth infinite because AC will be simply a rapidly varying DC level to the ideal amplifier.
  • Offset: The amplifier output will be zero when a zero signal appears between the inverting and non-inverting inputs.

The Ideal Op Amp and Its Attributes

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First, an operational amplifier (hereafter simply op amp) is a differential input, single ended output amplifier. This device is an amplifier intended for use with external feedback elements, where these elements determine the resultant function, or operation. This gives rise to the name “operational amplifier,” denoting an amplifier that, by virtue of different feedback hookups, can perform a variety of operations. At this point, note that there is no need for concern with any actual technology to implement the amplifier. Attention is focused more on the behavioral nature of this building block device.

An op amp processes small, differential mode signals appearing between its two inputs, developing a single ended output signal referred to a power supply common terminal. Summaries of the various ideal op amp attributes are given in the figure. While real op amps will depart from these ideal attributes, it is very helpful for first-level understanding of op amp behavior to consider these features.

It is also worth noting that this op amp is shown with five terminals, a number that happens to be a minimum for real devices. While some single op amps may have more than five terminals (to support such functions as frequency compensation, for example), none will ever have less. By contrast, those elusive ideal op amps don’t require power, and symbolically function with just four pins of ideal op amp.

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