Chapter 11: The Operational Amplifiers

Chapter 11: The Operational Amplifiers

Operational Amplifiers (op-amp)
Op-amp is an electronic device that amplify the difference of voltage at its two inputs. It has two input terminals, one of the inputs is called the inverting input (-) and the other is called the non-inverting input. Usually there is a single output. Most op-amps operate with two dc supply voltages, one positive and the other negative, as shown in Figure 1, although some have a single dc supply. Usually these dc voltage terminals are left off the schematic symbol for simplicity but are understood to be there.

Figure 1:Op-amp symbols.

The Ideal Op-Amp
Ideally, op-amps have characteristics (used in circuit analysis):
• Infinite voltage gain.
• Infinite input impedance (does not load the driving sources).
• Zero output impedance (drive any load).
• Infinite bandwidth (flat magnitude response, zero phase shift).
• Zero input offset voltage.

The ideal op-amp has characteristics that simplify analysis of op-amp circuits. The input
voltage, Vin, appears between the two input terminals, and the output voltage is AvVin, as
indicated by the internal voltage source symbol. The concept of infinite input impedance is particularly a valuable analysis tool for several op-amp configurations.


Figure 2: Basic op-amp representations.
The Practical Op-Amp
Practical op-amps have characteristics that often can be treated as ideal for certain situations, but can never actually attain ideal characteristics. In addition to finite gain, bandwidth, and input impedance, they have other limitations.
• Finite open loop gain.
• Finite input impedance.
• Non-zero output impedance.
• Input current.
• Input offset voltage.
• Temperature effects.
Characteristics of a practical op-amp are very high voltage gain, very high input impedance, and very low output impedance. Another practical consideration is that there is always noise generated within the op-amp. Noise is an undesired signal that affects the quality of a desired signal. Today, circuit designers are using smaller voltages that require high accuracy, so low-noise components are in greater demand. All circuits generate noise; op-amps are no exception, but the amount can be minimized.