In 1958. the age of the integrated circuit was ushered in by Jack Kilby of Texas Instruments. From the two hand-built circuits which he fabricated. the variety and quantity of integrated circuits have mushroomed at an ever increasing rate. One type of integrated circuit is the operational amplifier that is characterized by its high gain and versatility. Because of its versatility and ease of application, the operational amplifier has become one of the most widely used linear integrated circuits. Operational amplifiers are designed to be used with external components to provide the desired transfer functions.
The rapid evolution and versatility of the operational amplifier is shown by its initial development and use. One of the two hand-built integrated circuits which Jack Kilby built was a phase shift oscillator, the first linear integrated circuit. This was soon followed by the introduction of the uA702 and SN523 operational amplifiers. Even with their lack of short-circuit protection and their requirements for complex compensation they quickly gained acceptance. Among the improved designs which quickly followed was the uA741 single operational amplifier which required no external compensation. Conversely, the uA748 was designed for compensation by external components to change the frequency response for applications requiring wider bandwidth and higher slew rate.
Operational amplifier capabilities and versatility are enhanced by connecting external components to change the operating characteristics. Typical operational amplifier characteristics include frequency response, signal phase shift, gain and transfer function. The external components are placed in one or more feedback networks and/or the circuits that terminate the input.
To adequately evaluate the potential of an operational amplifier for a specific application. an understanding of operational amplifier characteristics is required. Figure 2-1 represents an equivalent operational amplifier circuit and its parameters. The parameters illustrated in Figure 2-1 are as follows:
- Input bias currents (lIBI and 1182)- the current flowing into both operational amplifier inputs. In an ideal condition. IIBI and 1IB2 are equal.
- Differential input voltage (VOl) – the differential input voltage between the non inverting (+) input and the inverting ( – ) input.
- Input offset voltage (VIO) an internally generated input voltage identified as the voltage that must be applied to the input terminals to produce an output of 0V.
- Input resistance (RI) – the resistance at either input when the other input is grounded.
- Output voltage (V0) – normal output voltage as measured to ground.
- Output resistance (RO) – resistance at the output of the operational amplifier.
- Differential voltage gain (AYD) or open-loop voltage gain (AOL) – the ratio of the input voltage to the output voltage of the operational amplifier without external feedback.
- Bandwidth (BW) – the band of frequencies over which the gain (VO/VOI) of the operational amplifier remains within desired limits.
The generator symbol (G) in Figure 2-1 represents the output voltage resulting from the product of the gain and the differential input voltage (AVD VDl).
An ideal operational amplifier (see Figure 2-2) provides a linear output voltage that is proportional to the difference in voltage between the two input terminals. The output voltage will have the same polarity as that of the noninverting ( +) input with respect to the voltage at the inverting ( – ) input. When the noninverting input is more positive than the inverting input. the output voltage will have a positive amplitude. When the noninverting input is more negative than the inverting input. the output voltage will have a negative amplitude.
An operational amplifier having no external feedback from output to input is described as being in the open-loop mode. In the open-loop mode, the characteristics of the ideal operational amplifier are as follows:
- Differential gain = -> ∞
- Common-mode gain = -> 0
- Input resistance = -> ∞
- Output resistance = -> 0
- Bandwidth = -> ∞
- Offset and drift = -> 0