Introduction#
Operational amplifiers (OpAmps) are high-gain voltage amplifiers that have a differential input and usually a single-ended output and that operate from DC. They were originally developed as controllers in feedback circuits that performed mathematical operations in analog computers.
Before integrated circuit operational amplifiers became available, there already existed operational amplifiers with vacuum tubes (Fig. 270), and operational amplifier modules constructed with discrete semiconductors.
The first integrated-circuit operational amplifier, realized in a bipolar IC technology, was the uA702 (Bob Widlar, Fairchild 1964). Since then, JFET (1970s), MOSFET (1980s) and BiMOS operational amplifiers have become available, and their performance is continuously improved.
The operational amplifier is a general-purpose amplifier, intended to be used as loop amplifier or controller in circuits that exploit negative feedback. Its intended function thus differs from the one given in section Functionality. For its application as loop amplifier or controller , it needs to provide a huge gain over a wide frequency range, with less than \(180\) degrees of phase shift. This will become clear after studying the chapter on the modeling of negative feedback amplifiers (Chapter Modeling of negative feedback circuits).
In modern embedded systems (systems that incorporate a microprocessor), operational amplifiers are mainly used in applications such as:
Loop amplifiers in analog active filters
Input amplifiers for amplification of low-level signals before A/D conversion can take place
D/A converter output amplifiers for driving actuators and transmission lines
Voltage buffers, summing amplifiers, etc.
Operational Amplifier types#
Although the operational amplifier was originally intended to be a general-purpose building block, a huge number of versions have become available. Obviously, the operational amplifier is not particularly ‘general-purpose’ at all, and the immense diversity of its application justifies this apparently unlimited number of device types. Manufacturers of operational amplifiers provide the designers with selection tables to narrow the search for devices.
Another consequence of this trend is that data sheets of operational amplifiers often specify the devices for typical applications. If the application to be designed strongly deviates from this typical application, simulation and breadboarding are required to ensure performance to requirements.
Architecture#
Integrated circuit operational amplifiers have been designed such that a positive current flow through the load (output source current ) is delivered by the positive power supply and the negative current flow through the load (output sink current ) is delivered by the negative power supply. This is shown in Fig. 271. A small and almost signal-independent quiescent current flows through the operational amplifier from the positive to the negative power supply terminal (Class AB output stages).
There exist different architectures for operational amplifiers:
Voltage-feedback operational amplifiers
Voltage-feedback operational amplifiers have a balanced input stage that converts the differential input voltage into a current that drives an output stage with a transimpedance character. The output current of this type of input stage is limited. Parasitic capacitances, as well as the limited current-drive capability of the input stage, may seriously limit the rate of change of the output voltage (slew-rate). Since the input stage of voltage-feedback operational amplifiers is well-balanced, the input offset voltage and the offset current of these operational amplifiers can be very low. This makes these amplifiers very well suited for applications in which a high low-frequency (DC) accuracy is required.
Voltage-feedback operational amplifiers may have rail-to-rail input stages. In fact, such amplifiers have two input stages in parallel. One operates at common-mode voltages up to the positive supply voltage and the other down to the negative supply voltage. Due to this architecture, the behavior of these devices may change with the common-mode input voltage.
Current-feedback operational amplifiers
Current feedback operational amplifiers have a push-pull class AB input stage with a high-current-drive capability. This input stage drives an output stage with a transimpedance character. Due to the high current-drive capability of the input stage, current-feedback operational amplifiers can have a very high rate of change of the output voltage. This makes them very well suited for high-speed applications. The input stage of the current-feedback operational amplifier, however, is strongly asymmetrical. This results in a relatively large offset voltage and offset current when compared to a voltage-feedback type.
Auto-zero operational amplifiers
Auto-zero techniques are often applied for compensation of offset (zero) errors. Auto-zero operational amplifiers have very low offset and are mainly intended for low-frequency applications in which DC accuracy and temperature stability are of utmost importance.
Fully-differential operational amplifiers
Relatively new to the family of operational amplifiers are the so-called fully-differential operational amplifiers. They are mainly intended for driving high-speed differential-input ADCs in digital radio and high-speed instrumentation systems with sampling frequencies in the GHz range. Fully differential operational amplifiers usually offer a limited design flexibility.
Idealized models#
As mentioned earlier, operational amplifiers are intended to be used as loop amplifiers or controllers in negative-feedback circuits. In Chapter Design of feedback amplifier configurations we introduced the nullor as the ideal controller. The operational amplifier is a practical implementation of the nullor. In fact, an operational amplifier can be regarded as a high-gain voltage-controlled voltage source, which is one of the possible controlled source approximations of the nullor.
Single-ended output operational amplifier#
Fig. 272 shows the nullor, the VCVS approximation of the nullor and the implementation with a single-ended output operational amplifier. One of the terminals of the norator is connected to the power supply terminals. In other words, the return path for the output current is the power supply.
Fig. 272 The operational amplifier can be regarded as an implementation of the nullor. One of the terminals of the norator is connected to the supply ground. Please notice that the nullator does not have inverting and non-inverting terminals. See Chapter Modified Nodal Analysis for the network equations of the nullor.#
Fully differential operational amplifiers#
Fig. 273 shows an idealized model for a fully-differential operational amplifier. These amplifiers usually have a control input for the common-mode output voltage.
Fig. 273 Implementation of the nullor with a fully-differential operational amplifier.#
This chapter#
In section Characterization of operational amplifiers, we will briefly discuss the characterization of operational amplifiers. Most of the parameters that describe the characteristics of these devices have already been introduced in Chapter Modeling and specification of amplifiers.
In section Modeling of the operational amplifier, we will discuss the modeling of operational amplifiers. Although it seems attractive to use so-called macro models for numeric simulation with SPICE, we will not pay much attention to this. This is because manufacturers of operational amplifiers still encourage the use of data sheets for design, and prototyping for design verification. Moreover, not all aspects have been modeled correctly in these macro models. Verification of the macro models with test circuits that correspond to those given in the data sheets , is indispensable for reliable design.
Instead of working with one complete model for numeric simulation, we will pay attention to the modeling of individual performance aspects, both symbolically and numerically. This gives the designer much more control over the level of complexity of the models used at different stages of design.