Power operational amplifiers (POA) are used to increase the power of low-level signals in applications that drive low impedances or reactive loads. They dissipate excess energy as heat, deliver extensive current, and can sustain relatively high supply voltages. Most devices have impedances that are 10 to 100 times lower than small-signal operational amplifiers and can deliver larger amounts of current and dissipate more power. There are several operational classes for power operational amplifiers. Class A devices are the most linear and the least efficient. Class AB designs provide increased efficiency and excellent linearity. Class B amplifiers are used almost exclusively in low-power applications. Class C devices are used with radio frequency (RF) transmissions. Class D amplifiers switch on and off at least two times per cycle while Class E units are designed for rectangular input pulses rather than sinusoidal audio waveforms. Class F power operational amplifiers include both RF and microwave devices. Glass G designs change the power supply voltage from a lower level to a higher level when larger output swings are required. Class H models modulate the higher power supply voltage through the input signal.
Selecting Power Operational Amplifiers
Selecting power operational amplifiers requires an analysis of performance specifications. Output peak current is the maximum current that can be delivered in the output. Output voltage swing is the maximum voltage output. Open-loop gain is the ratio between the input and output signals. Gain bandwidth product is the minimum value of the small-signal gain bandwidth at a specified frequency under reference conditions. Slew rate is the maximum rate of change of the output signal in response to a step input. Common mode rejection ratio (CMRR) measures a device’s ability to output 0 V when two inputs have the same voltage. Input offset voltage is the amount of DC voltage that power operational amplifiers produce when 0 V is applied to the input. Input bias current is the DC current required by the inputs to drive the first stage. Input impedance is the total impedance from the input terminals. Other performance specifications for power operational amplifiers include supply voltage range, internal power dissipation, quiescent current, and power bandwidth.
Features for power operational amplifiers include on-chip electrostatic discharge (ESD) protection, rail-to-rail outputs, and embedded current limits. Some devices are designed for audio, video, or stereo applications. Others allow small signal stages to be operated at higher supply voltages. Suppliers indicate when amplifiers meet temperature and operating range requirements for automotive, commercial, industrial, medical, or military applications.
Power operational amplifiers are available in a variety of integrated circuit (IC) package types and with different numbers of pins. Basic IC package types include single in-line package (SIP), dual in-line package (DIP), discrete package (DPAK), small outline package (SOP), and quad flat package (QFP). Many packaging variants are available. For example, common SOP variants include shrink small outline package (SSOP) and thin shrink small outline L-leaded package (TSSOP). Small outline integrated circuit (SOIC) packaging is also available for PWM amplifier chips. TO-3 is a transistor outline (TO) package with three leads. TO-92, another transistor outline package, is often used for low power devices. By contrast, TO-220 is suitable for high power, medium current, and fast-switching power devices.
Many suppliers specify power operational amplifiers according to EIA-724, a product lifecycle model from the Electronics Industry Alliance (EIA), a trade association that establishes standards for electrical and electronics products. The numbered lifecycle stages in EIA-724 describe activities such as product ramp-up, rapid growth, maturity, saturation, decline, and phase-out. Some manufacturers also specify lifecycle stages that are not part of EIA-724. Examples include product introduction, last shipments, and removal.