COVER STORY
Operational amplifiers adapt to the requirements of modern circuit design
Whilst the principle of the op-amp remains the same, their practical implementation has witnessed significant transformation. Today’s devices offer improved performance, enhanced precision, higher speeds, reduced power consumption, and increased versatility, addressing the requirements of today’s complex applications. In this month’s edition, Andrew Pockson, engineering manager at Anglia, brings you up to date with how this basic component has evolved, discussing the key considerations when specifying an op amp and offering an overview of products available from STMicroelectronics that cater to the requirements of modern applications.
Introduction
The enduring appeal of op amps lies in their ability to provide essential functions such as signal amplification, buffering, filtering, and mathematical operations. Their versatility and reliability have made them indispensable in various fields, including telecommunications, audio systems, industrial control, instrumentation, medical devices, and many more. The operational amplifier (op amp), a true cornerstone of circuitry, continues to play a significant role in a wide array of applications. With a history dating back to its invention by Karl Dale Swartzel Jr. in 1941, op amps have maintained their relevance and are still extensively utilized today. These powerful devices serve numerous tasks that include amplifying both AC and DC signals, providing signal buffering to maintain signal integrity, implementing effective filtering techniques, delivering gain and level shifting capabilities, enabling robust signal driving capabilities, and even performing mathematical operations.
Important op amp parameters
Op amps have gained widespread popularity and are often seen as commodity components. However, it’s important to recognize that they are far from generic. Due to the diverse requirements of demanding applications, op amps must exhibit remarkable versatility. Unfortunately, there isn’t a one-size-fits-all op amp that can fulfil every design need. Hence, modern high-performance op amps come
with a wide range of performance metrics. To ensure optimal performance, designers must consider the key parameters when selecting the right op amp for a specific application. These parameters are crucial for determining suitability and are defined as follows: Supply Voltage (V): The effective voltage range the device can operate over. ST offer products such as the TSB182, a precision amplifier that can operate over a wide voltage range up to 36V.
10 July/August 2023 Components in Electronics
Quiescent Current (µA): The amount of current the device consumes when it’s not actively driving any load or in an idle state. ST provides op amps with extremely low current consumption, which helps extend battery life significantly. For instance, the TSU series offers devices with ultra-low power consumption, typically consuming only 580 nA and at maximum 750 nA per channel when supplied with 1.8V. Input offset voltage (mV): The voltage difference between the input terminals required to nullify the output voltage. The TSZ122 offers low input offset voltages for precision measurement with very high accuracy, ideal for amplification of very small signals. Input bias current (pA): Average of the current drawn by the input terminals necessary for normal operation of the op amp.
Gain bandwidth product (MHz): Product of an op amp’s gain and bandwidth. It is measured at 20 dB gain and is defined for small signals. Devices such as the TSV792 offer high bandwidth (50MHz) combined with a low offset voltage and Rail-to-Rail capability, ideal for handling high speed signals and high order filters. Slew rate (V/µs): The maximum rate at which the output voltage can change in response to a rapid input change. The output rate of change is limited to the slew rate value, signal distortion can occur if the signal to be amplified is too fast. The TSV782 from ST features a slew rate of 20V/µs,
this makes it suitable for low-side current measurement and overcurrent protection.
Rail-to-rail input and output: Rail-to-rail input op amps can handle input signals from Vcc- to Vcc+ and/or have capability to drive the output very close to the power supply rails. TSV772 and TSV792 mentioned above are just a couple of examples of Rail-to-Rail op amps available from ST, these are important for low voltage applications. Noise level (nV/VHz): The unwanted fluctuations or disturbances that affect the clarity of the amplified signal. Op amps generate noise at the output even when there is no signal applied on the input, this noise can come from the thermal noise (white noise) or 1/f noise, also called flicker noise. Noise level may become considerable in applications with high gain or bandwidth. Stability: For precise measurement, stability of the op amp is critical, some of the key parameters which effect stability are Gain margin: which is the point where there has already been a phase shift of 180°. If the gain at this point is more than unity the op amp will be unstable. Phase margin: the difference between the phase when the gain is unity (0 dB) and 180°, if at 0 dB the phase lag is greater than 180° the op amp will be unstable. Compensation: To stabilize an op amp, it may be necessary to use compensation methods, following are 3 common methods employed.
Fig.1 - Out-of-the-loop compensation schematic
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