WEARABLE TECH Power Performance Trade-
Offs in Operational Amplifiers Thomas Brand, Field Applications Engineer, Analog Devices
Figure 1. An example of a simplified signal chain for a high-resolution data acquisition system.
H
igh performance with low power consumption; this requirement is coming
to bear in more and more applications, especially when it comes to mobile, battery- operated devices. Particularly in the times of IoT, Industry 4.0, and digitalisation, these handhelds make many aspects of daily life easier. This is true in applications ranging from mobile vital sign monitoring to the monitoring of machines and systems in an industrial setting. Demands for higher performance and maximum battery life are also seen in end user products, such as smartphones and wearables.
The limited battery energy available for supplying power necessitates efficient components with minimal current in active mode to maximise device runtime. Alternatively, lower power consumption allows one to achieve the same battery life with a lower capacity battery, reducing size, weight, and cost. Temperature management also should not be neglected. Here, too, more efficient components play a positive role. Cooling management, which takes up space, can be reduced because of the lower amount of generated heat. There is an extensive range of low power and even ultralow power (ULP) components available. This article focuses specifically on low power op amps.
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Trade-Offs Between Power Consumption and Performance
There are often trade-offs related with the power consumption of an operational amplifier that need to be considered in the selection of a suitable amplifier. Lower power often also means lower bandwidth. However, this also depends on the given amplifier architecture and stability requirements. The higher the parasitic capacitances and inductances, usually the lower the bandwidth. Thus, for example, transimpedance amplifiers (current feedback amplifiers) offer a relatively high bandwidth, but with lower precision. With a few tricks, the bandwidth-to-power ratio can be improved. For example, the gain bandwidth (GBW) is typically as follows:
Usually, the compensation capacitance should set the dominant pole, so ideally the load capacitance wouldn’t affect the bandwidth at all.
A lower capacitance yields typically to a higher bandwidth, which is then limited by the physical characteristics of the amplifier, but it will also hurt the stability, whereas it usually results in improved stability at a low noise gain. Nevertheless, in reality we can’t drive as large of a purely capacitive load at lower noise gains.
Gm is the transconductance, or the ratio between the output current and the input
voltage (IOUT/VIN), and C is the internal compensation capacitance.
The classic way to increase bandwidth is to increase bias currents, which will
increase Gm at the expense of more power consumption. We don’t want to do that for lower power.
APRIL 2024 | ELECTRONICS FOR ENGINEERS
Another trade-off in the use of low power op amps is the often-higher voltage noise. However, input referred voltage noise will typically be the amp’s dominant contributor to the total output broadband noise, but it could be dominated by resistor noise. The total noise is typically dominated by the noise sources in the input stage; for example, collectors have shot noise and drains have thermal noise. The 1/f noise (flicker noise) varies depending on the architecture and is caused by special defects in the component materials, among other things. Thus, it is typically dominated by the component size. In contrast, the current noise is usually lower at lower power levels. However, especially in bipolar amplifiers, it should also not be neglected. In the 1/f region, 1/f current noise can be the dominant contributor to total
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