Open! Help you master the trade-off between power consumption and performance of operational amplifiers~

Because the battery that provides the power source has limited energy, it is necessary to use components that consume the least current to maximize the operating time of the device. Alternatively, by reducing power consumption, a lower capacity battery can achieve the same battery life while reducing size, weight and cost. Temperature management is also not to be ignored. Again, more efficient components play a positive role. Cooling management takes up space, and if less heat is generated, the space occupied will also be reduced. Currently, there are many low-power and even ultra-low-power (ULP) components available on the market. This article focuses on low-power operational amplifiers.

When choosing an appropriate amplifier, the power consumption of the operational amplifier often needs to be considered and a trade-off needs to be made.

Low power consumption often also means low bandwidth. However, this also depends on the given amplifier architecture and stability requirements. The higher the parasitic capacitance and inductance, the lower the bandwidth generally. For example, current feedback amplifiers offer relatively high bandwidth, but with low precision. There are some tricks we can use to improve the bandwidth-to-power ratio.

For example, the gain-bandwidth product (GBW) is generally given by:

Gm represents transconductance, or the ratio of output current to input voltage (I _OUT /V _{IN ), and} C represents the internal compensation capacitance.

The typical way to increase bandwidth is to increase the bias current, which increases G _m but consumes more power. To keep power low, we do not want to do this.

Typically, the compensation capacitor sets the dominant pole, so ideally the load capacitance does not affect the bandwidth at all.

Due to the physical characteristics of the amplifier, lower capacitance generally results in higher bandwidth, but this also affects stability, which improves at lower noise gains. However, in practice, we cannot drive large purely capacitive loads at lower noise gains.

Another trade-off when using low-power op amps is the typically higher voltage noise. However, input voltage noise is the most dominant noise in the amplifier (part of the total output broadband noise), but it can also be resistor noise. The most dominant part of the total noise can come from noise sources in the input stage (for example, scattering noise from the collector and thermal noise from the drain). 1/f noise (flicker noise) varies by architecture and is caused by specific imperfections in the component materials. Therefore, it is generally dependent on the size of the component. In contrast, current noise is generally lower at lower power levels. However, it is not negligible, especially in bipolar amplifiers. In the 1/f region, 1/f current noise is the dominant contributor to the total 1/f noise at the amplifier output. Other trade-offs include distortion performance and drift values. Low-power op amps generally exhibit higher total harmonic distortion (THD), but like current noise, input bias and offset currents in bipolar amplifiers decrease as the supply current decreases. Offset voltage is another important specification for op amps. The impact can usually be reduced by adjusting the components at the input so that performance is not greatly degraded at low power, so VOS and VOS drift are constant over the power range. The external circuitry and feedback resistors (R _F ) also affect the performance of the op amp. Higher resistor values ​​reduce dynamic power and harmonic distortion, but they increase output noise and errors associated with the bias current.

To further reduce power consumption, many devices offer standby or sleep capabilities. This allows important device functions to be deactivated when not in use and reactivated as needed. Low-power amplifiers typically have longer wake-up times. Table 1 summarizes and summarizes the trade-offs discussed previously.