Low Power Design Strategies for Operational Amplifiers

Introduction

In recent years, the popularity of battery-powered electronic products has made power consumption an increasingly important issue for analog circuit designers. This article will introduce how to use low-power operational amplifiers for system design, also covering low-power operational amplifiers with low supply voltage capabilities and their applications, discussing how to correctly understand the specifications in operational amplifier datasheets, and comprehensively considering energy-saving technologies in circuit design to achieve more efficient component selection.

Understanding Power Consumption in Operational Amplifier Circuits

First, we will discuss amplifiers with low quiescent current (I_Q) and the relationship between increasing the feedback network resistance and power consumption.

Let’s first consider an example circuit that may require attention to power: a battery-powered sensor generating a 50mV amplitude and 50mV offset analog sine wave signal at 1kHz. The signal needs to be amplified to a range of 0V to 3V for signal conditioning (Figure 1), while saving battery power as much as possible, which will require a gain of 30V/V in a non-inverting amplifier configuration, as shown in Figure 2. So, how should we optimize the power consumption of this circuit?

Low Power Design Strategies for Operational Amplifiers

Figure 1: Input and Output Signals in the Example Circuit (Image Source: Texas Instruments)

Figure 2: Sensor Amplifying Circuit (Image Source: Texas Instruments)

The power consumption of operational amplifier circuits consists of various factors, namely static power, output power of the operational amplifier, and load power. Static power (or P_Quiescent) is the power required to keep the amplifier on, typically represented by I_Q (quiescent current) in the datasheet, as shown in the display of the Texas Instruments OPA391 datasheet below.

Figure 3: Quiescent Current of TI OPA391 (Image Source: Texas Instruments)

Output power (P_Output) is the power consumed when the operational amplifier’s output stage drives the load. Finally, load power (P_Load) is the power consumed by the load itself.

In this example, we have a single-supply operational amplifier with a sine wave output signal having a DC voltage offset. Therefore, we will use the following equation to calculate the total average power (P_{total avg}). The supply voltage is represented by V₊, V_off is the DC offset of the output signal, V_amp is the amplitude of the output signal, and R_Load is the total load resistance of the operational amplifier. It is important to note that the average total power is directly proportional to I_Q and inversely proportional to R_Load.

Selecting Components with SuitableI_Q Values

Due to the multiple variables in the above equations 5 and 6, it is best to consider only one variable when selecting components. Choosing amplifiers with low I_Q is the most direct strategy to reduce overall power consumption. Of course, there are some trade-offs in this process. For example, devices with lower I_Q typically have lower bandwidth, higher noise, and may be more difficult to stabilize.

Since the I_Q of different types of operational amplifiers can vary by orders of magnitude, it is worthwhile to take the time to select the appropriate amplifier. The following compares TI’s TLV9042, OPA2333, OPA391, and TLV8802. Purely from a numerical analysis, for applications requiring maximum power efficiency, TLV8802 would be an excellent choice.

Table 1: Comparison of Various Low-Power Operational Amplifiers

Reducing the Resistance Value of Load Networks

Now let’s consider the remaining terms in equations 5 and 6. The V_amp term cancels out and has no effect on P_total,avg and V_off, which is typically predetermined by the application. In other words, the system cannot use V_off to reduce power consumption. Similarly, the V₊ rail voltage is usually set by the available supply voltage in the circuit. Additionally, R_Load is also predetermined by the application. However, R_Load includes any output components of the load, not just the load resistor R_L. In the case of the circuit shown in Figure 1, R_Load will include R_L and the feedback components R1 and R2. Thus, R_Load will be defined by equations 7 and 8 as follows.

By increasing the value of the feedback resistor, the output power of the amplifier in the system is also correspondingly reduced. This technique is particularly effective when P_output dominates P_Quiescent, but it also has its limitations. If the feedback resistor becomes significantly larger than R_L, then R_L will dominate R_Load, causing the power consumption to stop decreasing. Large feedback resistors can also interact with the input capacitance of the amplifier, making the circuit unstable and generating significant noise.

To minimize the noise generated by these components, it is best to compare the thermal noise of the equivalent resistance seen at each operational amplifier input (see Figure 4 below) with the voltage noise spectral density of the amplifier. A rule of thumb is to ensure that the input voltage noise density specification of the amplifier is at least three times greater than the voltage noise from the equivalent resistance observed at each input of the amplifier.

Figure 4: Resistor Thermal Noise (Image Source: Texas Instruments)

Real-World Examples

Using these low-power design techniques, let’s return to the initial question: a battery-powered sensor generating a 0 to 100mV analog signal at 1kHz requires a signal amplification of 30V/V. Figure 5 compares two designs. The design on the left uses a typical 3.3V power supply, resistors not considering energy savings, and the TLV9002 general-purpose operational amplifier. The design on the right uses larger resistor values and a lower power TLV9042 operational amplifier. Note that when the equivalent resistance at the inverting input of the TLV9042 is about 9.667kΩ, the noise spectral density is less than one-third of the amplifier’s broadband noise, ensuring that the noise of the operational amplifier dominates any noise generated by the resistors.

Figure 5: Typical Design vs. Subtle Design (Image Source: Texas Instruments)

Using the values, design specifications, and the specifications of the two operational amplifiers in Figure 5, we can use equation 6 to derive P_total,avg for the TLV9002 design and the TLV9042 design respectively. The results are shown in equations 9 and 10.

From the above results, it can be seen that the power consumption of the TLV9002 design is more than four times that of the TLV9042 design. This is the result of the higher amplifier I_Q, which also shows that using operational amplifiers with high I_Q, even when attempting to use low feedback resistor values, will not yield significant power savings. In the above example, we have two strategies: increasing the resistance values and selecting operational amplifiers with lower quiescent current. Both strategies are applicable in most operational amplifier applications.

Saving Power with Low Voltage Rails

Revisiting equations 1 and 6 defines the average power consumption of a single-supply operational amplifier circuit with a sine wave signal and DC offset voltage:

Additionally, V₊ in equation 6 represents the supply rail (V₊), which is directly proportional to power consumption, so setting the supply rail (V₊) to the lowest available supply voltage in the circuit is also a method to reduce power consumption. The minimum supply voltage range for many operational amplifiers is 2.7V or 3.3V. This limitation is due to the minimum voltage required to keep the internal transistors operating within the desired range. Some operational amplifiers are designed to work at voltages as low as 1.8V or even lower. For example, the TLV9042 general-purpose operational amplifier can operate at a 1.2V rail.

Battery-Powered Applications

Most of today’s sensors and smart devices are battery-powered, and the terminal voltage of the battery decreases from its nominal rated voltage during discharge. For example, a standard alkaline AA battery has a nominal voltage of 1.5V. The actual terminal voltage may be close to 1.6V during the first unloaded measurement. As the battery discharges, that terminal voltage may drop to 1.2V or even lower.

Using operational amplifiers that can work at voltages as low as 1.2V instead of those requiring higher voltages provides the following advantages:

1. Operational amplifier circuits will continue to function longer even when the battery is nearing the end of its charge cycle and its terminal voltage drops.

2. Operational amplifier circuits can operate with a single 1.5V battery instead of requiring two batteries to form a 3V supply rail.

To understand why operational amplifiers with lower operating supply voltages can achieve longer battery life, consider the battery discharge curve shown in Figure 6. Batteries typically exhibit a discharge cycle similar to this curve. The terminal voltage of the battery will start close to its nominal rated value. As the battery discharges over time, the terminal voltage gradually decreases. Once the battery approaches the end of its charge, the terminal voltage will drop rapidly. If the operational amplifier circuit is only designed to operate at voltages close to the battery’s nominal voltage, such as V₁, the operating time t₁ will be very short. However, using operational amplifiers that can work at slightly lower voltages, such as V₂, can significantly extend the battery’s operating time t₂.

Figure 6: Typical Discharge Curve of a Single Cell Battery (Image Source: Texas Instruments)

While this effect will vary depending on battery type, load, and other factors, it is clear that operational amplifiers with low operating supply voltages will have longer operating times.

Low Voltage Digital Logic Levels

Applications using low voltage rails for both digital and analog circuits can also benefit from low-power operational amplifiers with low supply voltage capabilities. Digital logic has standard voltage levels ranging from 5V to 1.8V and below (Figure 7). Similar to operational amplifier circuits, digital logic becomes more energy-efficient at lower voltages. Therefore, lower digital logic levels are often preferred.

To simplify the design process, you may choose to use the same supply voltage level for both your analog and digital circuits. In this case, operational amplifiers with 1.8V capability (such as high-precision, wide-bandwidth OPA391 or cost-optimized TLV9001) can prove advantageous. However, it is important to note that if the design is required to operate at a 1.2V digital rail, the system must ensure that any noise that may leak from the digital circuit to the analog device power pins is cleared.

Figure 7: Standard Logic Levels (Image Source: Texas Instruments)

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