Low-Power Design of Operational Amplifiers

Ⅰ. Understanding Power Dissipation in Op Amp Circuits

First, we'll look at amplifiers with low quiescent current (I_Q) and how changing the feedback network's resistor value affects power dissipation.

Consider the following circuit, which uses a battery-powered sensor to generate an analog sinusoidal signal with 50mV amplitude and 50mV offset at 1kHz. For signal conditioning, the signal must be increased to the 0V to 3V range (Figure 1). To save battery power as much as possible, a noninverting amplifier architecture with a gain of 30V/V is required (Figure 2). So, how should we reduce the circuit's power consumption?

Figure. 1 Input and Output Signals in an Example Circuit

Figure. 2 Sensor Amplifier Circuit

Static power, op amp output power, and load power all contribute to the power dissipation of an op amp circuit. Quiescent power (or P_Quiescent for short) is the power required to keep the amplifier turned on, and it's usually written in datasheets as I_Q (quiescent current), like the one below.

Figure. 3 Quiescent Current of an Op Amp

The power utilized by the op amp output stage when driving a load is known as output power (P_Output). Finally, load power (P_Load) refers to the amount of energy consumed by the load.

We have a single-supply op amp with a sinusoidal output signal with a DC voltage offset in this example. As a result, we'll apply the equation below to figure out the overall average power (P_total,avg). V+ is the supply voltage, V_off is the output signal's DC offset, V_amp is the output signal's amplitude, and R_Load is the op amp's total load resistance. It's worth noting that overall power is related to I_Q and inversely proportional to R_Load.

Figure. 4

Ⅱ. Choose Components with the Right I_Q

Because there are so many variables in Equations 5 and 6, it's preferable to focus on just one when choosing materials. The most obvious technique for reducing total power consumption is to select an amplifier with a low I_Q. There are, of course, certain sacrifices in the process. Devices with a lower I_Q, for example, often have less bandwidth, and more noise, and are more difficult to stabilize.

Because the I_Q of different types of op amps might differ by a factor of ten, it's important to spend the time to find the perfect one. The following references are for comparison: TI TLV9042, OPA2333, OPA391, and TLV8802. The TLV8802 would be an excellent choice for applications that require maximum power efficiency based on purely numerical analysis.

Figure. 5 Comparison of Various Types Of Low-Power Op Amps

Ⅲ. Reduce the Resistance Value of the Load Network

Proceed to Equations 5 and 6 to consider the remaining terms. The V_amp terms cancel each other out, therefore P_{total, avg}, and V_off, which are normally predetermined by the application, are unaffected. In other words, the system is unable to cut power usage by using V_off.

Similarly, the V+ rail voltage is normally determined by the circuit's supply voltage. Furthermore, R_Load is determined by the program. R_Load, on the other hand, refers to any load output, not simply the load resistor R_L. R_Load would contain R_L and the feedback components R1 and R2 in the circuit shown in Figure 1. As a result, equations 7 and 8 will define R_Load as follows:

Figure. 6

The output power of the amplifier in the system is reduced by raising the value of the feedback resistor. This approach is very useful when P_output outnumbers P_Quiescent, but it has its drawbacks. If the feedback resistance exceeds R_L by a substantial amount, R_L will take control of R_Load, causing the power dissipation to plateau. The enormous feedback resistor interacts with the input capacitance of the amplifier, causing the circuit to become unstable and noisy.

It's recommended to compare the thermal noise of the corresponding resistor seen at each op amp input (see Figure 7 below) with the voltage noise spectral density of the amplifier to reduce noise contribution from these components. As a general rule, the amplifier's input voltage noise density requirements should be at least three times the voltage noise of the equivalent resistors measured from each input.

Figure. 7 Resistor Thermal Noise

Ⅳ. Real World Example

Let's return to the original challenge, which demands a signal amplification rate of 30V/V for a battery-powered sensor that provides a 0 to 100mV analog signal at 1kHz. The two designs are shown in Figure 8 below. A standard 3.3V supply, power-saving resistors, and the TLV9002 general-purpose op amp are used in the design on the left. The TLV9042 op amp is used in the design on the right, which has a bigger resistor value and lower power.

The noise spectrum density is less than one-third of the amplifier's broadband noise when the equivalent resistance at the TLV9042's inverting input is roughly 9.667kΩ, ensuring that the op amp's noise dominates any noise caused by the resistors status.

Figure. 8 Typical Design and Subtle Design

Equation 6 may be used to get P_total,avg for the TLV9002 and TLV9042 designs, respectively, based on the numbers in Figure 8, the design parameters, and the specifications of the two op amps. Equations 9 and 10 illustrate the outcomes for each.

Figure. 9

The power consumption of the TLV9002 design is more than four times that of the TLV9042 design, based on the preceding findings. This is due to the amplifier's greater I_Q, and it also demonstrates that even when using low feedback resistor values, there are no substantial power savings with a high I_Q op amp. We can use two strategies in the example above: raising the resistor value and selecting an op amp with a lower quiescent current. In most op amp applications, both techniques are possible.

Ⅴ. Use Low Voltage Rails to Save Power

Revisit Equations 1 and 6 again to define the average power dissipation for a single-supply op amp circuit with a sinusoidal signal and a DC offset voltage:

Figure. 10

Also, as shown in Equation 6, V+ is the power rail (V+) that symbolizes the line, and power dissipation is directly proportional to V+, therefore lowering V+ to the lowest attainable supply voltage in the circuit is another technique to reduce power consumption Methods.

The minimal supply voltage range for many op amps is 2.7V or 3.3V. The minimum voltage required to keep the inside transistors within the intended functioning range is the reason for this restriction. Some op amps are capable of operating at voltages as low as 1.8V or even lower. The TLV9042 general-purpose op amp.  for example, may function from a 1.2V rail.

Ⅵ. Battery Powered Applications

The majority of today's sensors and smart gadgets are powered by batteries, and when they drain, their terminal voltages fall below their nominal voltage ratings. A 1.5V alkaline AA battery, for example, has a nominal voltage of 1.5V. The actual terminal voltage during the initial no-load measurement may be close to 1.6V. The voltage at this terminal will drop to 1.2V or even lower when the battery empties.

Designing using an op amp that can operate at 1.2V instead of a higher voltage op amp has the following benefits:

1. Even as the battery nears the end of its charge cycle and its terminal voltage lowers, the op amp circuit will continue to work for longer.

2. Instead of using two 1.5V batteries to generate the 3V rail, the op amp circuit can use just one 1.5V battery.

Consider the battery discharge diagram in Figure 11 to see why a lower voltage op amp can get more life out of a battery. Discharge cycles for batteries are often similar to this graph. The battery's terminal voltage will begin to approach its nominal rating. The terminal voltage will steadily decrease as the battery discharges. The terminal voltage of the battery will rapidly drop as it approaches the end of its charging cycle. The op-amp circuit's working period t1 will be short if it is only meant to operate at a voltage close to the battery's nominal voltage, say V1. Using an op amp that can operate at slightly lower voltages, such as V2, can greatly lengthen the battery's operating life, t2.

Figure. 11 Typical Discharge Curve for a Single Cell

While the effect varies depending on the type of battery, the load on the battery, and other variables, it is obvious that op amps with low operational power supply have longer working times.

Ⅶ. Low Voltage Digital Logic Levels

Low power op amps with low supply voltage capability can be used in applications that employ low voltage rails for digital and analog circuits. Standard voltage ranges for digital logic range from 5V to 1.8V and below (Figure 12). At lower voltages, digital logic, like op amp circuits, becomes more energy efficient. As a result, lower levels of digital logic are often preferred.

You might choose to use the same supply voltage levels for your analog and digital circuits to make the design process easier. An op amp with 1.8V capabilities, such as the high-precision, wide-bandwidth OPA391 or the cost-optimized TLV9001, could be useful in this situation. It's worth noting, though, that if a design must work with a 1.2V digital rail, the wiring system must clean up any noise that may escape from the digital circuitry to the analog device power supply pins.

Figure 12. Standard Logic Levels

Ⅷ. Summarize

In this post, we'll show you how to rapidly identify a low-power op amp by looking at the op amp's specifications. Picking an op amp with low quiescent current, as bandwidth allows, and selecting an op amp in a feedback circuit are two of these ways. resistors with a higher resistance value Other considerations to consider while assuring low-power op amps are the usage of low-voltage rails and low-voltage digital logic levels.

References

Analog Devices. (2003, February 7). One resistor takes heat from single-supply op amps. https://www.analog.com/en/resources/technical-articles/one-resistor-takes-heat-from-singlesupply-op-amps.html

Texas Instruments. (2014, August 8). Top questions on op amp power dissipation – Part 2. https://e2e.ti.com/blogs_/archives/b/precisionhub/posts/top-2-questions-on-op-amp-power-dissipation-part-2

All About Circuits. (2021, January 5). How much power does an op amp dissipate? https://forum.allaboutcircuits.com/threads/how-much-power-does-an-op-amp-dissipate.175561/

ELE Times. (2021, April 15). Power-saving techniques for op-amp circuits. https://www.eletimes.com/power-saving-techniques-for-op-amp-circuits