Power-Saving Techniques for op-amp Circuits

Part 1: In recent years, the popularity of battery-powered electronics has made power consumption an increasing priority for analog circuit designers. With this in mind, this article is the first in a series that will cover the ins and outs of designing systems with low-power operational amplifiers (op amps).

In the first installment, I will discuss power-saving techniques for op-amp circuits, including picking an amplifier with a low quiescent current (IQ) and increasing the load resistance of the feedback network.

Understanding power consumption in op-amp circuits

Let’s begin by considering an example circuit where power may be a concern: a battery-powered sensor generating an analog, sinusoidal signal of 50 mV amplitude and 50 mV of offset at 1 kHz. The signal needs to be scaled up to a range of 0 V to 3 V for signal conditioning (Figure 1), while saving as much battery power as possible, and that will require a noninverting amplifier configuration with a gain of 30 V/V, as shown in Figure 2. How can you optimize the power consumption of this circuit?

Figure 1: Input and output signals

Figure 2: A sensor amplification circuit

Power consumption in an op-amp circuit consists of various factors: quiescent power, op amp output power, and load power. The quiescent power, P_Quiescent, is the power needed to keep the amplifier turned on and consists of the op amp’s IQ, which is listed in the product data sheet. The output power, P_Output, is the power dissipated in the op amp’s output stage to drive the load. Finally, load power, P_Load, is the power dissipated by the load itself. My colleague, Thomas Kuehl, in his technical article, “Top questions on op-amp power dissipation – part 1,” and a TI Precision Labs video, “Op Amps: Power and Temperature,” define various equations for calculating power consumption for an op amp circuit.

In this example, we have a single-supply op amp with a sinusoidal output signal that has a DC voltage offset. So we will use the following equations to find the total, average power, P_total,avg. The supply voltage is represented by V₊. V_off is the DC offset of the output signal and V_amp is the output signal’s amplitude. Finally, R_Load is the total load resistance of the op amp. Notice that the average total power is directly related to I_Q while inversely related to R_Load.

Picking a device with the right IQ

Equations 5 and 6 have several terms and it’s best to consider them one at a time. Selecting an amplifier with a low IQ is the most straightforward strategy to lower the overall power consumption. There are, of course, some trade-offs in this process. For example, devices with a lower IQ typically have lower bandwidth, greater noise and may be more difficult to stabilize. Subsequent installments of this series will address these topics in greater detail.

Because the IQ of op amps can vary by orders of magnitude, it’s worth taking the time to pick the right amplifier. TI offers circuit designers a broad selection range, as you can see in Table 1. For example, the TLV9042, OPA2333, OPA391 and other micropower devices deliver a good balance of power savings and other performance parameters. For applications that require the maximum power efficiency, the TLV8802 and other nanopower devices will be a good fit. You can search for devices with your specific parameters, such as those with ≤10 µA of IQ, using our parametric search.

Table 1: Notable low-power devices

Reducing the resistance of the load network

Now consider the rest of the terms in Equations 5 and 6. The V_amp terms cancel out with no effect on P_total,avg and V_off is generally predetermined by the application. In other words, you often cannot use V_off to lower power consumption. Similarly, the V₊ rail voltage is typically set by the supply voltages available in the circuit. It may appear that the term R_Load is also predetermined by the application. However, this term includes any component that loads the output and not just the load resistor, R_L. In the case of the circuit shown in figure 1, R_Load would include R_L and the feedback components, R₁ and R₂. Hence, R_Load would be defined by Equations 7 and 8.

By increasing the values of the feedback resistors, you can decrease the output power of the amplifier. This technique is especially effective when P_output dominates P_Quiescent, but has its limits. If the feedback resistors become significantly larger than R_L, then R_L will dominate R_Load such that the power consumption will cease to shrink. Large feedback resistors can also interact with the input capacitance of the amplifier to destabilize the circuit and generate significant noise.

To minimize the noise contribution of these components, it’s a good idea to compare the thermal noise of the equivalent resistance seen at each of the op-amp’s inputs (see Figure 3) to the amplifier’s voltage noise spectral density. A rule of thumb is to ensure that the amplifier’s input voltage noise density specification is at least three times greater than the voltage noise of the equivalent resistance as viewed from each of the amplifier’s inputs.

Figure 3: Resistor thermal noise

Real-world example

Using these low-power design techniques, let’s return to the original problem: a battery-powered sensor generating an analog signal of 0 to 100 mV at 1 kHz needs a signal amplification of 30 V/V. Figure 4 compares two designs. The design on the left uses a typical 3.3-V supply, resistors not sized with power-savings in mind and the TLV9002 general-purpose op amp. The design on the right uses larger resistor values and the lower-power TLV9042 op amp. Notice that the noise spectral density of the equivalent resistance, approximately 9.667 kΩ, at the TLV9042’s inverting input is more than three times smaller than the broadband noise of the amplifier in order to ensure that the noise of the op amp dominates any noise generated by the resistors.

Figure 4: A typical design vs. a power-conscious design

Using the values from Figure 4, the design specifications and the applicable amplifier specifications, Equation 6 can be solved to give P_total,avg for the TLV9002 design and the TLV9042 design. For your reading convenience, Equation 6 has been copied here as Equation 9. Equations 10 and 11 show the numeric values of P_total,avg for the TLV9002 design and the TLV9042 design, respectively. Equations 12 and 13 show the results.

As can be seen from the last two equations, the TLV9002 design consumes more than four times the power of the TLV9042 design. This is a consequence of a higher amplifier I_Q, demonstrated in the left terms of Equations 10 and 11, along with smaller feedback resistors, as accounted for in the right terms of Equations 10 and 11. In the case where more I_Q and smaller feedback resistors are not needed, implementing the techniques described here can provide significant power savings.

Conclusion

I’ve covered the basics of designing amplifier circuits for low power consumption, including picking a device with low IQ and increasing the values of the discrete resistors. In part 2 of this series, I’ll take a look at when you can use low-power amplifiers with low voltage supply capabilities.

Low-power op amps for low-supply-voltage applications

In part 1 of this series, I covered issues related to power consumption in single-supply operational amplifier (op-amp) circuits with a sinusoidal output and DC offset. I also discussed two techniques for reducing power consumption in these circuits: increasing resistor sizes and picking an op amp with a lower quiescent current. Both tactics are available in most op-amp applications.

In this installment, I’ll show you how to use low-power op amps with low supply-voltage capabilities.

Saving power with a low-voltage rail

Recall that part 1 included definitions of the average power consumption of a single-supply op-amp circuit with a sinusoidal signal and DC offset voltage using Equations 1 and 2:

I didn’t address the supply rail (V₊) in part 1 since it is “typically set by the supply voltages available in the circuit.” While this is true, there are applications where you may be able to use an exceptionally low supply voltage. In such cases, choosing a low-power op amp capable of operating within these supply rails can lead to significant power savings. You can see this in Equation 2, where P_total,avg is directly proportional to V₊.

Many op amps have minimum supply voltages in the range of 2.7 V or 3.3 V. The reason for this limitation has to do with the minimum voltage needed to maintain the internal transistors in their desired operating ranges. Some op amps are designed to work down to 1.8 V or even lower. The TLV9042 general-purpose op amp, for example, can operate with a 1.2-V rail.

Battery-powered applications

Many of today’s sensors and smart devices are powered by batteries with terminal voltages that degrade from their nominal voltage rating as they discharge. For example, a single alkaline AA battery has a nominal 1.5-V potential. When first measured without a load, the actual terminal voltage may be closer to 1.6 V. As the battery discharges, this terminal voltage will fall to 1.2 V and further. Designing with an op amp capable of operating down to 1.2 V, instead of a higher-voltage op amp, offers these advantages:

• The op-amp circuit will continue to work for longer, even as the battery approaches the end of its charge cycle and its terminal voltage degrades.
• The op-amp circuit can work with one 1.5-V battery, rather than needing two batteries to form a 3-V rail.

To see why a lower-voltage op amp can get more life from a battery, consider the battery discharge plot shown in Figure 1. Batteries typically have discharge cycles that resemble this curve. The battery’s terminal voltage will begin near its nominal rating. As the battery discharges with time, the terminal voltage will gradually degrade. Once the battery approaches the end of its charge, the terminal voltage of the battery will then decline rapidly. If the op-amp circuit is only designed to work with a voltage near the battery’s nominal voltage, such as V₁, then the operating time of the circuit, t₁, will be short. Using an op amp capable of working at a slightly lower voltage, however, such as V₂, significantly extends the operating life of the battery, t₂.

Figure 1: Typical discharge curve of a single-cell battery

This effect will vary with battery type, battery load and other factors. Still, it is clear to see how a 1.2-V op amp such as the TLV9042 could get more life out of a single 1.5-V AA battery than a 1.5-V op amp.

Low-voltage digital logic levels

Applications that use low-voltage rails for both digital and analog circuits can also take advantage of low-power op amps with low-supply-voltage capabilities. Digital logic has standard voltage levels from 5 V down to 1.8 V and below (Figure 2). As with op-amp circuits, digital logic becomes more power-efficient at lower voltages. Thus, a lower digital logic level will often be preferable.

To simplify the design process, you may choose to use the same supply-voltage levels for your analog and digital circuits. In this case, having a 1.8-V-capable op amp, such as the high-precision, wide-bandwidth OPA391 or cost-optimized TLV9001, can prove beneficial. To future-proof a design to a potential 1.2-V digital rail, the TLV9042 may be more appropriate. If you choose to take this approach, make sure to clean any noise that may leak into the power pins of the analog devices from the digital circuitry.

Figure 2: Standard logic levels

Conclusion

In this article, we covered applications where low-power op amps with low voltage supply capabilities can bring additional benefits.  In the next installment of this series, I’ll take a look at how to use op amps with shutdown circuitry to save power.