Technical Article

Understanding Output Signal Swing in Op Amps

March 17, 2024 by Robert Keim

Learn about the characteristics and limitations of an operational amplifier’s output voltage range.

Operational amplifiers, or ‘op amps’, are fundamental to a large number of analog designs. As we learned in the preceding article, all real-life op amps place limitations on the allowable voltage range for input signals (input signal swing) and the available voltage range for output signals (output signal swing. Previously, we discussed input swing specifications—both how to interpret them and what happens if we exceed them. In this article, we’ll focus on the output voltage range.

Output voltage specifications indicate the points at which the output signal reaches its saturation limit, meaning the voltage can’t go any closer to that supply rail. The signal is then referred to as “clipped.” An example of signal clipping can be seen in Figure 1, which depicts simulated output saturation for the LT1880 op amp when powered by 2.5 V and –2.5 V.

 

Operational amplifier output saturation produces a clipped signal.

Figure 1. Op amp output saturation produces a clipped signal. Image used courtesy of Robert Keim

 

Saturation leads to a complete loss of correspondence between the input waveform and the output waveform. However, output swing performance can degrade even before the saturation point is reached. Applications that require very low distortion may be negatively affected by subtle nonlinearity that occurs as the output signal approaches the limits specified on its datasheet. If you’re not able to use simulations or lab testing to verify that distortion performance is acceptable, it’s a good idea to assume that the output signal needs to stay a couple hundred millivolts away from the saturation limits.

Like the input voltage range, the output voltage range depends on the op amp’s supply voltages and is specified in relation to the higher-voltage (V+) and lower-voltage (V) power rails. In the next section, we'll take a closer look at those specifications.

 

Reading the Datasheet

Table 1 gives the output swing specs for the LT1880 from Analog Devices—the same op amp whose performance we simulated above. Analog Devices describes this op amp as offering rail-to-rail output swing.

 

Table 1. Output signal swing for the LT1880. Data used courtesy of Analog Devices

Output signal swing specs for the LT1880 op amp from Analog devices.

 

When reading this table, keep the following in mind:

  • The values for VOL indicate swing relative to the V supply rail (0 V). VOH is likewise given in relation to V+.
  • The “typical” and “maximum” columns don’t describe typical and maximum output voltage, but rather output voltage deviation.
  • Only the amount of deviation is given, not the direction.

The two equations below represent, respectively, the upper and lower boundaries of the specified output voltage range:

$$V_{high}~=~V_{+}~-~V_{OH(max)}$$

 

$$V_{low}~=~V_{-}~+~V_{OL(max)}$$

 

These equations remind us that manufacturers use the term “rail-to-rail” somewhat loosely. Even with no load current, the output can’t swing all the way to either voltage rail, and the gap widens substantially as output current increases. Later in this article, we’ll discuss the effect of output current on available output swing at length. For now, let’s look at a different part of the datasheet.

 

Output Swing Plots

When describing output swing limitations, datasheet specification tables use only a few examples of load-current conditions. We can gain a more complete picture of an op amp’s output swing abilities by examining the performance plots included in its datasheet.

Exactly how this information is reported varies between devices. For example, Figures 2 and 3 plot the output swing characteristic of two different op amps—the LT1224 and the LT6023, respectively—from Analog Devices.

 

Output voltage swing vs. load resistance for the LT1224.

Figure 2. Output voltage swing vs. load resistance for the LT1224. Image used courtesy of Analog Devices

 

Output saturation voltage vs. load current for the LT6023.

Figure 3. Output saturation voltage vs. load current for the LT6023. Image used courtesy of Analog Devices

 

The LT1224’s datasheet reports peak-to-peak voltage swing relative to load resistance. By contrast, the LT6023’s datasheet reports “saturation voltage,” which appears to be the difference between the saturation limit and the supply voltage, in place of voltage swing. It also uses current, not resistance, as the independent variable.

Figure 4, taken from the datasheet of the Texas Instruments OPAx277, uses a different visual format to plot voltage swing versus current. Personally, I find it clear and concise.

 

Output voltage swing vs. output current for the OPAx277.

Figure 4. Output voltage swing vs. output current for the OPAx277. Image used courtesy of Texas Instruments

 

Output Swing vs. Load Current

As the datasheet excerpts we’ve been examining make clear, the voltage range available for the output waveform isn’t a single specification that’s valid across all operating conditions. Instead, it’s significantly influenced by the amount of current that the op amp is sourcing or sinking. As more current flows through the output terminal, more voltage is dropped across the semiconductor components that are connected between the op amp’s power nodes and its output node.

The relationship between load impedance and output range differs according to which semiconductor technology is used for the output stage. For example, Figure 5 shows the simplified schematic of an op amp used in a TL08x integrated circuit. The input stage uses field-effect transistors; the output stage uses complementary bipolar transistors.

 

Simplified schematic of an op amp used inside a TL08x integrated circuit.

Figure 5. Simplified schematic of a TL08x op amp. Image used courtesy of Tony R. Kuphaldt

 

A bipolar output stage will generally require at least 200 mV—the approximate saturation voltage of a bipolar junction transistor—between the output voltage and the supply rail. A CMOS output stage can provide much lower drain-to-source voltages, but its load-current dependence is more severe. This is because the channel of an NMOS or PMOS transistor acts like a resistor, dropping more and more voltage as load current increases. The saturation voltage of a bipolar transistor doesn’t result from a resistive mode of operation, so it’s comparatively stable with respect to output current.

 

Wrapping Up

Operational amplifiers are versatile components that can often be implemented without the need for complex simulations or long journeys through datasheet specs. Sometimes, though, an op amp’s non-ideal behaviors will noticeably affect performance. For that reason, we need to study and address them during the design process. I hope that this series has helped you understand some of the fine points of managing an op amp’s input and output signals.

 

Featured image used courtesy of All About Circuits