Single-supply amplifier outputs don't swing rail to rail
(Features, 04 Jan 2008 )
By Bonnie Baker, Texas Instruments Inc.
Single-supply amplifiers do not truly swing rail to rail at the output. Near the rail, the amplifier is nonlinear. For linear operation, the output of single-supply amplifiers can come within only 50mV to 300mV of each rail (Figure 1).
Single-supply-amplifier, rail-to-rail-output ads can give a false sense of security. Figure 1 shows a typical single-supply amplifier’s output swing as you drive the output to the rails.
The amplifier’s linearity starts to degrade long before reaching the output-swing maximums, and the amplifier output never reaches either rail.
The conditions of the dc-open-loop-gain specification define the amplifier’s linear operating output range. The dc open loop gain in decibels is 20 log(ΔVout/ΔVos), where Vout is the output voltage and Vos is the input offset voltage. When you drive the output high, VH is the maximum voltage level at the output in the dc-open-loop-gain measurement. VOH is the absolute maximum voltage level with respect to VDD (drain-to-drain voltage) that the output can reach. VL is the minimum voltage level at the output in the dc-open-loop-gain measurement, and VOL is the absolute minimum voltage level that the output can reach. VH is less than VOH, and VL is greater than VOL.
From a signal-chain perspective, you can see an op amp’s output limitations to swinging rail to rail when the op amp is driving an ADC. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1kHz signal in a 5V system. The amplifier’s typical closed-loop bandwidth is about 3MHz with a typical slew rate of 2.3V/µsec. The amplifier output voltage swings from 140mV to 4.66V. In this 5V-supply system, the headroom between the signal and rails is 140mV. For this amplifier, the VOL minimum specification is 15mV above ground. The VOH maximum specification is VDD - 20mV.
Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2kHz, 3kHz, and 4kHz and so on. By reducing the amplifier’s output signal to 272mV from each rail, the data looks perfect with only the ADC distortion (Figure 2b).
When using a single-supply amplifier, read the fine print! Some single-supply amps have output-stage charge pumps, allowing the amplifier’s output swing to go to and well beyond the power-supply rails. In every case, read your data sheet and refer to the conditions on the open-loop-gain test.
Author Information Bonnie Baker is a senior applications engineer at Texas Instruments. You can reach her at bonnie@ti.com.
