We are considering using an opamp that is rail-to-rail on both its input and output for an audio electronics project I’m working on. This is my first serious encounter with such devices, and it turns out that they have quirks that you need to be aware of.
One of the quirks is input stage crossover. A lot of opamps with rail-to-rail inputs use complementary NMOS/PMOS or NPN/PNP pairs in the input stage, with one handing off to the other depending on the input voltage. This leads to the possibility of crossover artifacts — just like crossover artifacts in class-B or class-AB output stages but involving potentially more fragile input signals.
In many of these, both N and P pairs operate over a transition region — making them class-AB input stages. And while you might hope that the class-AB-ness would be enough to render the crossover effects negligible, apparently it’s not: the Texas Instruments OPA2156 spec sheet admonishes that, "Within this [transition] region PSRR, CMRR, offset voltage, offset drift, noise, and THD performance are degraded compared to operation outside this region."
So, let’s try to avoid the transition region.
What follows is a case study using the OPA2156 for the design constraints presented by my project. However the content should be adaptable to other devices and contexts.
Transition region bounds
The transition region for the OPA2156 is stated to be typically (V+) – 2.25V to (V+) – 1.25V. So if you power the device from a single 5V rail (a common condition for rail-to-rail opamp applications), the transition range is 2.75V to 3.75V. Further, if this circuit is biased at 2.5V, midway between ground and the rail, you’ll be out of the transition region as long as the audio signal remains below 0.25Vpeak. This is not unreasonable for some applications but certainly not all.
Avoiding the transition region
In my application, the opamp will be working as a unity gain non-inverting filter, the output needs to be biased at 2.5V, and the signal amplitude is 2.8Vpp. The easiest way to achieve the needed 2.5V output bias is to bias the input by the same amount. Under these conditions, the input signal will have a range of 1.1V to 3.9V. This easily encroaches on the transition region, which suggests we should explore some ways to address this.
Rebiasing the input
One approach we might take to avoiding the transition region is to re-bias the input stage. If we bias the input at 1.65V instead of 2.5V, the input will span 0.25V (the device’s minimum output voltage) to 3.05V. This keeps the signal out of the transition region for most of the range.
This may be an improvement, but it comes dangerously close the output stage limits and it will require additional circuitry to achieve the 2.5V output bias required in the application. Still, this can be an easy way to move the signal into a less sensitive area, especially if the signal amplitude is modest and the output bias voltage isn’t important.
Adjusting the rail voltage
Another approach to avoiding the transition region is to increase the device’s rail voltage.[^1]
Given Vtlo is the lower bound of the transition region and Vimax is the maximum input voltage, we want:
Vimax < Vtlo
Vtlo = Vs – V1, where Vs is the supply rail and V1 is voltage between Vs and the lower bound of the transition region (2.25V in the case of the OPA2156), and
Vimax = Vb + Vip, where Vb is the input bias voltage (2.5V in this case) and Vip is the peak amplitude of the input signal (1.4V in this case).
Vb + Vip < Vs – V1,
and solving for Vs yields:
Vs > Vb + Vip + V1
For Vb = 2.5, Vip = 1.4, and V1 = 2.25,
Vs > 2.5 + 1.4 + 2.25 = 6.15V.
The downside to this approach is that it may add complexity to the power supply design, especially if the product is intended to operate off a USB input or 5VDC adapter.
Computer scientists often talk about "leaky abstractions." The concept applies to the real world as well. Real world opamps are leaky abstractions, and new technologies tend to come with a set of leaks that differ from established ones.
In the case of input/output rail-to-rail opamps, one potential leak to be aware of is input stage crossover effects. Depending on the application, it may be possible to avoid regions where the input stage is crossing over by rebiasing the input and/or adjusting rail voltage(s).
[^1]: Many rail-to-rail opamps have limited maximum rails. Be sure to stay within the device’s limits!