# How do I get rail-to-rail output from an op-amp?

As far as I know, a rail-to-rail op-amp is capable of providing output up to the power supply rails (i.e. from +VDD to -VDD.)

I've been using a TLC2272 op-amp to understand this concept.

According to the datasheet, the maximum allowable input voltage is VDD - 1.5 V. My input is 0-5 V, but the output is not showing the expected result. The peak has been clipped just like any ordinary op-amp. A zero-volt output is obtained for zero-volt input.

Check the schematic. I've built and tested the circuit practically, so ignore the simulation result.

simulate this circuit – Schematic created using CircuitLab

The output voltage for various inputs is shown in the table below.

As expected, the output reaches zero volt for zero-volt input (ignoring 2.2 mV as it is a very negligible value.) For 5 V input, the output was supposed to be 10 V, but it is only 8.61 V. The input voltage is below the maximum allowed input voltage.

Sl No. Input Output
1 0 V 2.2 mV
2 1 V 1.98 V
3 2 V 3.98 V
4 3 V 5.95 V
5 4 V 7.93 V
6 5 V 8.61 V

Can anyone explain to me under which conditions I'll get rail-to-rail output?

Is my understanding of the rail-to-rail concept wrong?

• @Linkyyy As you write, VDD+ is only 5V above the midpoint, so it's within the 8V limit.
– pipe
Commented Jan 21, 2023 at 8:42

This opamp has very little output drive capability, check Fig.14 in the datasheet.

It reaches the lower rail because no output current is necessary for that.

For high outputs, the resistor divider drains current to ground. Therefore the output can't quite keep up. However, the output should still be above 9 V according to the datasheet, so there could be another issue.

Try with higher (e.g. 100 kOhm) resistors.

• ok , checking with 100k. Commented Jan 21, 2023 at 8:39
• Using 100k the output gets 8.68 volts. Just 0.07 volts have increased. Commented Jan 21, 2023 at 10:43
• Ok.. and have you measured the actual positive rail voltage ? @FalgoonsenApu Commented Jan 21, 2023 at 11:28
• Yes. Precisely 10 volts on power supply. Commented Jan 21, 2023 at 13:04
• Used very reliable multimeter too. Commented Jan 21, 2023 at 13:05