Non-inverting amplifier with stable gain

I'm working with a LM358 op-amp as a non-inverting amplifier to get a 0-24 V output from a 0-5 V input. When the input is 1.5 to 5 V, the gain is 4.8 which is OK (set by resistors), but below 1.5 V this gain decreases more and more until 0 V.

How can I make this 4.8 gain stable along the range 0-5 V?

Picture of the circuit I'm using, R1 = 1 kΩ, R2 = 3.8 kΩ.

• How do you figure its gain drops at below 1.5V input? Commented Jan 18, 2023 at 2:47
• Because I'm using an osciloscope(Analog Discovery 2). CH1 for the input, CH2 for the output and a MATH function dividing CH2 value into CH1 value, resulting in the gain. So, over 1.5V the MATH function is near 4,8, but below 1.5V the gain decreases with the input. Commented Jan 18, 2023 at 2:57
• What are the positive and negative power supply voltages? Commented Jan 18, 2023 at 3:03
• Positive supply voltage is 27V and negative supply voltage is GND Commented Jan 18, 2023 at 3:04
• Is the output connected to anything other than R2 and the oscilloscope? Commented Jan 18, 2023 at 3:09

but below 1.5 V this gain decreases more and more until 0 V.

This sounds very much like the input offset voltage not being factored into your calculations. For the LM358 it is 2 mV typically but, can be as high as 3 mV. Some op-amps are even worse. Some op-amps boast an input offset error of only 5 μV. However, your circuit gain is 4.8 hence, it manifests as a DC error on the output of 9.6 mV. This can be a positive or negative error voltage.

• ~10 mV in 1.5 volts is 0.667 % error
• ~10 mV in 150 mV is a 6.67% error
• ~10 mV is 15 mA is a 66.7% error

As you can see, the closer you get towards zero, the error magnifies.

Then there are your measurement errors to consider. All-in-all what you say doesn't surprise me but, it's not that the gain is reducing it's just that it appears to reduce because of DC offset errors.

If you want to measure gain, use an AC signal and decent meters to take readings of the AC voltages.

• If the resistances of the feedback resistors are too high then the input bias current (025uA max) affects the output voltage at low levels. Commented Jan 18, 2023 at 16:55
• @Audioguru as stated in the question: R1 = 1 kΩ, R2 = 3.8 kΩ Commented Jan 18, 2023 at 18:25

... but below 1.5 V this gain decreases more and more until 0 V.

As pointed out by @Andy aka ...

I think you talk about voltage output, because, in fact, it is something like 0.1 V at input.

Here is a simulation also in the case of the opamp being supplied with -5 V (in place of the ground).

DC Dynamic Analysis

TRANsient Analysis: see the variable "gain", curves in "red" ...