Amplify DC signal: how to deal with real life problem ==> output is close to Vs when input signal too small OK otherwise

Question

Hi I believe this problem must be classical and well known but I cannot find a place where it is specifically discussed: I have build a simple non inverting 2X amplifier following exactly the classical and simple circuit as discussed for example in the answer to this question: Amplify 0 - 3.3V DAC signal to 0 - 10V as well as in many textbooks. I use a LF351DT opamp with Vs+=12V and Vs- to GND. and 2 10k resistor to get a 2X amplification. I feed it with a variable 0-5V DC and measure output. This is all there is in the circuit.

The circuit works as it should when the input is large enough (>~1.5V) that means Vout=2X Vin but for small value the opamp output is constant ~10V. I tried different chips ( same model) and this is reproducible. I cannot find a place where this effect is discussed. Does anybody have an idea on what's going on and how to deal with it? Thanks

Answer 1

Does anybody have an idea on what's going on and how to deal with it? Thanks

It's called signal inversion (also phase reversal) and happens when the input goes beyond the upper or lower limit specified in the data sheet. Basically, the output polarity flips and things become fairly meaningless: -

2nd picture from here.

My advice is to add a small negative supply rail to the op-amp negative supply power pin or, use a rail-to-rail device.

Answer 2

According to the datasheet this is not a rail-to-rail device:

The datasheet does not provide detailed specs for the voltage rails you are using, but you can see that the output can't reach the positive rail, and this is worse the larger the output current.

You should also not expect correct behavior with 0V inputs.

Answer 3

You need an op-amp that can function with the input at the lower supply rail, or you need a negative rail. Once you go outside the input limits (which you are doing with an input so close to the negative rail) anything can happen.

Since you have 12V available you don't need a Rail-to-rail op-amp, a single-supply type will likely do, or one with input common mode range that includes the negative rail and rail-to-rail output.

For example, an LM358 has an input range that includes the negative rail and goes up to about 10V with a 12V supply (see Fig 1). It can supply typically almost 20mA while dropping less than 2V (so 10V out) but I would not depend on more than 5-7mA, so a minimum 1.8K load or so.