# Why Does Op-Amp Output Voltage Always Saturate (in a Non-Inverting Amplifier Circuit)?

I am trying to amplify the voltage measured across a shunt resistor (Rs). I am using a non-inverting amplifier circuit (as shown in the figure). The expected voltage across the shunt is in the 0-50mV range, and I would like an amplification gain of around 100 (40dB), so I can measure it with an A2D channel with an input range of 0-5V. To achieve this, the resistors I am using are Rf=10,000Ohm and Rg=100Ohm, giving a closed loop gain of 101. The op-amp I am using is a NTE778A. I am powering this circuit using a 12V DC power supply. The problem I am facing is, whatever voltage there is across the shunt, the output voltage always saturated to a value that is (very) slightly lower than Vcc. I am not sure what mistake I am making, either in the construction of the circuit or in my basic understanding of this amplification concept, and any help is appreciated. If this is a common rookie mistake, please do excuse me.

• Looking at the datasheet, the op amp you're using has a typical input offset voltage of 2V, which seems like it can't be right, because the LM741, the classic example of a "bad" op amp, has an offset voltage of 6mV maximum. May 18, 2017 at 23:42
• Either this op amp is designed for some very strange operating conditions, or there's a typo in the datasheet, or it's just phenominally bad. May 18, 2017 at 23:43
• @Felthry Typo:should be 6mV not 6 V. Substituting a LM158/LM258/LM358 dual op-amp should give better results. With a single-ended supply, input and output swings much closer to ground. A rail-to-rail op amp is also acceptable. May 18, 2017 at 23:58
• I haven't looked at the datasheet but it's sounds like input phase reversal when you violate the input common mode range of the amplifier. May 19, 2017 at 0:02
• @sstobbe is pretty much bang on. Qualifies as a rookie mistake. This op-amp doesn't allow you to go to within 2V of the lower power rail. If you want to use this op-amp 's inputs near ground, you should get at least -5V supplied to it. May 19, 2017 at 0:28