Op amp Voltage offset problem

Question

_{(source: ecetutorials.com)}

This is not an assignment.

1. Calc Maximum output offset voltage caused by the input offset voltage Vios

2. Calc Maximum output offset voltage caused by the input bias current Ib. For an inverting amplifier with R1 = 100 k and Rf = 10k . Here 741 Op amp is used with Vios = 6 mv and Ib = 500 nA.

In this question Vin is given in inverting end only.

In the book Roy Chaudary Linear Integrated Circuit. It is given that No matter where the input is given the resulting formula for the V0 will be V0 = (1+ Rf/R1) * Vos.

Roy Chaudary Go to page 122

However, in the solution set of this question it is given that V0s = - (Rf/R1) * Vios. What am I supposed to do which one is correct? What am I not considering

Answer 1

The solution set is in error.

To see this, simply insert a series voltage source in either input of the op-amp and determine the resulting output voltage- the magnitude will be as Chaudary says.

The sign of the result is somewhat arbitrary but normally a positive offset is assumed to be driving the amplifier positive at the output, so I would say the sign of the solution set answer is also incorrect.

Answer 2

This question makes me go back to my college years when a teacher was trying to "explain" operational amplifiers to me through definitions. When she told me "the input offset voltage (Vios) is the voltage that makes the output voltage be zero", I could not imagine it...

So, the output voltage Vos of the single op-amp is not caused by some input voltage but rather by something non-electric - the difference between the parameters (asymmetry) of the two parts of the differential circuit... and we cannot measure it directly. We can measure this imperfection indirectly by neutralizing its impact to the output voltage through an equivalent input voltage Vios.

For this purpose, we ground the circuit inputs, add the compensating voltage in series to the differential input voltage, and vary it until zero the output voltage. We can connect the compensating voltage source in series either to the inverting or non-inverting input.

In the discussed inverting configuration, we do the opposite trick - we add voltage equal to Vios (according to the op-amp specifications) in series to the differential input voltage with the purpose to emulate Vios. It acts as the only input voltage that is compensated by the voltage at the output of the negative feedback network (the voltage divider R1-Rf). We discern the well-known non-inverting configuration (a little odd if the floating compensating source is connected to the inverting input). Hence the expression for the resulting V0 = (1+ Rf/R1) * Vos.

Indeed, it is quite strange to consider an inverting amplifier as non-inverting... and maybe this is the reason for the author's mistake above.

Again, we can connect the compensating voltage source in series either to the inverting or non-inverting input... but the latter is more suitable since the source will be grounded.

The trick to connect a floating input voltage source in series to the op-amp differential input is old and it is shown in the famous Sheingold's paper (Fig. 4 on Page 7).