Op-Amp: Input Offset Voltage Explained

Hey friends, welcome to the Kohik.com ALL ABOUT ELECTRONICS. So, in this article, we will understand the input offset voltage of the op-amp. And it is one of the DC offsets that you find inside the op-amp.

Op-Amp: Input Offset Voltage Explained

So, this DC offset or the DC imperfections the unwanted DC signal that you find along with the output signal at the output terminal of the op-amp.

So, if you see the datasheet of any op-amp then you will find that three parameters related to these offsets are defined. The first is the input offset voltage, the second is the input offset current and the third is the input bias current.

So, in this article, we will understand this input offset voltage. And we will learn what is (what is input offset current) input offset voltage and how it can affect the output of the op-amp.

What is the input offset voltage

Now, to understand this input offset (input offset voltage of 741 op amp) voltage, let’s assume that both input terminals of this op-amp are at ground zero potentials. And if op-amp is ideal in that case, the output of the op-amp should be equal to zero volts.

What is the input offset voltage

But actually, if you see, you will find some finite voltage at the output terminal of the op-amp. So, let’s understand why this finite voltage appears at the output terminal.

internal structure

So, if you see the internal structure of the op-amp, the first stage of this op-amp is a differential amplifier.

So, actually, these two transistors should be identical to each other. But actually, there is a slight mismatch between the two transistors. And because of that, both transistors have different biasing voltage.

So, let’s say, this non-inverting terminal has biasing voltage VB1 and this inverting terminal has biasing voltage (input offset voltage comparator) VB2. So, because of a slight mismatch between these biasing voltages, there is a potential difference between the two terminals. And this potential difference used to get amplified at the latter stages. And because of that, we used to get some offset voltage at the output terminal.

What is the input offset voltage  internal structure

So, now suppose if we apply the differential voltage of opposite polarity at one of the op-amp terminals, then we can ensure that the output of the op-amp is zero volts.

So, the definition of this input offset voltage (input offset voltage formula) is the amount of differential input voltage that is required to apply between the two terminals such that the output of the op-amp will become zero volts.

So, this input offset voltage can be few millivolts for the general-purpose op-amp and it can be as low as 1 microvolt for the very high precision op-amps.

So, for example, if you take the case of LM324 and LM 741, the input offset voltage used to be in the range of few millivolts. While if you take very precise op-amps like AD 8538 and AD 8551, the input offset voltage used to be in the range of few microvolts.

Now, as I said before, it is very hard to predict the polarity of this input offset voltage. And even if you take the two op-amp ICs which are manufactured from the same wafer, it is quite possible that both ICs have different input offset voltage.

So, the manufacturers also used to provide the maximum value of this input offset voltage. So, whenever you are using that op-amp, it is ensured that the value of the input offset voltage will be less than the maximum value.

Effect of input offset voltage on the feedback circuits

So, that is being said, now let’s see the effect of input offset voltage on the feedback circuits. So, here we have op-amp which is configured in an inverting op-amp configuration. So, to find the effect of this input offset voltage, let’s assume that the input that is provided to the circuit is zero volts.

So, in this case, some input offset voltage will be present between the two terminals. And the same thing can be represented by applying the input offset voltage at one of the terminals.

So, here this input offset voltage is applied at the non-inverting terminal. And the circuit will look identical even in the case of the non-inverting op-amp. Because whenever we are considering the input voltage as zero, in that case inverting and the non-inverting op-amp configuration look identical.

So, now let’s find the effect of input offset voltage on the output. Now, let’s say the output voltage because of this input offset voltage is equal to Voo, that is output offset voltage. And it will be equal to 1 plus Rf divided by R1 times this input offset voltage.

Effect of input offset voltage on the feedback circuits

So, for the given op-amp if the value of input offset voltage is equal to 1 mV and the DC gain of this configuration is 10, in that case, the value of output offset voltage will be equal to 1mV. Now, in this circuit, suppose the input signal of let’s say 1mV is applied at this non-inverting terminal, in that case, the output of this op-amp will be equal to 10 mV.

So, because of this input offset voltage, there will be a 10 percent error in the output voltage. And as I said earlier, it is hard to predict the polarity of this offset voltage. So, the output voltage can be either 9mV or11mV.

So, as you can see over here, the DC error in the output voltage because of this input offset voltage is a function of the DC gain of this feedback circuit. And as the DC gain of this feedback circuit increases, the error in the output voltage will also increase. And this error is a very critical parameter when you are dealing with very small input voltages. For example, whenever you are using this op-amp for digital to analog conversion then it is required that the error at the output should be as minimum as possible.

So, to achieve that either you should use the op-amp which has a very low value of the input offset voltage like 1 UV, or by external means, you need to nullify the effect of this input offset voltage. So, in the same example, suppose input offset voltage is equal to 1 UV, then the output offset voltage will be equal to 10 times this1uV.

So, the error in the output voltage will be equal to only 10 UV. While your signal is equal to 1mV, then the output because of this input will be equal to 10mV. So, as you can see over here, in this case, the error in the output voltage has been reduced by 1000 times. So, in this way, by using the op-amp, with a very low value of input offset voltage, we can reduce the error in the output voltage.

Dc Offset Null

The second method to reduce the effect of this input offset voltage is to use the DC offset nulling circuits. And there are many op-amp ICs that used to come with these DC offset null pins.

So, just by connecting the external TRIM POT between the two terminals and adjusting the TRIM POT. we can nullify the effect of this input offset voltage. So, these are some of the methods by which we can either reduce or remove the effect of this input offset voltage.

Dc Offset Null

Offset voltage drift with temperature

So, even if we use these DC nulling circuits, then also it is quite possible that you will find some DC error at the output terminal. And this error arises because of the change in the ambient temperature.

Because this offset voltage used to get changed with the temperature. And usually, it is defined as an offset voltage drift in the datasheets. And it is defined by the units of uV/C.So, for general purpose op-amps, this offset voltage drift used to be in the range of 10to 20 uV/C.

While for very high precision op-amps, this offset voltage drift used to be in the range of either 1 or 0.1 uV/C.So, now let’s see the effect of this offset voltage drift on the output of the op-amp by taking one example.

FAQ

YouTube Video

So here are a video based on Input Offset Voltage. Which was uploaded by Techjunkie Jdb

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