D.C. Characteristics of Op-amp

D.C. Characteristics of Op-amp

The important d.c. characteristics of op-amp are,

1. Input bias current (I_b),        

2. foput offset current (I_ios)

3. Input offset voltage (V_ios),

4. Thermal drift

The input bias current, input offset current and input offset voltage are already discussed. The effect of these parameters is to add the error to the expected d.c. output voltage. These parameters produce output offset voltage V_oos.

1. Thermal Drift

The op-amp parameters input offset voltage V_ios, input bias current I_b and input offset current Ijos are not constants but vary with the factors :

i) Temperature ii) Supply voltage changes and iii) Time

The effect of change in temperature on the parameters is most severe. Let us discuss the effect of change in temperature on these parameters.

a. Effect on Input Offset Voltage

The effect of change in temperature on the input offset voltage is defined by a factor called thermal voltage drift. It is also called as input offset voltage drift.

The thermal voltage drift is defined as average rate of change of input offset voltage per unit change in temperature. Mathematically it is given by,

Input offset voltage drift = Δ V_ios / Δ T … (2.14.1)

where Δ V_ios = Change in input offset voltage

Δ T = Change in temperature

It is expressed in µV/°C. The drift is not constant and it is not uniform over specified operating temperature range. The value of the input offset voltage may increase or decrease with the increasing temperature.

The Fig. 2.14.1 shows the graph of normalized values of input offset voltage versus temperature, for MC1741 op-amp.

The input offset voltage is zero at room temperature of 25 °C Referring to the graph shown in the Fig. 2.14.1, the thermal voltage drift values can be obtained.

b. Effect on Input Offset and Bias Currents

Similar to the input offset voltage, input bias current and input offset current are not constants but vary with temperature. The effect of temperature on input bias current is defined by a factor called input bias current drift while effect on input offset current is defined by a factor called input offset current drift.

The average rate of change of Normalised input bias current per unit change Iio in temperature is called input bias current drift.

The average rate of input offset current per unit change in temperature is called input offset current drift. 

Mathematically these drifts are given by,

Thermal drift in input bias current = ΔI_b / ΔT ...(2.14.2)

Thermal drift in input offset  current = ΔI_ios / ΔT  ...(2.14.3)

Both the drifts are measured in nA/ °C or pA/ °C. These parameters vary randomly with temperature i.e. they may be positive in one temperature range and negative in another.

The Fig. 2.14.2 (a) and 2.14.2 (b) show the graphs of normalized values of input bias current and input offset current versus temperature, for MC1741 op-amp. These curves are different for different types of op-amps and are generally provided by the manufacturers.

The input offset current is assumed to be zero at room temperature of 25 °C.

Practically no information is available about the change in input bias current versus temperature. Infact when compensating resistance Rcomp is used, there is no need to consider the change in input bias current as a function of change in temperature.

Review Question

1. List the d.c. characteristics of op-amp and define each of them.

May-07, 08, 11, Dec.-09, 16, Marks 8

2. Explain thermal drift.