Op-amp IC 741

Op-amp IC 741

A very popular IC version of op-amp is μA741. The manufacturer of pA741 is Fairchild Semiconductor.

Number of other manufacturers also produced 741 op-amp IC version namely MC1741 by Motorola, LM741 by National Semiconductor etc. For convenience, this widely used op-amp IC is called simply IC 741 op-amp dropping the prefixes.

Pin Diagram :

The IC 741 is 8 pin IC available in dual in line package (DIP). The pin diagram of IC 741 op-amp is shown in the Fig. 2.19.1.

The pins 1 and 5 are offset null pins. These are used to nullify offset voltages and provide offset voltage compensation.

The pin 2 is inverting input while pin 3 is noninverting input terminal. 

The output is to be taken from pin 6.

The pin 4 is for - V_EE supply while pin 7 is for + V_CC supply.

The pin 8 is the dummy pin and no connection are to be made to this pin externally.

Ideal Vs Practical Characteristics of 1C 741 Op-amp :

The Table 2.19.1 lists the ideal op-amp characteristics and the typical values of these characteristics for 741 IC.

Features of IC 741 :

i) No frequency compensation required.

ii) Short circuit protection provided.

iii) Offset voltage null capability.

iv) Large common mode and differential voltage range.

v) No latch up.

Realistic Simplifying Assumptions

We can make two assumptions which are realistic and simplify the analysis of op-amp circuits to a great extent. The assumptions are useful and can be used to obtain the output expressions in variety of linear applications.

1. Zero Input Current

The current drawn by either of the input terminals (inverting and noninverting) is zero. 

In practice, the current drawn by the input terminals is very small, of the order of μA or nA. Hence the assumption of zero input current is realistic.

2. Virtual Ground

This means the differential input voltage V_d between the noninverting and inverting input terminals is essentially zero.

This is obvious because even if output voltage is few volts, due to large open loop gain of op-amp, the difference voltage V_d at the input terminals is almost zero.

e.g. if output voltage is 10 V and the A_OL i-e- open loop gain is 10⁴ then

V_o = V_d A_OL V_d = V_o / A_OL = 10 / ∞ = 0

Hence V_d is very small. As A_OL → ∞ difference voltage v_d → 0 and realistically assumed to be zero for analysing the circuits.

Thus we can say that under linear range of operation there is virtually short circuit between the two input terminals, in the sense that their voltages are same. No current flows from the input terminals to the ground. The Fig. 2.20.1 shows the concept of the virtual ground. The thick line indicates the virtual short circuit between the input terminals.

Now if the non-inverting terminal is grounded, by the concept of virtual short, the inverting terminal is also at ground potential, though there is no physical connection between the inverting terminal and the ground. This is the principle of virtual ground.

Key Point Thus from the equation (2.20.1), the voltage at the one input terminal of an op-amp can be realistically assumed to be equal to the voltage at the other input terminal.

This concept is very much useful in the analysis of closed loop op-amp circuits. The steps of analysis based on the assumptions are,

Step 1 : Input current of the ideal op-amp is always zero. Using this, the current distribution in the circuit is obtained.

Step 2 : The input terminals of the op-amp are always at the same potential. Thus if one is grounded, the other can be treated to be virtually grounded. From this, the expressions for various branch currents can be obtained.

Step 3 : Analyzing the various expressions obtained, eliminating unwanted variables, the output expression interms of input and circuit parameters can be obtained.

Review Question

1. Explain the significance of virtual ground. How would you explain the existence of virtual ground ?