Lecture 6

Lecture - 6: Practical Operational Amplifier

The symbolic diagram of an OPAMP is shown in fig. 1.

741c is most commonly used OPAMP available in IC package. It is an 8-pin DIP chip.

Parameters of OPAMP:

The various important parameters of OPAMP are follows:

1.Input Offset Voltage:

Input offset voltage is defined as the voltage that must be applied between the two input terminals of an OPAMP to null or zero the output fig. 2, shows that two dc voltages are applied to input terminals to make the output zero.

V_io = V_dc1 – V_dc2

V_dc1 and V_dc2 are dc voltages and R_S represents the source resistance. V_io is the difference of V_dc1 and V_dc2. It may be positive or negative. For a 741C OPAMP the maximum value of V_io is 6mV. It means a voltage ± 6 mV is required to one of the input to reduce the output offset voltage to zero. The smaller the input offset voltage the better the differential amplifier, because its transistors are more closely matched.

Fig. 2

2. Input offset Current:

The input offset current I_io is the difference between the currents into inverting and non-inverting terminals of a balanced amplifier.

I_io = | I_B1 – I_B2 |

The I_io for the 741C is 200nA maximum. As the matching between two input terminals is improved, the difference between I_B1 and I_B2 becomes smaller, i.e. the I_io value decreases further.For a precision OPAMP 741C, I_io is 6 nA

3.Input Bias Current:

The input bias current I_B is the average of the current entering the input terminals of a balanced amplifier i.e.

I_B = (I_B1 + I_B2 ) / 2

For 741C I_B(max) = 700 nA and for precision 741C I_B = ± 7 nA

4. Differential Input Resistance: (R_i)

R_i is the equivalent resistance that can be measured at either the inverting or non-inverting input terminal with the other terminal grounded. For the 741C the input resistance is relatively high 2 MΩ. For some OPAMP it may be up to 1000 G ohm.

5. Input Capacitance: (C_i)

C_i is the equivalent capacitance that can be measured at either the inverting and noninverting terminal with the other terminal connected to ground. A typical value of C_i is 1.4 pf for the 741C.

6. Offset Voltage Adjustment Range:

741 OPAMP have offset voltage null capability. Pins 1 and 5 are marked offset null for this purpose. It can be done by connecting 10 K ohm pot between 1 and 5 as shown in fig. 3.

Fig. 3

By varying the potentiometer, output offset voltage (with inputs grounded) can be reduced to zero volts. Thus the offset voltage adjustment range is the range through which the input offset voltage can be adjusted by varying 10 K pot. For the 741C the offset voltage adjustment range is ± 15 mV.

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