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. Vio = Vdc1 – Vdc2 Vdc1 and Vdc2 are dc voltages and RS represents the source resistance. Viois the difference of Vdc1 and Vdc2. It may be positive or negative. For a 741C OPAMP the maximum value of Vio 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.

7. Input Voltage Range :

Input voltage range is the range of a common mode input signal for which a differential amplifier remains linear. It is used to determine the degree of matching between the inverting and noninverting input terminals. For the 741C, the range of the input common mode voltage is ± 13V maximum. This means that the common mode voltage applied at both input terminals can be as high as +13V or as low as –13V.

8. Common Mode Rejection Ratio­ (CMRR).  

CMRR is defined as the ratio of the differential voltage gain A_d to the common mode voltage gain A_CM

CMRR = A_d / A_CM.

For the 741C, CMRR is 90 dB typically. The higher the value of CMRR the better is the matching between two input terminals and the smaller is the output common mode voltage.

9. Supply voltage Rejection Ratio: (SVRR)

SVRR is the ratio of the change in the input offset voltage to the corresponding change in power supply voltages. This is expressed in m V / V or in decibels, SVRR can be defined as

SVRR = D V_io / D V

Where D V is the change in the input supply voltage and D V_io is the corresponding change in the offset voltage.

For the 741C, SVRR = 150 µ V / V.

For 741C, SVRR is measured for both supply magnitudes increasing or decreasing simultaneously, with R₃ £ 10K. For same OPAMPS, SVRR is separately specified as positive SVRR and negative SVRR.

10. Large Signal Voltage Gain:

Since the OPAMP amplifies difference voltage between two input terminals, the voltage gain of the amplifier is defined as

Because output signal amplitude is much large than the input signal the voltage gain is commonly called large signal voltage gain. For 741C is voltage gain is 200,000 typically.

11. Output voltage Swing:

The ac output compliance PP is the maximum unclipped peak to peak output voltage that an OPAMP can produce. Since the quiescent output is ideally zero, the ac output voltage can swing positive or negative. This also indicates the values of positive and negative saturation voltages of the OPAMP. The output voltage never exceeds these limits for a given supply voltages +V_CC and –V_EE. For a 741C it is ± 13 V.

12. Output Resistance: (R_O)

R_O is the equivalent resistance that can be measured between the output terminal of the OPAMP and the ground. It is 75 ohm for the 741C OPAMP.

Example - 1

Determine the output voltage in each of the following cases for the open loop differential amplifier of fig. 4:

1. v_{in 1} = 5 m V dc, v_{in 2} = -7 µV_dc
2. v_{in 1} = 10 mV rms, v_{in 2}= 20 mV rms

Fig. 4

Specifications of the OPAMP are given below:
A = 200,000, R_i = 2 M Ω , R O = 75Ω, + V_CC = + 15 V, - V_EE = - 15 V, and output voltage swing = ± 14V.

Solution:

(a). The output voltage of an OPAMP is given by

Remember that v_o = 2.4 V dc with the assumption that the dc output voltage is zero when the input signals are zero.

(b). The output voltage equation is valid for both ac and dc input signals. The output voltage is given by

Thus the theoretical value of output voltage v_o = -2000 V rms. However, the OPAMP saturates at ± 14 V. Therefore, the actual output waveform will be clipped as shown fig. 5. This non-sinusoidal waveform is unacceptable in amplifier applications.

Fig. 5

13. Output Short circuit Current :

In some applications, an OPAMP may drive a load resistance that is approximately zero. Even its output impedance is 75 ohm but cannot supply large currents. Since OPAMP is low power device and so its output current is limited. The 741C can supply a maximum short circuit output current of only 25mA.

14. Supply Current :

I_S is the current drawn by the OPAMP from the supply. For the 741C OPAMP the supply current is 2.8 m A.

15. Power Consumption:

Power consumption (PC) is the amount of quiescent power (v_in= 0V) that must be consumed by the OPAMP in order to operate properly. The amount of power consumed by the 741C is 85 m W.

16. Gain Bandwidth Product:

The gain bandwidth product is the bandwidth of the OPAMP when the open loop voltage gain is reduced to 1. From open loop gain vs frequency graph At 1 MHz shown in. fig. 6, It can be found 1 MHz for the 741C OPAMP frequency the gain reduces to 1. The mid band voltage gain is 100, 000 and cut off frequency is 10Hz.

Fig. 6

17. Slew Rate:

Slew rate is defined as the maximum rate of change of output voltage per unit of time under large signal conditions and is expressed in volts / m secs.

To understand this, consider a charging current of a capacitor shown in fig. 7.

Fig. 6

If 'i' is more, capacitor charges quickly. If 'i' is limited to I_max, then rate of change is also limited.

Slew rate indicates how rapidly the output of an OPAMP can change in response to changes in the input frequency with input amplitude constant. The slew rate changes with change in voltage gain and is normally specified at unity gain.

If the slope requirement is greater than the slew rate, then distortion occurs. For the 741C the slew rate is low 0.5 V / m S. which limits its use in higher frequency applications.

18. Input Offset Voltage and Current Drift:

It is also called average temperature coefficient of input offset voltage or input offset current. The input offset voltage drift is the ratio of the change in input offset voltage to change in temperature and expressed in m V /° C. Input offset voltage drift = ( D V_io / D T).

Similarly, input offset current drift is the ratio of the change in input offset current to the change in temperature. Input offset current drift = ( D I_io / D T).

For 741C,

D V_io / D T = 0.5 m V / C.
D I_io/ D T = 12 pA / C.