The symbolic diagram of an OP-AMP is shown in Figure 1.
Figure 1
741c is most commonly used OPAMP available in IC package. It is an 8-pin DIP chip.
Parameters of OP-AMP:
The various important parameters of OP-AMP are follows:
1. Input Offset Voltage (Vio)
Input offset voltage is defined as the voltage that must be applied between the two input terminals of an OP-AMP to null or zero the output Figure 2, shows that two dc voltages are applied to input terminals to make the output zero.
Vdc1 and Vdc2 are dc voltages and RS represents the source resistance. Vio is the difference of Vdc1 and Vdc2.
Figure 2
It may be positive or negative. For a 741C OP-AMP the maximum value of Vio is 6 mV. 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.
2. Input offset Current (Iio)
The input offset current Iio is the difference between the currents into inverting and non-inverting terminals of a balanced amplifier.
Iio = | IB1 – IB2 |
The Iio for the 741C is 200 nA maximum. As the matching between two input terminals is improved, the difference between IB1 and IB2 becomes smaller, i.e. the Iio value decreases further. For a precision OP-AMP 741C, Iio is 6 nA
3.Input Bias Current
The input bias current IB is the average of the current entering the input terminals of a balanced amplifier i.e.
IB = (IB1 + IB2 ) / 2
For 741C IB(max) = 700 nA and for precision 741C IB = ± 7 nA
4. Differential Input Resistance (Ri)
Ri 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 OP-AMP it may be up to 1000 G ohm.
5. Input Capacitance (Ci)
Ci is the equivalent capacitance that can be measured at either the inverting and non-inverting terminal with the other terminal connected to ground. A typical value of Ci is 1.4 pf for the 741C.
6. Offset Voltage Adjustment Range
741 OP-AMP 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 Figure 3.
Figure 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 non-inverting 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 Ad to the common mode voltage gain ACM.
CMRR = Ad / ACM
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 µV / V or in decibels, SVRR can be defined as
SVRR = ∆Vio / ∆V
Where ∆V is the change in the input supply voltage and ∆Vio is the corresponding change in the offset voltage.
For the 741C, SVRR = 150 µV / V.
For 741C, SVRR is measured for both supply magnitudes increasing or decreasing simultaneously, with R3 ≤ 10K. For same OP-AMPS, SVRR is separately specified as positive SVRR and negative SVRR.
10. Large Signal Voltage Gain
Since the OP-AMP 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 OP-AMP 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 OP-AMP. The output voltage never exceeds these limits for a given supply voltages +VCC and –VEE. For a 741C it is ± 13 V.
12. Output Resistance (RO)
RO is the equivalent resistance that can be measured between the output terminal of the OP-AMP and the ground. It is 75 ohm for the 741C OP-AMP.
13. Output Short circuit Current
In some applications, an OP-AMP may drive a load resistance that is approximately zero. Even its output impedance is 75 ohm but cannot supply large currents. Since OP-AMP is low power device and so its output current is limited. The 741C can supply a maximum short circuit output current of only 25 mA.
14. Supply Current
IS is the current drawn by the OP-AMP from the supply. For the 741C OP-AMP the supply current is 2.8 m A.
15. Power Consumption
Power consumption (PC) is the amount of quiescent power (Vin = 0 V) that must be consumed by the OP-AMP in order to operate properly. The amount of power consumed by the 741C is 85 mW.
16. Gain Bandwidth Product
The gain bandwidth product is the bandwidth of the OP-AMP when the open loop voltage gain is reduced to 1. From open loop gain vs frequency graph at 1 MHz shown in Figure 6, It can be found 1 MHz for the 741C OP-AMP frequency the gain reduces to 1. The mid band voltage gain is 100, 000 and cut off frequency is 10 Hz.
Figure 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 / µsecs.
To understand this, consider a charging current of a capacitor shown in Figure 7.
Figure 7
If 'i' is more, capacitor charges quickly. If 'i' is limited to Imax, then rate of change is also limited.
Slew rate indicates how rapidly the output of an OP-AMP 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 / µS. 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 µV /° C. Input offset voltage drift = ( ∆Vio / ∆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 = ( ∆Iio / ∆T).
For 741C,
∆Vio / ∆T = 0.5 µV / C
∆Iio/ ∆T = 12 pA / C
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