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Electronics-communication engineering analog-electronics practical-operational-amplifier notes
Physics
Kendriya Vidyalaya Hebbal
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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:
- 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
Vio = Vdc1 � Vdc
, 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. V io is 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
- Input offset Current:
The input offset current Iio 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. the I io value decreases further a precision OPAMP 741C, I io is 6 nA
3 Bias Current:
The input bias current I B is the average of the current entering the input terminals of a balanced amplifier i.
I B = (IB1 + IB2 ) / 2
For 741C I B(max) = 700 nA and for precision 741C I B = ± 7 nA
- 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 OPAMP it may be up to 1000 G ohm.
- Input Capacitance: (Ci)
Ci 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 Ci is 1 pf for the 741C.
- 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.
SVRR = D Vio / D V
Where D V is the change in the input supply voltage and D Vio 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 R 3 £ 10K. For same OPAMPS, SVRR is separately specified as positive SVRR and negative SVRR.
- 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.
- 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 +VCC and
�VEE. For a 741C it is ± 13 V.
- Output Resistance: (RO)
RO 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
a. vin 1 = 5 m V dc, vin 2 = -7 μVdc
:
b. vin 1 = 10 mV rms, vin 2 = 20 mV rms
Fig. 4
Specifications of the OPAMP are given below: A = 200,000, Ri = 2 M Ω , R O = 75Ω, + VCC = + 15 V, - VEE = - 15 V, and output voltage swing = ± 14V.
Solution:
(a). The output voltage of an OPAMP is given by
Remember that vo = 2 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 vo = -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.
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
- 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 V / m S. which limits its use in higher frequency applications.
- 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 Vio / 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 Vio / D T = 0 m V / C. D I io / D T = 12 pA / C.
Example - 1
A 100 PF capacitor has a maximum charging current of 150 μA. What is the slew rate?
Solution:
C = 100 PF=100 x 10-12 F I = 150 μA = 150 x 10-6 A
(ii) If A = 10 5 then the output offset voltage Vin(off) = 20 μ V x 10 5 = 2 volt
Output offset voltage = 2 volts.
Example - 4
R1 = 100Ω, Rf = 8 k, RC = 10 k. Assume that the amplifier is nulled at 25°C. If Vin is 20 mV peak sine wave at 100 Hz. Calculate Er, and Vo values at 45°C for the circuit shown in fig. 2.
Fig. 2
Solution:
The change in temperature ΔT = 45 - 25 = 20°C.
Error voltage = 51 mV
Output voltage is 1640 mV peak ac signal which rides either on a +51 mV or -51 mV dc level.
Example - 5
Design an input offset voltage compensating network for the operational amplifier μA 715 for the circuit shown in fig. 3. Draw the complete circuit diagram.
Fig. 3
Solution:
From data sheet we get vin = 5 mV for the operational amplifier μA 715.
V = | VCC | = | - VEE | = 15 V
Now,
If we select RC = 10Ω, the value of Rb should be Rb = (3000) RC = 30000Ω = 304Ω
- Infinite bandwidth so that any frequency signal from 0 to infinite Hz can be amplified without attenuation.
- Infinite common mode rejection ratio so that the output common mode noise voltage is zero.
- Infinite slew rate, so that output voltage changes occur simultaneously with input voltage changes.
There are practical OPAMPs that can be made to approximate some of these characters using a negative feedback arrangement.
Equivalent Circuit of an OPAMP:
Fig. 5, shows an equivalent circuit of an OPAMP. v 1 and v 2 are the two input voltage voltages. Ri is the input impedance of OPAMP. Ad Vd is an equivalent Thevenin voltage source and RO is the Thevenin equivalent impedance looking back into the terminal of an OPAMP.
Fig. 5
This equivalent circuit is useful in analyzing the basic operating principles of OPAMP and in observing the effects of standard feedback arrangements
vO = Ad (v1 � v2 ) = Ad vd.
This equation indicates that the output voltage vO is directly proportional to the algebraic difference between the two input voltages. In other words the OPAMP amplifies the difference between the two input voltages. It does not amplify the input voltages themselves. The polarity of the output voltage depends on the polarity of the difference voltage v d.
Ideal Voltage Transfer Curve:
The graphic representation of the output equation is shown in fig. 6 in which the output voltage vO is plotted against differential input voltage vd , keeping gain Ad constant.
Fig. 6
The output voltage cannot exceed the positive and negative saturation voltages. These saturation voltages are specified for given values of supply voltages. This means that the output voltage is directly proportional to the input difference voltage only until it reaches the saturation voltages and thereafter the output voltage remains constant.
Thus curve is called an ideal voltage transfer curve, ideal because output offset voltage is assumed to be zero. If the curve is drawn to scale, the curve would be almost vertical because of very large values of Ad.
Electronics-communication engineering analog-electronics practical-operational-amplifier notes
Course: Physics
University: Kendriya Vidyalaya Hebbal
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