Characteristics of Operational Amplifier
Experiment 1
Aim
To Measure the Input bias current, Input offset current, Input offset voltage and Slew rate of IC 741 Op-Amp
Apparatus Required
Theory
Input OFFSET Voltage : Ideal operational amplifiers have zero output voltage when the both inputs are same. However, real operational amplifiers do not meet this ideal goal. A small DC voltage must be applied to the input in order to drive the DC output voltage to zero. This small differential input voltage is called the Input OFFSET Voltage.
Input Bias Current: The DC current flowing into the input terminals of an ideal operational amplifier is zero. But in practice, the input current is not exactly zero there are always two input bias currents, IB+ and IB- . Input bias current IB is defined as the average of two input bias current IB1 and IB2 i.e. IB = (IB1 + IB2) / 2 . Where IB1 is the DC bias current flowing into the non inverting input and IB2 is the DC bias current flowing in to the inverting input
Input OFFSET Current: The mismatch or difference between the input bias currents flowing through the inputs of an operational amplifier. Input offset current Ii0 is defined as the algebraic difference between two input bias currents IB1 and IB2 . Ii0 = |IB1 – IB2|.
Slew rate: The maximum rate at which the Op-Amp is able to change to a different voltage is called the ``Slew rate'' because it's the rate at which the amplifier can slew to a different value. It's usually expressed in V/ - the bigger the number, the faster the Op-Amp. For example, µA 741 Op-Amp has a slew rate of 0.5V/µs. This means that the output from µA 741 Op-Amp can change by 0.5Vevery micro second.
Note: When a signal at a given frequency is applied to the Op-Amp, the maximum permissible output voltage is determined by the slew rate. And if we try to develop the output voltage higher than the maximum permissible value, the signal waveform will be distorted.
Circuit Diagram
Observations
Model Graph
Procedure
To measure the Input Bias Current IB :
Connecting the circuit in inverting mode as shown in figure measure the output voltage Vo then find IB1 using the formulas IB2 = Vo/Rf.
Connect the circuit in non inverting mode as shown in figure. Measure the output voltage Vo then find IB2 using the formulas IB1 = Vo/Rf then find IB using the formula IB = (IB1 + IB2 ) / 2
To measure the Input OFFSET Current Iio : From the calculated values of IB1 and IB2 the input offset current can be calculated using the formula Iio = |IB1 - IB2 |
To measure the OFFSET Voltage Vio : Connect the circuit as shown in figure. Measure the D.C. output voltage at pin 6. The gain is given by Rf/R1 thus for the given circuit it is 1000. Hence the input offset voltage is found using Vio = Vout /1000
To measure Slew rate SR: Connect the circuit as shown in figure. Use the CRO to find the output of the Op-Amp at pin 6. Set the audio oscillator to 10khz, adjust the signal level to over drive the Op-Amp, then adjust CRO timings the desired waveform. Trace the waveform on a graph sheet. Now measure the voltage change of waveform, then the slew rate is found by using the formula. SLEW RATE = 2пfV (V/ μs) Where, ‘f’ is frequency and ‘V’ is amplitude of the signal
Link to experiment
Go here and select Experiment 1
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