Measrement of IC741 OpAmp parameters
Prerequisite:
 Pin out of IC 741C with supply voltage rejection ratio.
 Immense knowledge of OpAmp’s AC and DC characteristics.
Objective:
 To study the pin configurations, specifications & functioning of IC 741 OpAmp.
 To measure the OpAmp parameters and compare them with ideal characteristics.
Apparatus:
 IC µA 741 OPAmp.
 CRO.
 Dual Regulated Power Supply
 Voltmeter 010mV  2 Nos.
 Ammeters 010 µA  2 Nos.
 Connecting Wires.
Pinout and Circuit diagrams:
IC µA741 OPAMP
Specifications:
Supply voltage  
µA 741A, µA 741, µA 741E  ± 22V 
µA 741C  ± 18V 
Internal power dissipation  
DIP package  310 mw 
Differential input voltage  ± 30V 
Operating temperature range  
Military (µA 741A, µA 741)  55^{0} to +125^{0} C 
Commercial (µA 741E, µA 741C)  0^{0} C to +70^{0} C 
Input offset voltage  1.0 mV 
Input Bias current  80 nA 
PSSR  30µV/V 
Input resistance  2MΩ 
CMMR  90dB 
Output resistance  75Ω 
Bandwidth  1.0 MHz 
Slew rate  0.5 V/µ sec 
Circuit Diagrams:
a) Input bias current and input offset current
Figure 1.
Input Bias current: (Ib1 + Ib2) / 2
Input offset current: Ib1  Ib2
b) Input offset voltage
Figure 2.
Viof = The input voltage for which the output Vo is zero. Otherwise, it is the output voltage for input zero with unit gain amplification.
c) CMRR (Common Mode Rejection Ratio)
Figure 3.
CMRR = Ad / Ac, Differential input voltage is given between V_{1} and V_{2} and common mode voltage is given as common to both the inputs.
d) PSRR (Power supply Rejection Ratio)
The variation of the output voltage due to variation of the power given to the IC. It is measured as µV / V.
e) Slew rate and Bandwidth
Figure 4.
Figure 5.
Theory:
An opamp is a high gain, direct coupled differential linear amplifier choose response characteristics are externally controlled by negative feedback from the output to input, opamp has very high input impedance, typically a few mega ohms and low output impedance, less than 100Ω.
Opamps can perform mathematical operations like summation integration, differentiation, logarithm, antilogarithm, etc., and hence the name operational amplifier opamps are also used as video and audio amplifiers, oscillators and so on, in communication electronics, in instrumentation and control, in medical electronics, etc.
OpAmp characteristics:
An ideal opamp draws no current from the source and its response is also independent of temperature. However, a real opamp does not work this way. Current is taken from the source into opamp inputs. Also the two inputs respond differently to current and voltage due to mismatch in transistors. A real opamp also shifts its operation with temperature. These nonideal characteristics are:
 Input bias current
 Input offset current
 Input offset voltage
 Thermal drift
 Slew rate
 Input and output voltage ranges
Procedure:
 Connect the IC 741 with supply voltages on a bread board.
 Connect the CRO as per the requirement for voltage and for current measurement.
 Measure the parameters of OpAmp and compare them with ideal one.
1. Input bias current and input offset current:
 Connect the circuit as shown in figure for Input bias current and input offset current.
 Measure the current at inverting and non inverting terminals of opamp as IB^{+} and IB^{}.
 Average these values to find out the input Bias current. Also, find the difference between these two currents to know the input offset current.
IB^{+}  IB^{}  Input Bias Current  Input Offset Current 

2. Input offset voltage:
 Connect the circuit as shown in Figure 2. for Input offset voltage measurement.
 Give zero volts as input and measure the output.
Output Voltage (V_{out})  Input Voltage at Inverting Terminal V_{1}(V)  Input Voltage at NonInverting Terminal V_{2}(V) 

3. Slew Rate:
 Connect the circuit diagram as shown in figure 4.
 Apply A square wave of low frequency at inverting terminal of opamp.
 The input amplitude of square wave is to be adjusted until the output is 20 volts peak to peak.
 The frequency is then to be adjusted until the output becomes triangular.
 The rising edge slope of the trianglewave was taken to be the slew rate that was being sought.
Input Voltage at Inverting Terminal V(V)  Output Slope V/µ Sec  Frequency (Hz) 

4. CMRR Measurement:
Configure the OpAmp in unit gain configuration, in differential mode operation as shown in figure 3.. Give 1V as differential input and observe the output as 1V. Keeping the differential input same as 1V, increase the common mode voltage from 0V to 10V insteps of 1V and observe the variation in the output. Calculate Ad and Ac and CMRR.
5. Bandwidth:
Connect the opamp in unit gain configuration as shown in figure 5. Apply 1V sinusoidal input at 10Hz and observe the output. Increase the frequency in steps (semilogarithemic scale) and note the output voltage values. Identify the 3dB cutoff point, which gives the band width.
Result:
The pin configuration, specifications & functioning of different integrated circuits used in the practical applications have been studied.

UpdatedOct 24, 2016

Views11,519