Drexel University Electrical and Computer Engineering Dept. Advanced Analog Electronics, ECE-E421 TITLE: Operational Amplifier Performance NAMES: Curtis King Salah Abushariefeh SECTION: 061 DATE PERFORMED: October 3, 2007 DATE DUE: October 10, 2007 DATE RECEIVED: Objective: The objective of this lab is to explore the electrical characteristics of operational amplifier and measure the performance of a 741-type Operational Amplifier IC. There were seven components to this lab used to study the characteristics of the uA741 Opamp. These components are detailed below. A. Inverting Amplifier: Figure 1 from “preparation” illustrates the inverting configuration of the Op-amp. The gain of the inverting Op-amp is (A = (-RF/R1)). The gain can be manipulated by using different values for the RF and R1 resistors. Figure 1. An Inverting Amplifier with VIN = 100mV and f = 500Hz. Resistor values vary with the corresponding gain. Inverting amplifiers were designed in the pre-lab with gains of -50 and 5 respectively using different values of input resistance. The above Op-amp configuration was built and operated with input signal of 500 Hz, 100mV. Figure 2. Input VIN (1) and Output VOUT (2) voltages measurements for the inverting op-amp circuit. Resistor values are provided in table 1. Figure 3. Input VIN (1) and Output VOUT (2) voltages measurements for the inverting op-amp circuit. Resistor values are provided in table 1. The following table summarizes the measurements obtained in this part: Gain of Inverting Operational Amplifier RIN(ideal)KΩ 2.00 100.00 RF(ideal)KΩ 100.00 500.00 G(ideal) -50.00 -5.00 RIN(actual)KΩ 1.96 98.52 RF(actual)KΩ 98.34 492.95 G(actual) -50.17 -5.004 Error % 0.34 0.00 Table 1. Inverting Operational Amplifier resistors used and the actual value of the resistors. It also shows the gain of the system and percent error. B. Non-inverting Amplifier: Figure 4 from “preparation” illustrates the non-inverting configuration of the Opamp. The gain of this Op-amp is (A = (1 + RF/R1)). The gain can be manipulated by using different values for the RF and R1 resistors Non-inverting Amplifiers were designed in the pre-lab with gains of 2 and 200 respectively. Figure 4 shows the non-inverting amplifier configuration. Figure 4. Non-Inverting Amplifier with VIN = 100mV and f = 500Hz. Resistor values vary with the corresponding gain. Resistors RF and R1 were chosen to realize a gain of 1 and 199 for gains or 2 and 200, respectively. The real values of the resistors were provided in the table below. Figure 5. VIN (Top) and VOUT (Bottom) for the non-inverting op-amp circuit showing a gain of 2.0 with RF = 9.74k and R1 = 9.83k. . Figure 6. Input (1) and Output (2) Voltages for the non-inverting op-amp circuit with gain of 182.9 using RF and R1 to be197.34 k and .99 k, respectively. The following table summarizes the measurements obtained in this part: Gain of Non-Inverting Operational Amplifier Rin(ideal)KΩ 10.00 1.00 Rf(ideal)KΩ 10.00 199.00 G(ideal) 2.00 200.00 Rin(actual)KΩ 9.74 0.99 Rf(actual)KΩ 9.83 197.34 G(actual) 2.01 200.74 Error % 0.46 0.37 Table 2. Non-Inverting Operational Amplifier resistors used and the actual value of the resistors. It also shows the gain of the system and percent error. C. Frequency Response: The expected frequency responses of two different inverting Op-amps were designed with frequency ranging from 10Hz to 1MHz in the pre-lab. The first one had a gain of -10 with and input impedance of 1KΩ, while the second one had a gain of -100 with an input impedance of 1KΩ (see fig 7). Figure 7. Input (1) and Output (2) for the inverting op-amp circuit with a gain is 968.8mV/101mV = 9.8. Real inverting Op-amps were constructed using resistors RF = 9.8KΩ and R1 = R2 = .99KΩ. The frequency response of was captured and recorded using LABVIEW. The frequency responses in terms of V/V and dB and is illustrated in figures 8 and 9, respectively. FREQUENCY RESPONSE (V/V) 11.00 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 100.0 Freq. resp (V/V) 1.0k 10.0k 100.0k 1.0M Figure 8. Frequency response in V/V of the inverting op-amp with a gain close to 10. FREQUENCY RESPONSE (dB) 25.0 20.0 15.0 10.0 5.0 0.0 -5.0 -10.0 -15.0 -20.0 100.0 Freq. resp (dB) 1.0k 10.0k 100.0k 1.0M Figure 9. Frequency response in dB of the inverting op-amp with gain close 10. The frequency response of the second circuit with a gain of -100 was built also. The frequency response of this op-amp was also captured using LABVIEW and is shown in figures 9 and 10, respectively. The break frequency at 6 kHz is apparent in fig 11. FREQUENCY RESPONSE (V/V) 110.00 100.00 90.00 80.00 70.00 60.00 50.00 40.00 30.00 20.00 10.00 0.00 100.0 Freq. resp (V/V) 1.0k 10.0k 100.0k 1.0M Figure 10. Frequency response (in V/V) of the inverting op-amp with a gain near -100. FREQUENCY RESPONSE (dB) 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 -5.0 100.0 Freq. resp (dB) 1.0k 10.0k 100.0k 1.0M Figure11. Frequency response (in dB) of the inverting op-amp with a gain near -100. D. Common Mode Rejection Ratio (CMRR) : The CMRR of a test circuit with both differential and common mode inputs was designed in the pre-lab was calculated by modifying the following formula The common mode gain ACM was calculated to be 47 whereas the differential mode gain was 47 also yielding a CMRR of 33.7dB. The circuit diagram of the test circuit is illustrated in figure 12. Figure 12. Circuit Diagram of the test circuit designed in the pre-lab. A hardwired version of this test circuit shown above was built with and input voltages of 50 mV and an output of 14.995V having a CMMR of 49.54dB. E. Input Offset Voltage (VOS): At test circuit, figure 13, was designed in the pre-lab and used to calculate the VOS. The VOS, a property of a particular op-amp, was predicted to be negligible using the following equation: VOUT = VOD + V01d The ideal circuit elements were R1 = R2 = 100Ω and RF = 100kΩ with a gain of -1000 (V/V). A hardwired version of the test circuit designed in pre-lab was constructed with R1 = 97Ω, RF = 99143Ω, and R2 = 97.94Ω having a gain of -1022. Figure 13. Test circuit designed in the Pre-lab to determine VOS. A value of 0.71 mV was calculated for the offset voltage VOS from the measured values. Amplifier Ideal Measured Gain (V/V) -1000 -1022.093 R1(Ω) 100 97 Rf(Ω) 100000 99143 R2(Ω) 100 97.94 Table 3: Test circuit containing data from both an ideal and real amplifier. F. Input Bias Current (IB): Another test circuit, figure 14, was designed to determine the input bias current (IB) of the non-inverting input of the operational amplifier. This was done by using the VOS calculated and the assumption that IB = IB1 = IB2. Tables 4 and 5 summarize the resistors values and gain of the test circuit shown below. Figure 14. Test circuit designed in the Pre-lab used to determine IB. Gain (V/V) Ideal Measured -10 -10.04 R1(Ω) 100 97.94 Rf(Ω) 1000000 984000 R2(Ω) 1000 983 Table 4. Test circuit in figure 14 summarizing data from both the ideal and real amplifier. IB1 (A) 6.17E-08 IB2 (A) 6.17E-08 IB (A) 7.67E-10 Vo (V) -0.67 Table 5. Test circuit in figure 14 containing data measured values of IB. G. Input Offset Current (IOS): The circuit shown in figure 14 was modified so that RF = 1MΩ while holding the gain constant at AV = 11. R2 (R2=RF//R1) was chosen such that the output voltage (VOUT) was very small. The hardwired version of the test circuit was built to measure the circuit elements and determine the value of IOS of a real op-amp. These values were compared to the theoretical design of the pre-lab. Using the circuit analysis obtained in the pre-lab and the observed value of output voltage (VOUT = 9 mV), IOS was calculated to be equal to 9.933E-8 A. Figure 15 shows the modified test circuit and table 5 summarizes the circuit element values and gain, while table 6 contains the values of the input offset current for the hardwired circuit. Figure 15. Modified test circuit diagram used to determine IOS. Gain (V/V) Ideal Measured -10 -10.03676 R1(Ω) 100 97.94 RF (Ω) 1000000 984000 R2 (Ω) 1000 983 Table 5: Altered circuit of figure 15 summarizing data from both the ideal and real amplifier. IB1 (A) 6.17E-08 IB2 (A) 6.17E-08 IB (A) 7.67E-10 Vo (V) -0.67 Table 6. Test circuit of figure 15 containing measured values of IB Conclusion: In conclusion, the 741 op-amp has limitations very similar to any electronic device. The 741 does not exhibit infinite input impedance as observed. It also has small output impedance where as the ideal op-amp has zero output impedance. In addition, gain values of 741 in both the inverting and non-inverting configurations were finite as opposed to infinite gain. Since this is not an Ideal op-amp, it can not be used as an impedance converter because this could cause significant error in circuit design and analysis.
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