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UNIVERSITI MALAYSIA PERLIS ANALOG ELECTRONICS II EMT 212 2009/2010 EXPERIMENT # 2 OP-AMP (PARAMETERS) MARKS T1 T2 G1 T3 G2 T4 T5 Q C Total 2 6 1 3 1 2 1 11 6 35 NAME signature MATRIK # 100% PROGRAMME GROUP DATE Analog Electronics II (EMT212) 2009/2010 Laboratory Module EXPERIMENT 2 Op-Amp (Parameters) 1. OBJECTIVE: 1.1 To investigate the bandwidth of an operational-amplifier as a function of gain. 1.2 To determine the slew rate of an operational-amplifier. 1.3 To investigate the output offset voltage at the output of an operationalamplifier. 2. INTRODUCTION: The operational amplifier has a few limitations. This experiment will investigate the operational-amplifier’s bandwidth as a function of gain, the slew rate of the operational-amplifier, and output offset voltages. Unlike a real operationalamplifier, the ideal operational amplifier has an infinite voltage gain, perfectly matched internal transistors, and no output offset voltage. The bandwidth of an operational-amplifier is inversely proportional to the closed-loop gain of the amplifier. The following equation shows the relationship between bandwidth and the feedback ratio, : fT where BWCL (2.1) fT is the gain-bandwidth product or unity-gain frequency BWCL is the closed-loop bandwidth of the amplifier is the feedback ratio: R1 R1 RF (2.2) Another factor that limits the high frequency response of an operational-amplifier is its maximum permissible slew rate. The maximum slew rate is the maximum value of: S where V t (2.3) V is a change in output voltage t is the time interval over which the output voltage changes The slew rate limits the high frequency response because at high frequencies there is a large rate-of-change of voltage. The maximum sinusoidal frequency at which an operational-amplifier having slew rate S can be operated without producing distortion is: f S (max) S 2K where fS (max) is the maximum frequency imposed by the slew rate limitation. K is the peak value of the output waveform. 2 (2.4) Analog Electronics II (EMT212) 2009/2010 Laboratory Module Output offset voltage is the dc voltage that appears at the output when both inputs are zero volts. The output offset voltage of an operational-amplifier is caused by input offset voltage, due to slightly mismatched transistors in the differential-amplifier input stage, and differences in input bias currents, I- and I+. The output offset voltage due to mismatched bias currents I+ and I- can be reduced by connecting a compensating resistor, RC, in series with the non-inverting input. This resistor does not affect the closed-loop gain of the amplifier. The optimum value of RC is : (2.5) RC R1 RF When this compensating resistor is used, the magnitude of the output offset voltage due to input offset current is: (2.6) V os ( I I ) R F I io R F where Vos is the magnitude of the output offset voltage I I I io is the input bias current at the inverting terminal is the input bias current at the non-inverting terminal is the input offset current Note that Vos may be either positive or negative, depending on which of I+ and I- is the larger. The 741 operational-amplifier has externally-accessible terminals that can be used to null, or balance, the amplifier, i.e. to adjust the output offset to zero when the inputs are zero. A potentiometer is connected across pins 1 and 5 for this purpose, as shown in Figure 4.5. 3. COMPONENTS & EQUIPMENTS: 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Resistors: 3.1.1 1 M (2) 3.1.2 470 k 3.1.3 100 k 3.1.4 47 k 3.1.5 10 k (2) Variable Resistor : 3.2.1 100 k LM 741 OP-AMP DC Power Supply Function Generator Oscilloscope Breadboard Digital multimeter 3 Analog Electronics II (EMT212) 2009/2010 4. Laboratory Module PROCEDURE: 4.1 To measure the unity-gain frequency of the operational-amplifier, connect the following circuit as shown in Figure 4.1. Apply a 100 Hz, 50 mVpk sinewave input signal to the circuit. +15V _ 7 2 741 3 + 6 4 -15V Vout Vs GND Figure 4.1 Unity-Gain Frequency Measuring Circuit 4.2 Using a dual-trace oscilloscope observe Vs and Vout. Increase the frequency of the signal generator, and record the frequency where Vout decreases to 0.707 times its value at 100 Hz. This frequency is the unity-gain frequency of the amplifier, or its gain-bandwidth product. Complete TABLE 1. 4.3 To demonstrate that the gain-bandwidth product is constant, connect the following circuit as shown in Figure 4.2. RF +15V 10 k _ 7 2 741 3 Vs + 6 4 -15V Vout GND Figure 4.2 Gain-Bandwidth Product Measuring Circuit 4 Analog Electronics II (EMT212) 2009/2010 4.4 4.5 Laboratory Module Apply a 100 Hz, 50 mVpk sinewave input signal to the circuit. With RF = 47 k, measure and record the output voltage, Vout and complete TABLE 2. Increase the frequency of the signal generator until the output voltage decreases to 0.707 times its value at 100 Hz. Remove the signal generator and make sure its output voltage has not changed. Record this frequency in TABLE 2. Repeat this procedure with RF = 100 k. To measure the slew rate of the operational-amplifier, connect the circuit in Figure 4.3. Apply a 1 kHz, 1Vpk square-wave input signal to the circuit. 100 k +15V 10 k _ 7 2 741 Vs 3 + 6 4 -15V Vout GND Figure 4.3 Slew Rate Measuring Circuit 4.6 With a dual-trace oscilloscope, measure the slew rate in the following manner. Adjust the time-base of the oscilloscope so that only one changing edge of the output waveform Vout can be viewed (either a low-to-high voltage change or a high-to-low change). Expand the time base on the oscilloscope so that the change in time t can be observed. Record this waveform in GRAPH 1. Then measure both the change in voltage V and the change in time t. Use these values to calculate the slew rate S 4.7 V . Record the result in TABLE 3. t Calculate the maximum frequency imposed by the slew rate using the equation in the Introduction part. Now, apply a 1 kHz, 10Vpk sinewave input signal and use a 10 k resistor for RF. Increase the signal generator frequency beyond this calculated maximum frequency and note the changes in the waveform of Vout. Record this waveform in GRAPH 2. 5 Analog Electronics II (EMT212) 2009/2010 4.8 Laboratory Module To measure the total output offset voltage of the operational amplifier, connect the following circuit as shown in Figure 4.4(a). 1 M +15V 1 M _ 7 2 741 3 + 6 4 Vout -15V GND Figure 4.4(a) Output Offset Voltage Measuring Circuit 4.9 Using a digital voltmeter, measure and record the dc output voltage Vout in TABLE 4. 4.10 Now replace the short-circuit to ground on the non-inverting terminal with a 470 k resistor to ground as shown in Figure 4.4(b). Repeat procedure step 4.9. 1 M +15V 1 M _ 7 2 741 3 470 k + 6 4 -15V Vout GND Figure 4.4(b) Total Output Offset Voltage Measuring Circuit 6 Analog Electronics II (EMT212) 2009/2010 Laboratory Module 4.11 To demonstrate how a 741 amplifier can be balanced, connect a potentiometer as shown in Figure 4.5. 4.12 While measuring Vout with a digital voltmeter, adjust the 100 k potentiometer until the output offset voltage is as close to 0 V as possible. Measure and record this total output offset voltage in TABLE 5. 1 M +15V 1 M _ 7 2 741 3 5 + 4 1 470 k 6 -15V Vout 100 k RC GND -15V Figure 4.5 Balancing The Total Output Offset Voltage Circuit 7 Analog Electronics II (EMT212) 2009/2010 Laboratory Module Name :_____________________________ Matric No. : _______________________________ 5. Date : _______________ RESULTS: TABLE 1 Voltage (calculated) 0.707Vout Frequency (2 marks) TABLE 2 RF Vout Voltage (calculated) Frequency 0.707Vout 47 k 100 k (6 marks) GRAPH 1 8 (1 mark) Analog Electronics II (EMT212) 2009/2010 Laboratory Module Name :_____________________________ Matric No. : _______________________________ Date : _______________ TABLE 3 V (Volt) t (sec) S (3 marks) GRAPH 2 (1 mark) TABLE 4 Vout (V) Without 470 k With 470 k (2 marks) TABLE 5 Vos (V) (1 mark) 9 Analog Electronics II (EMT212) 2009/2010 Laboratory Module Name :_____________________________ Matric No. : _______________________________ 6. Date : _______________ QUESTION: mark Q1 Using the results of procedure steps 4.2 – 4.4, verify that the gain-bandwidth product is constant. Compare this constant to the manufacturer’s specifications. A1 (3) A2 Compare the slew rate calculated in procedure step 4.6 with the manufacturer’s specifications. Using the results of procedure step 4.7, describe the effects that exceeding the maximum frequency that the slew rate imposed had on the output waveform Vout. Q3 Which is better op-amp when it comes to slew rate? Q2 (3) (2) A3 Q4 A4 Compare the total output offset voltage measured in procedure step 4.9 with the manufacturer’s specifications. How well did the modification of procedure step 4.9 decrease the total output offset voltage? Explain why. 10 (3) Analog Electronics II (EMT212) 2009/2010 Laboratory Module Name :_____________________________ Matric No. : _______________________________ 7. Date : _______________ CONCLUSION: (6 marks) Based on your measurement data and graph, make an overall conclusion by referring to the objective of this experiment in terms of: 1. Bandwidth and gain? 2. The relationship between slew rate and frequency? 3. The effect of output offset voltage of an op-amp? 11