OBJECTIVES The objectives of the experiments are as follow:
To understand the properties of an ideal operational amplifier.
To understand the limitations of a practical operational amplifier.
To be familiar with the typical applications of an operational amplifier.
EXPERIMENT PROCEDURE AND LAB RESULT 1. Inverting Amplifier a) Connect the circuit of KHz and compare it with the predicted value. b) Figure 1, using both the positive and negative power supplies. c) Adjust the sine wave generator on the E&L CADET II Ruggedized Electronic Circuit Trainer to give an output of 1
at 1 KHz.
d) Monitor and with the dual channel oscilloscope. Measure the voltage gain of the operational amplifier at 1 KHz and compare it with the predicted value.
Table 1: Actual gain vs. Predicted Gain for inverting Amplifier
2. Frequency response of Amplifier Measure the frequency response of the amplifier by noting the voltage gain at the frequencies indicated below: At the “half power cut-off frequency”, the gain drops to at 1 KHz, determine this frequency.
of the gain
Table 2: Frequency response of Amplifier Frequency (kHz) Gain =
9.53 9.53 9.53 9.22 8.59 7.03
Frequency Response of Amplifier 10 9 8
7 6.739 6 5 4 3 2 1 0
1000 Frequncy (Hz)
of the gain at 1 KHz
42.3 kHz 100000
, by linear interpolation, we
have: Half power cut-off frequency 3. Input Bias Current a) Remove the sine wave generator and short the input
to ground. Measure the DC
output voltage with the voltmeter. b) Ground the non-inverting, input with a 10 kΩ resistor instead of the straight wire. Measure the DC output voltage again. Table 3: Effect of input bias current balancing resistance No Bias Current balancing
With 10 kΩ Bias Current
Output Voltage 4. Non-inverting Amplifier
Connect the circuit shown in Figure 2, measure the gain of the amplifier at 1 KHz by monitoring
on the oscilloscope.
Table 4: Actual gain vs. Predicted Gain for inverting Amplifier 1.00V
5. Comparator Circuit Connect the comparator circuit shown in Figure 3, determine its transfer characteristics (relationship between
Table 5: Input vs. Output
3.45 -0.24 -0.29 -0.31 -0.34 -0.37
Transfer Characteristics 6 5
4 3 2 1 0 -1 -15.0 -12.5 -10.0 -7.5
0.0 2.5 Input (V)
switches from “High” to “Low” when
In this experiment, the
6. Digital to Analog Converter Connect the Digital-to-Analog converter in Figure 4, verify the circuit using the switches available on the test system.
Table 6: Digital to Analog Converter BINARY BIT THEORETICAL(V) EXPERIMENTAL(V)
In the experiment, S6 is the MSB and S8 is the LSB.
Digital to Analog Converter 0.0 -0.5
y = 0.9911x - 0.0099 R²= 1.0000
-1.0 -1.5 -2.0
-2.5 -3.0 -3.5 -4.0 -4.5 -5.0 -5.0
-3.0 -2.5 -2.0 Theoretical (V)
From above chart, we can see the analog output matches with theoretical value very well. (k=0.9911 and R2=1) DISCUSSION 1. Experiment 1: Actual Gain vs. Theoretical Gain The gain obtained from experiment result is quite closed to the predicted value. The minor difference (~5%) is caused by the following factors: a) We use ideal Op Amp assumption when calculate the close loop gain, but the characteristic of Op Amp (741) used in the experiment is not ideal: Parameters Idealized Op Amp
Actual Op Amp (741)
Open Loop Gain, Avo
Input impedance, Zin
Output impedance, Zout
Input Current. Iin
Offset Voltage, Vi
1V (in Experiment)
b) During experiment, the input voltage is not stable, it has approx. 2% fluctuation (1.000~1.023V). c) The measurement equipment like oscilloscope also contributes system errors when read the output pk-pk value. 2. Experiment 2 : Frequency Response of Amplifier This experiment shows that the actual Op Amp has a limited bandwidth, at higher frequency (>20 KHz) the close-loop gain is lower than at lower frequency condition. The frequency
response analysis of the circuit illustrated that there is a tradeoff between gain and bandwidth. This trade off must be recognized when designing with op amps. From the frequency response chart, it’s able to find out the half power cut-off frequency, which is the frequency where the close-loop gain is 70.7% or -3dB of the maximum gain. 3. Experiment 3 : Input Bias Current Error Under ideal conditions, the output voltage should be zero when the input is connected to ground. However, this is not true for real life Op Amp. Consider the inverting amplifier circuit shown below:
If the input voltage is zero, there should be zero current coming into the inverting input of the op-amp. However, there is a small bias current, I1, which goes through Rf. This current creates a voltage at the output equal to I1Rf. This is the error voltage. The same voltage will be seen at the output of a non-inverting amplifier. If we look at the voltage follower circuit shown below, it is easy to see that the output error voltage is –I1Rs.
In a non-inverting amplifier we add a resistor Rc. The compensating resistor value equals the parallel combination of Ri and Rf. The input creates a voltage drop across Rc that offsets the voltage across the combination or Ri and Rf. Thus, the output is reduced. The same is done for the inverting amplifier.
In the experiment, when an input bias current compensating resistor (10K) is added to the non-inverting pin, the output voltage is decreased from 8.8mV to 3.0mV, which proves the effectiveness of compensating resistor. 4. Experiment 4: Non Inverting Amplifier The gain obtained from experiment result is quite closed to the predicted value. The minor difference (~5%) is caused by the same reasons explained in Experiment 1. 5. Experiment 5: Comparator Circuit This experiment examines the principle behind the comparator circuit, which compares input value Vi against the reference voltage Vref and determines the operation of the op-amp (either on or off) due to the Zener diode’s properties (forward or reversed bias). The actual cut-off value of the input voltage is not equal to the reference voltage (2.5V instead of 3.0V), possibly caused by the forward offset voltage. 6. Experiment 6: Digital to Analog converter From the experiment result, we can see the analog output match with theoretical calculation very well.
CONCLUSION In conclusion, with this experiment I have better understanding about the application of Operational Amplifier; it can be used as an inverting amplifier, non-inverting amplifier, comparator and a digital-to-analog converter. I also have better understanding about the limitations of Operational Amplifier, such as limited band width and input bias current error.