ECE 3235 Electronics II

ECE 3235 Electronics II
Experiment # 2
Basic Application of OpAmp I
Note: you might want to bring a calculator to the lab
1. General Information
Figure 1 OpAmp 741 Pinout.
Figure1 shows the pin configuration for the famous 741 operational amplifier (OpAmp).
Note that there is no ground pin. As shown in figure 2, you must define ground as the
common connection between the positive and negative DC supplies, which will be ±15 V
throughout this and following experiments. This ground node must always be your
common ground for all instruments and elements in the circuit.
Figure 2 OpAmp 741 Pinout with Power Supply.
2. The Non-Inverting Amplifier
Construct the following non-inverting amplifier circuit of Figure 3 (note that power
supply connections are not shown in the figure using the 741 OpAmp, but you should
keep in mind that they always exist). Apply a low-frequency (f=100 Hz) sinusoid at the
Figure 3 Noninverting Amplifier
1) Measure the voltage gain and the phase shift at that frequency. Limit the voltage at the
input to 80 mV peak-to-peak maximum.
2) Measure accurately the –3 dB frequency fc (sometimes called corner or break
frequency) and the phase shift there.
3) Additionally, measure the voltage gain and phase shift at 10fc and 100fc. How many
dB/decade is the high-frequency roll-off?
4) Measure the voltage gain and phase shift at a number of sampled frequencies (you
select the sample points) and make an approximate plot for gain and phase shift versus
Now apply a 10-kHz square wave at the input. Increase the amplitude to about 8 V peakto-peak, causing the output to go deeply into saturation (limiting should be experienced in
both half-cycles at about ±12 to ±14V).
5) Measure the slope of the rising and falling edges in V/µsec. This is the approximately
the slew-rate (SR) of your op-amp. Use the highest possible oscilloscope sweep speed for
maximum accuracy.
6) What is the difference in the magnitudes of these two slopes?
Re-apply the 100-Hz sinusoid and adjust its amplitude for a peak-to-peak value of 10V at
the output. Now, keeping the output constant at 10V(p-p), increase the input frequency
until the output waveform appears not to be sinusoidal any longer (it will begin to appear
somewhat triangular). Note that in this part, the amplitude of the input must be varied to
keep the peak-to-peak value of the output constant at 10 V.
7) Record the value of the frequency where distortion starts. (note: Displaying Vin and
Vout simultaneously on the same axis should help you detect any significant distortion
more accurately).
8) With the maximum allowed output of 10V(p-p) and the slew rate you measured, what is
the theoretical maximum allowed input frequency? Compare the theoretical value to the
one you record in 7).
9) Repeat 7)-8) if the output is kept constant at 18V(p-p).
10) Replace resistor R2 by 10k, 1k, what are the gains for these two cases?Measure the
3db bandwidth for these two cases and compute the product of DC gain magnitude and
3db bandwidth. Compare the products with that when R2=100k.
3. The Inverting Amplifier
Figure 4 Inverting Amplifier.
Build the circuit in Figure 4. Apply a low-frequency (f=100 Hz) sinusoid to the input.
1) Measure the gain and phase shift at the frequency (f=100 Hz). Limit Vinto 80mV (p-p).
2) Adjust the frequency to measure accurately the –3 dB frequency (fc) and the phase
shift there. Is it the same as in the non- inverting amplifier?
3) Measure the voltage gain and phase shift at 2fc, 4fc and 8fc. What is the highfrequency roll-off in dB/octave?
4. The Summing Amplifier
Apply a low-frequency (f = 100 Hz) sinusoid of about 50-mV peak-to-peak amplitude to
the input labeled Vin and, using a separate power supply, a +50mV dc voltage to the
input labeled V'in in the circuit of Figure 5.
Figure 5 Summing Amplifier.
1) Using DC coupling on your oscilloscope, sketch Vout and Vin showing the actual
phase relation between these two voltages.
2) The average value of Vout should be about -5 V. If not, could the input offset voltage of
your OpAmp be causing this discrepancy?
3) To find out, set Vin = V'in = 0 V and measure the DC value of Vout, this value
represents the contribution of the op-amp input offset voltage to Vout.
4) With Vin and V’in back to their original values, vary the frequency of Vin and
accurately measure the –3 dB (cut-off) frequency of the ac portion of Vout (use AC
coupling on your oscilloscope for this measurement).