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LAB 12: Op-amp invertor and follower Names___________________________ This lab introduces the op amp in two of its simplest (and most common) modes: the inverting amplifier and the voltage follower. We’ll be using the LF411, which is an FET version of the famous bi-polar 741. The pin-out is as follows: Pin 7 is always connected to the positive supply voltage +VS! Pin 4 is always connected to the negative supply voltage -VS! PART A: Open-loop gain. Hook up the following circuit: 1) Use the 10 k pot from one of our previous labs, or solder up a new one. Set the supply voltages to ±10 V. Be sure you always remember to connect pins 7 and 4 to their proper supply voltages! These connects are seldom indicated on circuit diagrams. 1 2) Experiment with the pot, changing the input voltage from + VS to – VS. You’ll see that for all input voltages the output will be saturated at a large negative or positive voltage respectively. What are these ‘saturation’ limits? How close are they to the supply voltages? positive Saturation negative Saturation V V PART B: DC closed-loop gain. Now we’ll include negative feedback by including two resistors: a feedback resistance RF and an input resistance RI. Hook up the following circuit: 1) As usual, measure carefully the resistor values you use! As we discussed in lecture, this circuit acts as an inverting amplifier whose gain is given by 𝐺= RI RF Gain G 𝑉𝑂 𝑅𝐹 =− 𝑉𝐼 𝑅𝐼 () () 2 2) In steps of about 0.5 V, vary the input voltage VI (using the pot) from -10 V up to +10 V. For each of these values of VI measure the output voltage VO. Note that the input and output voltages are measured relative to ground. 3) In Excel, enter your data and plot a graph of output voltage verses input voltage. You will see that when the output voltage approaches either of the supply voltages the transfer function becomes non-linear, i.e. saturates. 4) In another column, calculate the ratio VO / VI. From the portion of your data in the linear region, calculate the average of this ratio. Compare your measured gain with what you calculate from the measured resistances. G (theory) G (measured) SHOW ME YOUR WORK …….. APPROVAL? PART C: AC Distortion and frequency response. Replace the voltage-divider input with a sine-wave generator, and now measure VI and VO (peak-peak) with your oscilloscope ( Ch 1 = VI and Ch 2 = VO ): 3 1) Set the frequency of the input sine wave to about 1 kHz. Note that the input and output are 180⁰ out of phase with the input. This simply reflects the inverting character of the amplifier. If you haven’t done so yet, adjust the input voltage to be about 1 V peak-topeak. Now gradually increase the input voltage. You’ll notice that as the output increases to large amplitudes (like the supply voltages) the amplification is no longer linear and the output signal becomes “clipped”. THE LESSON: always work in the linear region! 2) Now, set your input wave to have an amplitude of about 1 V (peak-to-peak). Let’s look at the frequency response of our amplifier on a logarithmic scale! Measure VI and VO as a function of frequency in the following steps: 1 kHz, 5KHz, 10 kHz, 50 kHz, and 100 kHz. Between 100 kHz and 1 MHz you’ll see the gain VO / VI start to drop, so measure at more frequencies in this region, say ... 200 kHz, 400 kHz, 600 kHz, 800 kHz, and as close to 1 MHz as possible. Plot the gain as a function of log (frequency). You should also notice that in this region the output becomes distorted and shifted in phase. Show me your graphs, if your results meet my standards I will check the following BOX: PART D: The voltage follower. Another op-amp configuration using feedback is the voltage follower. Despite its simplicity, it’s one of the most useful op-amp applications. The follower provides a buffer between one circuit stage and the next. It has a large input resistance (that of an FET input) and so does not load the input circuit; a vast simplification in design. Alternately, it has a low output resistance and can drive reasonable loads. Hook it up: 4 1) For various input voltages, measure the output. Compare the accuracy. How close to the plus or minus supplies can the output swing before the output differs by 10% from the input? VI VO % error = 100 ∗ (𝑉𝑂 − 𝑉𝐼 )/𝑉𝐼 2) Measure the output resistance RO of your LF411: Set the input to some fixed voltage VREF, say about 5 V. Measure the output voltage when you load the output with a small load resistor 𝑅𝐿 ≈ 100 . For a small enough load resistance, VO will deviate from VREF. You can then figure the output resistance from the Thevenin equivalent circuit: 5 RL VREF VO () (V) (V) 𝑅𝑂 = 𝑅𝐿 RO 𝑉𝑅𝐸𝐹 − 𝑉𝑂 𝑉𝑂 () SHOW ME YOUR WORK …….. 6