Physics 427 Lab # 7



` OP-AMPS III Physics 427Low-level signalsLab # 7 1) Current-to-voltage converter For the current-to-voltage converter shown below, the output voltage is given by Vout = – (I in )(RF) + [ (I – )(RF) ] -37973011430Note that the term enclosed in the square brackets is caused by the negative bias current of the op amp. Let us call this term VO – the output voltage when the input current is zero. VO = (I –)(RF) For an ideal op amp, this term would be zero. For our real op amp, it will be non-zero. The current that we wish to measure is given by the following equation: Iin = (VO – Vout)/RFYou should first determine the value of VO for the OP-07 op amp you are using. This is done as follows: Allow the input terminal to float (i.e., don’t connect it to anything and don’t ground it) so that Iin = 0. Measure and record Vout. This will be equal to VO. Now use the current-to-voltage converter in the circuit shown on the next page to measure the reverse leakage current of a signal diode. Measure Iin for the following diode bias voltages (the bias voltage is the voltage measured at the point where the potentiometer connects to the diode): 0.0, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 1.0, 2.0, 3.0, 4.0, 5.0 volts. (This circuit is very sensitive. Don’t touch it during measurement.) Make sure that the diode is in the circuit backwards! The black stripe should be facing the potentiometer. -313055114302) Noise due to electric field coupling (capacitive coupling) Connect a coax cable (one with red and black alligator clips on the end) to one of the DSO input channels. Position the cable so that the leads (the alligator clips or grabbers) are in the middle of the table. DO NOT TOUCH EITHER OF THE LEADS DURING YOUR MEASUREMENTS. Turn up the vertical sensitivity of the DSO until you can clearly see the noise voltage. Sketch the noise and measure its frequency and peak-to-peak amplitude. (If you can’t get a waveform, try to trigger off of LINE.) Repeat this measurement for the following cases: Move the leads next to the power strip. Move the leads as far away from the power strip as possible. Place the leads on the table and touch the red lead. Move your other hand around so that it is near the power strip, the overhead lights (not too close), etc. Can you determine the sources of the electric fields that cause the noise voltage? (Hint: consider the frequency that is dominant in the noise.) 3) Noise due to magnetic field coupling (inductive coupling) Now connect a large coil to the coax cable and place the coil in the middle of the table. Connect the primary of the 120V/6.3V transformer to the power strip, leaving the secondary open (no load). Move the coil near the transformer. Turn the coil until the noise voltage is a maximum. (How is the coil oriented relative to the transformer?) Sketch the noise voltage observed on the DSO and measure its amplitude and frequency. Rotate the coil until the noise voltage reaches its minimum value. How is the coil oriented relative to the transformer? 4) Common mode noise rejection using a difference amplifier Construct a non-inverting amplifier with a gain of 1000. Connect a microphone and Frequency Generator in series to the input of the amplifier as shown below. -370205140970Set the FG to produce a 5000 Hz sine wave of the lowest possible amplitude (you will need to set the FG to its -30 dB mode). The FG voltage will be considered as “noise” and the microphone output will be the “signal”. Make some sounds into the microphone (vowel sounds sustained for several seconds work well). A DSO sweep speed of 2 ms/div will give you a clear view of the microphone voltage. Observe the output voltage of the amplifier using the DSO. The waveform you see is the sum of the signal and noise voltages. Both the signal and noise have been amplified 1000 times. (The gain will actually be less than 1000 because of the bandwidth of the amplifier). Increase the noise by turning up the FG amplitude. Now you will see how a difference amplifier will magically remove virtually all traces of the noise voltage and leave a clean signal. Construct the difference amplifier shown on the next page and observe the output voltage. You should see a noise-free waveform. Note that the inputs to the differential amplifier are connected across the microphone while the noise voltage (the FG voltage) is common to both inputs (hence it is called common-mode noise). Thus, V2 = Vnoise V1 = Vnoise + Vsignal Since the difference amplifier amplifies the difference between the two input voltages the output voltage will be Vout = 1000(V1 – V2) = 1000 Vsignal The noise voltage is subtracted out and only the signal voltage is amplified. ................
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