Department of Physics, Stanford University Lab 7.12 Final
Physics 105, Intermediate Lab Seminar: Analog Electronics Page 5
Lab 7: OpAmps and
Negative Feedback
Read: Day 1: Meyer Ch 8 Sections 6.1-6.4 all
Day 2 Meyer Ch 8 Sections 6.5-6.6
PRELAB
Part 1 . Find and/or calculate the range of the following properties of the LF411 (not the "A" version) and the LM741 from their datasheets, posted on Coursework:
AVOL, “large signal voltage gain”, dimensionless -- also calculate this in dB
CMRR (in dB) -- also calculate open loop common mode gain, ACM
Slew rate
Power supply requirements
Output voltage capability
Short circuit output current for the 741
Input resistance
Part 2 For the inverting amplifier in Figure 2:
` a. Calculate the closed-loop gain both as a voltage ratio and in dB.
b. What is the input impedance, Zin, of the circuit? (can be answered by inspection).
Part 3 Calculate the closed-loop gain of the non-inverting amplifier in Figure 3, both as a voltage ratio and in dB.
Part 4 In Figure 4, you will measure Zout by attaching a 10 ohm resistor to the output. If the opamp current output is limited to 20mA, what is the peak voltage you can use for this measurement?
Part 6 For the phototransistor circuit in Figure 6:
a. Derive and predict the gain (current-to-voltage) of the phototransistor circuit. Note that the gain is output voltage/input current, ie, it is not a dimensionless quantity. This is called a “transimpedance amp” because the gain is in units of impedance.
b. What is VCE for the [phototransistor? The phototransistor is like a regular BJT, except that incoming photons take the place of the base.
Part 8 Calculate the peak value of a sine wave which will drive a 45ohm speaker at .25W avg. (hint: remember RMS vs peak values).
Part 9 In the integrator in Fig 9, predict the output shape and amplitude from an input of a 2Vpp, 500Hz square wave.
Homework Problems
1. Meyer Ch 6, p. 238: Problem 1
2. Meyer Ch 6, p. 239: Problem 11 a, b, c, only (don’t do part d of this problem)
LAB
CARE AND FEEDING OF INTEGRATED CIRCUITS (IC’s):
When inserting an IC into the breadboard, do so carefully. The pins are thin and easily bent. The IC should seat firmly in the socket, so that the body of the IC just contacts the breadboard, but do not force the IC into place. For removal from the board, always use an IC extractor (green or yellow tweezers-looking thingy). And remember: op amps ALWAYS require an external power supply, even if not shown on the schematic.
LF411
Lab Day 1
Part 1 (5pts) Open Loop Opamp
1. Use a 25-turn pot (upright square with small metal screw) for this part.
Connect the opamp open loop as in Fig 1. Adjust the pot until you get 0 volts at the output. Explain, in terms of the opamp gain specification (200V/mV), what happens when you do this. A couple of sentences should be enough.
Lab note: for clarity, the power connections to the opamp , pins 4 and 7, will be omitted from the schematics from now on. However, you always need to provide this power.
Part 2 (10) Inverting Amp
Build the inverting amplifier as in Fig 2. Drive it with a sine of 1kHz.
1. Submit a plot of input and output signals (on the same plot), showing the gain of the amp @1kHz. Compare with Prelab prediction.
2. Sweep the frequency (every decade) and measure the gain. Plot your gain vs. frequency on the gain-bandwidth curve for the LF411 (last page of this lab). Draw this by hand on the curve—don’t take the time to re-plot on computer.
3. Add an additional 1k resistor in series with the input of the circuit (you will have two 1k’s in series now). Measure Zin using a 1kHz sine. Submit your measurement data and voltage divider calculation for Zin. Compare with Prelab prediction.
Part 3 (10) Non-Inverting Amp
Build the non-inverting amplifier in Figure 3. Note the terminals are reversed. Test with a 1kHz sine.
1. Report your measurement of the voltage gain (sine amplitudes), compare with Prelab prediction. No plots needed.
2. Add a 1M resistor in series with the input. Try to measure Zin , and explain why “10Mohms” is incorrect (see the input impedance of the LF411 on its data sheet).
3. Find the f3dB point and estimate Cin (see handout on Coursework/Materials/Lab 7 on figuring out all the capacitances in the system.
Part 4 (5) Follower
Build and admire the follower in Figure 4.
1. State the gain you measure (no plots needed).
2. Explain why the follower is a special case of the inverting amp
3. Use a 10 ohm resistor at the output and attempt to measure Zout using the voltage divider method. Use your Prelab calculation to limit the output voltage and thus avoid the current limitation of the opamp. Don’t spend too much time measuring this as it is very low and nearly impossible to measure, so just put an upper limit on it.
Part 5 (10) Slew Rate
Add a 10k resistor to the input of the follower as in Figure 5.
1. Drive the input with a 1kHz square wave, and measure the slew rate (V/usec) by looking at the maximum slope of the transition; . Compare with the datasheet spec (Prelab for Part 1)
2. How does this change as the input amplitude is varied?
3. Now switch to a sine wave with an amplitude of 3-5 V pk. Find the frequency where the output begins to drop. Show that this is consistent with the slew rate you measured above. (look at the maximum slope of the sine)
4. Repeat #1 using the 741 opamp.
LAB DAY 2
Part 6 (10) Phototransistor Current-Voltage Converter
A phototransistor produces a collector current, called photocurrent, proportional to the intensity of incoming photons. The photons replace the base connection, as shown in Figure 6a. You will use the SDP8405 phototransistor, shown in Figure 6b. This type has only two terminals, C and E (no B), and spec sheets are posted on Coursework. Careful, it looks like a clear LED but is a smaller size.
Build the circuit in Figure 6a, which converts the photocurrent to a voltage. Point the SDP-8405 straight up at the room lights.
1. Submit a waveform showing the DC and AC components of the output. If the AC component is very noisy, use the Average function of the scope (“Display key”)
2. From the output voltage, calculate the DC and AC components of the photocurrent. This was your Prelab calculation
3. What is the percent modulation (AC amplitude/DC amplitude), and the frequency of the modulation?
4. Measure the voltage at the summing junction (point “x” in Fig. 6a) with full light, and with no light (just put your hand over it), and compare with your Prelab prediction for VCE .
Part 7 (10 pts) Summing Amplifier
Drive the circuit in Figure 7 with a 1kHz sine, 1Vpeak. Choose your own DC offset at the potentiometer.
1. What is the DC offset at the potentiometer? Use the DMM to measure this.
2. Use the scope to look at the voltage at pin 2 of the opamp. What do you expect to see, and do you see it? (remember the Golden Rules)
3. Submit the output waveform confirming summing operation of the circuit: AC gain and DC offset at the output.
Part 8 (10) Push Pull Buffer (solves crossover distortion)
Connect an inverting opamp to a push-pull buffer (follower) as in Figure 8 ; use the connection labeled “Part 8.1”. Drive the circuit with a 440Hz sine. Use the amplitude you calculated in the Prelab (to drive a 45ohm speaker @.25W).
1. Measure the signal (on the scope) at the opamp output, and at the push-pull output. Make sure you see the crossover distortion. Submit both waveforms.
2. Connect the speaker to the output, and listen to the tone on the speaker
3. Disconnect the speaker. Reconnect the 100k feedback resistor to the push-pull output as in Figure 8 (“Part 8.2”). Repeat Step 1, and explain how the opamp fixes the crossover distortion.
4. Listen again on the speaker. You should hear an improvement without the crossover distortion.
Part 9: (10) OpAmp Integrator
Build the integrator in Figure 9. Start by driving it with a 1kHz square wave. Make sure it integrates. If it saturates at the rails, you may need to adjust the DC offset at the function generator by a tiny amount.
1. Drive the integrator with a 2Vpp 500Hz square wave. Confirm your prelab prediction for the maximum output amplitude. Show the input and output waveforms demonstrating integration.
2. Find the 3dB frequency of this circuit. Compare with Prelab.
3. Explain what happens, and why, when you remove the 10Meg resistor
4. How does this circuit differ from the passive RC integrator/filter?
LF 411