P123 HW 3: op amps I 1
Total points: 20 (plus 3 optional); due Monday, Feb. 23, 2015
REV 0 1 ; February 14, 2015
1 Bidirectional Current Source (1 point)
2 Odd Summing Circuit (3 points)
3 V to I (4 points, total)
3.1 Choose An Op Amp (2 points) . . . . . . . . . . . . . . . . . . . . . .
3.2 Design the Circuit (2 points) . . . . . . . . . . . . . . . . . . . . . . .
4 Bad Circuits?
(5 points)
5 Differential Amp (4 points)
5.1 Gain (1/2 point) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 (quasi-) differential input 1 point . . . . . . . . . . . . . . . . . . . . .
5.2.1
Hold non-inverting input (B) fi xed: . . . . . . . . . . . . . . . .
5.2.2
Hold inverting. input (A) fi xed . . . . . . . . . . . . . . . . . .
5.3 common-mode input (1 1/2 points) . . . . . . . . . . . . . . . . . . . .
5.4 A question about input impedances 1 point . . . . . . . . . . . . . . . .
6 Wien Bridge (3 points)
6.1 Why is the lamp needed?
(1/2 point) . . . . . . . . . . . . . . . . . . .
6.2 Why, when everything’s working right, does the circuit deliver a clean sinewave
6.3.1
6.3.2
?
(1/2 point)
6.3 Adjustments:
. . . . . . . . . . . . . . . . . . . . . . . . . . .
(2 points) . . . . . . . . . . . . . . . . . . . . . . . . .
frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . .
amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Designing a Wien Bridge Circuit (optional: 3 points)
7.1 Question: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8 Links to Datasheets
8.1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 Links to Op Amp Datasheets . . . . . . . . . . . . . . . . . . . . . . .
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Show how to use a current-limiting “diode,” like the one you used in the tail of Lab 5’s difference amp (1N5294: really an FET), to make a bidirectional 2-terminal current source.
Hint: a “bridge” circuit like the one familiar to you from power supplies can help. A sketchy data sheet describing this part appears at the end of these HW papers.
Question: voltage limit
With your additional circuitry, what is the minimum voltage across the bidirectional current source that will permit it to work properly? (See sketchy data sheet for the 1N5294.)
1 Revisions: cut table of “recommended op amps” as distracting (2/14).

P123 HW 3: op amps I 2
Suppose you would like to sum the signals provided by two transducers, giving the two transducers equal full-scale ”weights.” The summed output is to drive an A/D, and the range of that device de fi nes the permitted output range for your summer . We will assume the transducers provide DC signals of just one polarity, as stated below. Here are the relevant speci fi cations:
A/D input range: 0 to +2.5V
signal source A: signal range: 0 to +0.5V
output impedance: ≤ 10
M
Ω signal source B: signal range: 0 to -1mA (current sink) output impedance: ≥ 10
M
Ω output voltage compliance: ≤ 0
.
1
V

P123 HW 3: op amps I 3
Your task, here, is to use an op amp to convert a voltage in the range 0 to +10V into a current, sourced from a +10V positive supply , in the range 0 to 100mA. You may use either an LMH6645 op amp or an LM358 (partial data sheets attached). Don’t read all this data sheet material, by the way. Instead, decide fi rst what are the few relevant specs (or one spec), then fi nd that speci fi cation.
We offer partial data sheets for two op amps—but note : only one of those two op amps can do the job. And here’s a hint: we’re proposing two “single-supply” op amps. In general, these devices are good at handling inputs close to the negative supply, on input and output; some can go close to the positive supply as well. Look closely at these speci fi cations.
Here are a couple of requirements:
• use just one supply: +10V.
• Let your current source work up to at least +8V (its “output voltage compliance” should be 0 to +8V, to say this in jargon).
3.1 Choose An Op Amp (2 points)
Which op amp do you choose, and why? (You will need to sketch your circuit before you’re likely to be able to answer this question.)
3.2 Design the Circuit (2 points)
Design the circuit, using the op amp of your choice. Among other tasks, check whether the op amp you choose can put out 100mA. If not, provide an appropriate circuit remedy.

P123 HW 3: op amps I 4
On the next two pages you will fi nd some bad circuits, and a few OK ones mixed in. Tell us brie fl y what’s wrong with the bad ones, and how to fi x them. Sometimes the fi x itself will be a suf fi cient answer.
Figure 1: bad circuits: I

P123 HW 3: op amps I
( More bad? circuits)
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Figure 2: bad circuits: II

P123 HW 3: op amps I
Here is a standard differential amp made with an op amp.
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Figure 3: diff amp
5.1 Gain (1/2 point)
First, what is the circuit’s differential gain?
Input impedances What is the input impedance at each of the circuit’s input terminals (not the op amp’s terminals, n.b.), for two cases—
5.2 (quasi-) differential input 1 point
Apply the signals to one input terminal at a time, while the other amp input sits still:
5.2.1 Hold non-inverting input (B) fi xed:
Assume that you apply the signal at the R that feeds the inverting terminal, while the other input sits still (such an arrangement in fact mixes a little common-mode signal with the differential signal; you could easily fi gure how much, if you were so inclined!).
:
5.2.2 Hold inverting. input (A) fi xed

P123 HW 3: op amps I
5.3 common-mode input (1 1/2 points)
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5.4 A question about input impedances 1 point
If you got the answer we hope you got for the two input impedances—differential and commonmode—then you discovered a happy fact, a fact that helps a diff amp to do what it’s built to do, viz., deliver good CMRR. Please explain, if you can, the relation between CMRR and the input impedances.
Here are some questions designed to help you admire this ingenious circuit:
6.1 Why is the lamp needed?
(1/2 point)
Why not use a fi xed resistor with R ≈ R F EEDBACK , to set the ampli fi er’s gain?
6.2 Why, when everything’s working right, does the circuit deliver a clean sinewave ?
(1/2 point)
Hint: Mr. Fourier knows!

P123 HW 3: op amps I
6.3 Adjustments: (2 points)
6.3.1 frequency
To change the sine’s frequency, what should you change?
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6.3.2 amplitude
To change the sine’s amplitude, what should you change? (This one is hard; please justify your answer brie fl y.)
Below is an I vs V plot for a particular #344 lamp.
Figure 4: I vs V for #344 lamp

P123 HW 3: op amps I 9
7.1 Question:
What R f eedback value would you choose, to get an output amplitude of about 9V peak-to-peak?
(One could answer this experimentally–but we’d like to see you calculate the result.)
Notes :
• it’s temperature that changes the lamp’s resistance, as you know;
• the plot above shows DC values (Paul and I just watched I and V on two DVM’s, and wrote down the pairs of readings);
• since the lamp responds rather slowly to heating by the sinusoidal waveform, it is the RMS I and V values that do the job: these heat the lamp the same way as the corresponding DC values we have plotted. For example, whatever I we found at 1.0V DC is the I RMS we would see for a 1
.
0
V RMS sine wave–about 1.4V peak.
• the relevant resistance value that we should take from the plot is not the local slope (the incremental resistance), but the average or overall relation between I and V at the operating amplitude (simpler, really).
8.1 Contents
• Current-limiting diode
• LF411 op amp data
• LMH6645 op amp data
• LM358 op amp data

P123 HW 3: op amps I
Sketchy Data Sheet
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Figure 5: Current-limiting diode: 1N5294
8.2 Links to Op Amp Datasheets
Datasheet for 3 Op Amps
We’re not asking you to read through these long documents. Instead, we’re hoping you’ll make a quick hunt for a single speci fi cation.
LF411: http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lf411-n&fileType=pdf
LMH6645 http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lmh6645&fileType=pdf
LM358: http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lm358&fileType=pdf hw3 feb15.tex; February 14, 2015