Wien bridge oscillator with diodes in feedback loops

When I was at GenRad I'd mark out a section of PCB and designate it as a no man's land... any attempt at digital violation would garner great grief... very rarely would anyone try :-) ...Jim Thompson

Some folks like debugging, I suppose. When my children were young, and were going on about how nice it would be to have some ridiculously huge house, I used to ask them if their hobby was vacuuming. ;)

Cheers

Phil Hobbs

So, what's your opinion of the wien bridge circuit in the LTspice examples (wien.asc)? It uses a jfet in the feedback loop and a LT1001 op-amp. Output is about 3 volts p-p from +/- 15 volt supplies.

-Bill

I remember I had a piece or gear years ago (Tube) that had a Wien bridge in it that used incandescent lamps to stabilized the bias.

Jamie

That's a well-known story--it was invented by Bill Hewlett for his master's thesis at Stanford, and then became HP's first product, the HP200A audio oscillator. See e.g.

Cheers

Phil Hobbs

Hi,

Thanks very much for the good feedback I received. I will try to change the= op-amp and put the pots as suggested.

Given that there was some criticism about taking circuits from application = notes can some suggest a good book on oscillators since I am lecturing on t= hat subject. I would appreciate if it has some practical circuits so that t= hey can be done as experiments for my students.

Thanks very much,

Regards,

jozamm

My god. You are lecturing on this stuff and you have absolutely no idea of the theory and application of the material.

Your university ought to be more careful on who they hire as faculty.

Jim

op-amp and put the pots as suggested.

notes can some suggest a good book on oscillators since I am lecturing on that subject. I would appreciate if it has some practical circuits so that they can be done as experiments for my students.

he op-amp and put the pots as suggested.

n notes can some suggest a good book on oscillators since I am lecturing on= that subject. I would appreciate if it has some practical circuits so that= they can be done as experiments for my students.

Wow, you are teaching this? Try this. Get rid of the diode and 5k pot. (Yeah and get a good opamp and run it bipolar not single supply.) Set the oscillator frequency at some low value... 10 Hz or so. Now with the pot on the feedback resistor slowly increase the gain... while watching the output. At some point you'll see the circuit just start to oscillate... the amplitude will grow slowly. (The low frequency will let you to watch this on the 'scope in real time. (I guess I'm assuming you've got a digital 'scope.))

Reduce the gain a bit and the amplitude should slowly go down... you may need a small 'tweaker' pot to get the fine control you will need. If you let the amplitude grow it eventually hits the power supply rails and clips.

George H.

he op-amp and put the pots as suggested.

n notes can some suggest a good book on oscillators since I am lecturing on= that subject. I would appreciate if it has some practical circuits so that= they can be done as experiments for my students.

Well, the Wien bridge is a classic, but should come relatively late in the = course, after filter networks. This particular circuit would benefit greatly from= a pullup resistor on the LM324 op amp output (to force it into class A operat= ion) or a different op amp (LT1013 was suggested). LM324 is power-stingy becaus= e of its output impedance dead-band, but that's a bad quality in a low distor= tion application. It would also be improved by a 'correct' setting procedure f= or the adjustable resistor. Actually, I'd lobby for a few precision resistors and= a resistor in parallel with the diodes. Forcing most of feedback current through the= =20 diodes seems ill-advised, IMHO. Your goal with that adjustment is exactly = a voltage gain of 3.000000000... (because higher gain makes the oscillator clip, lowe= r makes it stop oscillating). =20

Pay no attention to critics of 'taking circuits from application notes', it= 's VERY MUCH a part of our technology to do so. I was able for years to rebuild electr= onics with no access to the circuit diagrams, because nearly every user of IC jun= gle chips followed slavishly the manufacturer's application examples. Make su= re the students see, too, that some of the notes are ... incomplete.

op-amp and put the pots as suggested.

notes can some suggest a good book on oscillators since I am lecturing on that subject. I would appreciate if it has some practical circuits so that they can be done as experiments for my students.

course,

the

resistor

voltage

makes it stop

VERY MUCH

electronics

I know how strongly I worded things, but perhaps a better way of putting what I said would be that it is foolish to _unthinkingly_ use a circuit from an app note. App note circuits are often very good, but sometimes they're bad -- and the app note doesn't say. App notes also rarely say _when_ the circuit is good for one thing or another.

So app note circuits can be great starting points, but should never be used without question.

I like "Oscillator Design & Computer Simulation" by Randal W. Rhea, Prentice-Hall, 1990. The "computer" part of the simulation is excruciatingly dated, but you can take what he says and apply it to SPICE quite easily. Similarly, when he says "Oscillator" he's just assuming that you know he means "RF oscillator with resonant elements". But the principles still apply.

It is very instructive to analyze that circuit _on paper_, both for the circuit behavior with the diodes removed (simulating the low-amplitude case) and the circuit behavior with the diodes shorted (simulating the high-amplitude case). You can do both by getting the system characteristic equation in the Laplace domain with the feedback resistance left as a variable, then substituting in various values. Then do a root-locus of the system as the feedback resistance (or conductance) varies.

Find the critical value of feedback resistance for which the system is metastable, and the sensitivity of that value on the other component values in the system. At that point, you have about 98% of the story, and you only need to ask yourself what happens when you throw diodes into the mix, and how you might select different values of the feedback resistances to get lower distortion.

Remembering the usual precautions of depending on simulators, do you think that passing a simulation would be sufficient to warrant proceeding with a particular app note circuit?

John

A lot of the problems with datasheet circuits have to do with:

1. Unsuitable parts for the job. The mfg wants you to use their parts exclusively, regardless of whether they're cost-effective in the application.

1. Lack of protection networks. You can blow up a 7805 by shorting its input to ground, for instance.

2. Lack of attention to startup and transient situations.

1. Oversimplifying. An LM317 plus a pot makes an okay current source, unless the wiper of the pot bounces momentarily (which it's going to, eventually). When that happens, the 317 will very likely blow up whatever it's driving.

2. Other things that I've forgotten.

Cheers

Phil Hobbs

Rhea's other book, "HF Filter Design and Simulation," means "design and simulation for filtered typically at UHF and up..." :-)

But hey, he came from the cable TV industry, they talk funny there, it would seem!

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

I just found the above transformation helped me understand and analyse it. Replicating the divider was unnecessary.

Here are three (steady-state) equivalent re-arrangements in one sim:

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WIRE -400 -480 -400 -608 WIRE -144 -480 -144 -512 WIRE -144 -480 -400 -480 WIRE 272 -480 272 -624 WIRE 272 -480 224 -480 WIRE -144 -464 -144 -480 WIRE 160 -464 -144 -464 WIRE 512 -464 512 -512 WIRE 656 -464 656 -512 WIRE 736 -464 736 -512 WIRE -144 -448 -144 -464 WIRE -1808 -432 -1808 -528 WIRE -1808 -432 -1856 -432 WIRE -832 -432 -832 -528 WIRE -832 -432 -880 -432 WIRE -2016 -416 -2016 -528 WIRE -1920 -416 -2016 -416 WIRE -1040 -416 -1040 -528 WIRE -944 -416 -1040 -416 WIRE -1568 -400 -1568 -416 WIRE -1568 -400 -1856 -400 WIRE -800 -400 -800 -656 WIRE -800 -400 -880 -400 WIRE -592 -400 -592 -416 WIRE -592 -400 -800 -400 WIRE -1568 -384 -1568 -400 WIRE -592 -384 -592 -400 WIRE -144 -336 -144 -368 WIRE -1568 -272 -1568 -304 WIRE -592 -272 -592 -304 FLAG -1568 -272 0 FLAG -2304 -528 0 FLAG -2224 -528 0 FLAG -2096 -528 0 FLAG -1904 -688 V1a FLAG -592 -272 0 FLAG -1328 -528 0 FLAG -1248 -528 0 FLAG -1120 -528 0 FLAG -928 -688 V1b FLAG 656 -464 0 FLAG 736 -464 0 FLAG -144 -336 0 FLAG 512 -464 0 FLAG 112 -768 V1c SYMBOL Opamps\\opamp -1696 -608 M180 SYMATTR InstName U1 SYMBOL Opamps\\opamp -1888 -480 M0 SYMATTR InstName U2 SYMBOL res -1584 -400 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res -1584 -512 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res -2032 -656 R0 SYMATTR InstName R4 SYMATTR Value 8k06 SYMBOL res -1616 -544 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 7k87 SYMBOL res -2064 -784 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL cap -1872 -544 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1n SYMBOL cap -2080 -576 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C2 SYMATTR Value 1n SYMBOL diode -2288 -576 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1 SYMATTR Value 1N4148 SYMBOL diode -2240 -640 R0 SYMATTR InstName D2 SYMATTR Value 1N4148 SYMBOL Opamps\\opamp -720 -608 M180 SYMATTR InstName 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SYMATTR InstName R4c SYMATTR Value 8k06 SYMBOL cap 528 -512 R180 WINDOW 0 24 64 Left 0 WINDOW 3 24 8 Left 0 SYMATTR InstName C4c SYMATTR Value 1n SYMBOL diode 672 -512 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D1c SYMATTR Value 1N4148 SYMBOL diode 720 -576 R0 SYMATTR InstName D2c SYMATTR Value 1N4148 SYMBOL Opamps\\opamp 192 -544 R0 SYMATTR InstName U2c SYMBOL res 48 -640 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5c SYMATTR Value 7k87 SYMBOL res -192 -640 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R20 SYMATTR Value 100k SYMBOL cap 208 -640 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1c SYMATTR Value 1n SYMBOL res -160 -464 R0 SYMATTR InstName R1c SYMATTR Value 10k SYMBOL res -160 -608 R0 SYMATTR InstName R2c SYMATTR Value 10k TEXT -2304 -432 Left 0 !.lib opamp.sub TEXT -2312 -384 Left 0 !.tran 1m TEXT -2304 -336 Left 0 !.ic V(V1a)=1u TEXT -1328 -336 Left 0 !.ic V(V1b)=1u TEXT 240 -352 Left 0 !.ic V(V1c)=1u

I am now beginning to agree completely. I just simulated my second data sheet circuit and I have found (barring mistakes made by me) that it is trash. This one comes from AN-43, page AN47-24, from Linear as suggested by George Herold in the original post.

I was interested in Fig 27, the lock-in amplifier. It didn't look right to me so I simulated the front-end of it. I found that the bridge would not null.

If I am wrong, I would appreciate someone pointing out my error.

Thanks, John

I do remember a subtle difference in GBW depending on which of those nodes were merged. It was discussed here sometime in the past year. I'll see if I can find the message-ID. ...Jim Thompson

I don't think that it's a good idea to proceed with any circuit that simulates OK but that you don't understand. _If_ it simulates OK, _and_ you understand the circuit well enough to know that it'll probably work, _or_ if you have the time to spend on a board spin, then yes -- go ahead. OTOH, if you have no clue whatsoever of why it works or how, then having it simulate OK means nothing, whether the circuit came from a data sheet or your own fertile mind.

I liked that book. It had examples of oscillators with specifications I'd never thought of (mostly power oscillators, and oscillators that were intentionally made with a minimum of components. On top of that, I read the book, then I went and sat down and designed an oscillator in SPICE using his methods*, then I waltzed over to my bench, built the SPICE circuit, and it worked as predicted. I thought that was a pretty good indication of how well the book explained things.

• Meaning both that I never needed to get it oscillating in SPICE, nor did I ever have to do anything other than linearized analysis.