LM317 + Op Amp

[Suraj143]

Guys can somebody tell me whats the configuration of this op amp?I'm giving 0V-5V varying voltage, How does the output behaves?

 

[Anonymous_person]

Hello,

having done a brief google search I came up with this info:

https://tangentsoft.net/elec/opamp-linreg.html

It apears from the information it is simply a way of controlling the output voltage from the LM317 by adjusting the input voltage to the OpAmp. The OpAmp is constantly trying to make the referance voltage and the output voltage (via the resistor divider) equal.

I suppose its a case of small voltage and Current can drive a Larger Voltage and Even larger current.

There is more about how it works in the link.

Owen.

 

[bountyhunter]

This looks like somebody's "bench design" they never built up. The op amp has no external compensation which would be some kind of R-C going from it's output back to the inverting input. Also, putting an op-amp (gain block) in the fedback path of an LM317 will jack the loop gain up. I don't know if this circuit is stable. It might be possible to stabilize, but I doubt it would work right as drawn.

 

[MrAl]

Hi guys,

This is a voltage tracking regulator. The output tracks the input (5v) signal. The output however is twice that of the input, so with 5v input you get 10v output. Of course you need a decent input voltage supply to the regulator too, of at least 12.5v to get 10v out.

Compensation is not too hard to achieve. Change the NPN transistor to a PNP type with emitter to the ADJ pin collector to ground, reverse the connections to the input of the op amp.
Alternately, remove the transistor entirely, and replace with maybe a 10 ohm resistor from ADJ to the output of the op amp, again reversing the input connections to the op amp.

The whole idea with a tracking regulator is to use an op amp as the error amplifier so that it can detect very small deviations of the output and compensate by adjusting the signal going to the ADJ pin of the LM317. The LM317 has some minor problems which the op amp stage helps to correct. The op amp also allows for a variable control voltage to output voltage ratio by simply changing one resistor (one of the 10k units, for example to get a 1 to 4 ratio change the top 10k to 30k). Of course it also can 'track' another signal, and also helps with the temperature characteristic.

Whatever you do you should also do a quick simulation to make sure it works right

 

[WTP Pepper]

At a quick glance, the op amp is configured in positive feedback mode which means it will act as a comparator giving a nasty output switching voltage. I suspect the + & - of the op amp are swapped.

 

[MrAl]

Hi,

Actually though the transistor provides an inversion, so the op amp connections on the original schematic would be correct, but there's just too much loop gain.
Using a PNP transistor reduces the loop gain so stabilizes the output.
See the post just before yours #4 if you are interested.

 

[bountyhunter]

Whatever you do you should also do a quick simulation to make sure it works right

You have to build it. Sims are notoriously useless for compensation modeling.

As I said, I don't think this circuit is stable.

 

[Anonymous_person]

I don't think the Circuit will be stable, mind you that being said, I wasn't to good at Nyquist and Bode plots last year!!!

But It looks like it will constantly be adjusting. Heck it may even be a Chaos Generator!

I agree about Sims, Even during some uni work this year even software like Cadence (Virtuoso) was not always matching what we were told would be the case, and it would change depending on Which PC in the Lab you were using!

Owen.

 

[Suraj143]

Hi guys,

This is a voltage tracking regulator. The output tracks the input (5v) signal. The output however is twice that of the input, so with 5v input you get 10v output. Of course you need a decent input voltage supply to the regulator too, of at least 12.5v to get 10v out.

Compensation is not too hard to achieve. Change the NPN transistor to a PNP type with emitter to the ADJ pin collector to ground, reverse the connections to the input of the op amp.
Alternately, remove the transistor entirely, and replace with maybe a 10 ohm resistor from ADJ to the output of the op amp, again reversing the input connections to the op amp.

The whole idea with a tracking regulator is to use an op amp as the error amplifier so that it can detect very small deviations of the output and compensate by adjusting the signal going to the ADJ pin of the LM317. The LM317 has some minor problems which the op amp stage helps to correct. The op amp also allows for a variable control voltage to output voltage ratio by simply changing one resistor (one of the 10k units, for example to get a 1 to 4 ratio change the top 10k to 30k). Of course it also can 'track' another signal, and also helps with the temperature characteristic.

Whatever you do you should also do a quick simulation to make sure it works right

Very nice explanation MrAl.Its easier to understand.

I have some questions.

How did you calculated the gain= 2? inverting gain is -R1/R2 ??

Compensation is not too hard to achieve. Change the NPN transistor to a PNP type with emitter to the ADJ pin collector to ground, reverse the connections to the input of the op amp.

What do you mean by reverse the connections to the input of the op amp?

 

[crutschow]

You have to build it. Sims are notoriously useless for compensation modeling.

That's not necessarily true. It depends upon the accuracy of the simulation models. I have simulated numerous circuits where the simulated stability was sufficiently close, for engineering purposes, to the actual circuit stability seen after it was built.

 

[bountyhunter]

That's not necessarily true. It depends upon the accuracy of the simulation models. I have simulated numerous circuits where the simulated stability was sufficiently close, for engineering purposes, to the actual circuit stability seen after it was built.

Then you got lucky. Every designer I know would never rely on sims for stability data or phase margin. NEVER bet the design on a sim's phase data unless you just want to be an unemployed designer.

And BTW, where would you get accurate models of the LM317 and LM358 to simulate this circuit?

I actually worked at National and was invloved in spice model development. The models are a joke, even on the newer products. They are accurate only over a narrow range of voltage and current, if that.

SIMS have a use, but phase margin is not wherre they are accurate. We spent months arguing with the idiots developing the models for the LM2731 and LM2733. Their phase data never matched bench data. Their response? The bench data must be wrong. We ended up having to eliminate some applications from the data sheet because the sim was so far off. Next time you think a sim is accurate, remember that.

 

[bountyhunter]

Guys can somebody tell me whats the configuration of this op amp?I'm giving 0V-5V varying voltage, How does the output behaves?

As I said, I don't think the original circuit would be stable, but here are some suggestions:

Throw Q1 and R3 out and connect the output of the op amp to the bottom of R4 (also ADJ pin).

Reverse input polarity to the op amp to get correct function

connect a 0.22 uF ceramic cap from the op amp output to the inverting input

If the OP amp has both +/- supplies, it should then be able to pull the regulated output VOUT down to zero, the op-amp output will be at -1.2V.

NOTE: as shown, the regulated output VOUT will be TWICE the 0-5V program voltage. If you want it to track that voltage, eliminate R2.

ALSO: the lowest the original circuit's VOUT can go is 1.2V if the Q1 collector pulls down near ground the output is 1.2V up from there. It can never go to zero. It's a worthless circuit altogether.

 

[Suraj143]

Hi bountyhunter

Reverse input polarity to the op amp to get correct function

Here comes the secret.So this will make this op-amp as non inverting so the gain will be (1 + R1/R2) am I right?

 

[bountyhunter]

Hi bountyhunter

Reverse input polarity to the op amp to get correct function

Here comes the secret.So this will make this op-amp as non inverting so the gain will be (1 + R1/R2) am I right?

No. The circuit I described will have a capacitor as feedback element looking into the parallel combination of two 10K resistors. The op amp is then actually an "integrator" (acting as the error amp) not a DC amplifier.

As shown, the output is 2X the program voltage (0 - 5V). To get same V/V tracking, just delete R2.

The op amp must have both +/- supplies if you want the reg output Vout to come down to 0V. If the op amp has only positive rail, then it will only come down to about 1.2V.