The LF411 was my choiche when I was designing a similar circuit.

The ac parameter do matter in my opinion... the transients are handled directly by the op amp and the LM337 is just keeping a constant voltage (nominally 1.25 V) across its adj pin and out pin.

 

Which circuit did you build?

Please post a schematic.

Exactly the LM337 keeps the voltage constant, it's adj pin is being held at a particular voltage by the op-amp, if a load transient occurs the LM337 will just take care of it.

I have been slighltly tempted to add a low pass filter before/after the uA741 so load transients on the positive supply don't affect the negitive. I decided against it since the uA741 already has a small low pass filter built-in and the capacitors on the LM337 should take care of it.

Anyway, I'll need to do some tests to prove this, but I can't do them todat since I've left it at work.

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I didn't build the circuit beacuse I was not satisfied with my design.. I started with the same idea: a voltage divider (I used two 12k6 1% resistors) and an op amp (the LF411). I was suggeted to add the resistor to ensure a minimum load for the LM337 regulator.
The inverting input is a virtual ground under steady state operative conditions; any change is handled by the op amp, so it should be fast, do you agree?
The main challenge was the stability. The op amp is used in the open loop configuration and it adds a low frequency pole. The loop gain rolls off with -40 dB/decade slope but I haven't thought of a compensation network yet..
Do you have the chance to simulate the circuit?

 

What value did you use?

I've thought about this again and I don't really know about this. Normally when you build an LM337 circuit, the LM337 handels the transients. However we have added another amplifier and the LM337 is in its feedback loop. I think that if the uA741 is too slow and holds Vadj at the same voltage then the LM337 will react and compensate for this.

I think the LF411 is your problem here, I don't think it's completely stable when configured as a unity gain buffer.

Kind of, the software I use doesn't have a a model for the LM317 and LM337 so I subsituted them for discrete transistor regulators, it worked so I assembled it.

Don't you have an exact schematic of your circuit to post?

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I don't think so. It is compensated for unit gain, like the ua741.

The LM337 has a local feed-back (the 120 ohm resistor) and it keeps the constant voltage accross the adj pin and the out pin, as we said. With this configuration it does no react to transients as it does when the adj pin is grounded. I mean.. if the output of the op amp were a sine wave, the output of the LM337 would be a sine wave + 1.25 V

At this moment I don't.
I remember I was trying to reduce the output current of the op amp and I found this solution: consider the required load current for the positive regulator. I used a LM317 and it needs 5 mA (max). If you use the op amp around the LM317 you can use a 220 ohm resistor (instead of 120 ohm) and the current will be 1.25/220 = 5.7 mA

 

The role of the opamp here is to provide a steady voltage for the LM337 adj pin so that LM337 would function normally as a voltage regulator, and not to "regulate" the LM337 to cause it to react to output voltage transience. It is in effect a replacement of a variable resistor for setting the output voltage. Opamp speed is never an issue.

The worst thing to happen is the opamp output voltage varies or being modulated due to load variation. Use every filter that you can find to make the opamp inmute to input voltage transience. Slow opamp like 741 works best.

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L.Chung

 

So it is, I didn't bother to read the datasheet!


Of course it still reacts to transients when the adj terminal is grounded. Try setting the output to 1.25V and then varying the load, it still regulates fine.


But why would I want to?

The reason for using a 220ohm resistor was purely due to get the correct ratio as I only have a 2k5 10-turn pot available. Putting it on the negitive supply wasn't an option since the minimum load is 10mA and I don't have a 1k125 pot!

eblc1388,
You've put it far better than I have. I was beggining to think I was incorrect when I'm right. The regulator is doing the regulation not the op-amp! The adj pin needs to to be held at a fixed voltage and the LM337 handles the load variations.

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I totally agree with you.

When I talked about "transients", I was thinking of variations between the positive and negative outputs (tracking).
We want a constant output and this is handled by the regulator; another aspect is tracking and this is handled by the op amp.

I will check this thread and I look forward to view the pictures and read any news about this circuit.

Regards

 

I'm glad we now agree but I'm more interested in why your circuit didn't work and mine does. I'm sorry I didn't understand you, what was the problem? Did it oscillate? Perhapps Ron had a point about phase shift and stability. Just because the LM337 doesn't contribute any gain it doesn't mean it doesn't cause any phase shift.

I think you could try adding a lowpass filter or a slower op-amp like the uA741.

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I can't find an LM337 spice model, but an LM317 in a similar circuit behaves as below (keep in mind this is a sim, not hardware). I ran the sim with compensation cap values of 0 and 100nF. The load current is pulsing between 10mA and 100mA, as shown on the schematic. The 0.1 ohm resistor in series with the 10uF cap is ESR. I tried 0.01 ohms, and it still oscillated.
The diode on the output in my schematic keeps the current load from pulling the output several kV negative in the sim.

Have a look at the National LM337 datasheet's equivalent circuit. I think you'll see this is not much like a PNP emitter follower. It has loads of loop gain and an RC frequency compensation network.

Attached Images

	FB reg stability waves.PNG (26.3 KB, 91 views)
	FB reg stability sch.PNG (10.4 KB, 117 views)

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Ron

 

Hi Ron,

How about adding a 10uF capacitor from junction of R1 & R3 to 0V?

Can you post also the .asc file (I'm too lazy to redraw the circuit in Switchcad)?

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L.Chung

 

Here are (hopefully) all the files needed. Try the 10uF at the summing node. It causes some VERY low-level underdamped ringing. Not bad.

Attached Files

fb stability.zip (3.1 KB, 76 views)

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Ron

 

Thanks Ron. The simulation is working.

I have increased the value of the additional cap to 100uF but without the frequency compensating capacitor Cc and the output trace is now as good as your previous circuit with the value of Cc compensating capacitor = 100nF.

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L.Chung

 

That's good to know as an intellectual exercise, but it obviously is not as space- or cost-effective.

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Ron

 

I wonder why circuit worked so well then?

Perhapps it isn't working that well as I haven't looked at the output on a scope and my multimeter was just taking the average output.

I'm intrested about how the LM337 adds gain to the system. Whilst I'm aware it isn't just a PNP transistor, I was under the impression that it is in a situation similar to a common emitter amplifier with full negitive feedback. Suppose you increase Vadj with respect to 0V, Vload responds to keep Vadj with respect to Vload at 1.25, just like increasing Vb increases Vload to keep Veb at about 0.7V on a common emitter amplifier. Can you see my point?

I don't understand why in your simulation the load isn't acting as negitive feedback in the LM317 circuit like I'd expect.

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