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Old 18th November 2008, 03:01 AM   #11
Werner is offline Werner  Europe
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Better, but now increase the reference voltage, as presently
it sits too close to the opamp's neg supply (ground).
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Old 19th November 2008, 04:52 AM   #12
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Alright - here's the situation:

Two configurations shown attached. The one on the left, where the op amp is connected to split supply, works exactly like I expected. With 12 V rails, R3 and R4 produce an input voltage of 0.55 V, with ripple below 1 mV (my measurement threshold) due to the capacitor C2. The op amp takes that voltage and amplifies it by R2/R1+1 = 11 for an output voltage of 6.1V.

The ripple on the voltage rails is 1.8 V p-p positive and 0.2 V p-p negative, for a 470 ohm load (13mA output current drawn from the positive rail). The ripple on the output is less than 1 mV, estimated at less than 0.1 mV. Nothing but a bit of HF noise on my scope.

The ripple rejection of this circuit is therefore about 80 dB, certainly in line with what I was expecting.

The circuit on the right, meanwhile, is configured as a single supply. It does not give the same performance. The DC voltages are right: the V+ is now 24 V, the input voltage 1.1 V and the output 12.7V. The ripple on the V+ is again 1.8 V p-p, but now the ripple on the output is 400 mV p-p!

I don't understand why the ripple is so high.
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Old 19th November 2008, 07:08 AM   #13
Werner is offline Werner  Europe
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The input is with 1.1V too close to the opamp's V--. As I said, increase the input level and reduce the opamp gain to 2 or 3.
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Old 19th November 2008, 09:12 AM   #14
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Awesome! Yep, that does the trick. Even added a pass transistor to the mix.

(test point: V++ 24V, input 7V, output 14V)

Though with the lower gain, the op amp seems a little less stable. Needed a capacitor across the test load to settle it.

I have about 1mV p-p of ugly looking diode switching noise that I can't seem to shake, otherwise we really made some progress today.

/rjm
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Old 20th November 2008, 12:22 AM   #15
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Here's what the breadboarded test circuit looks like.

I added an additional RC filter to supply the op amp and voltage divider.

The NPN transistor I used is just what happened to be in the drawer.

As shown, the output voltage will be about 3/5th of the input.

In my test configuration, 24 V in, 15 V out. Ripple is 1.2 V p-p at the input, output noise is less than 0.2 mV, for a 470 ohm + 100 uF load.

The configuration is unstable without the 100 uF cap on the load.

There is some residual airborne pickup, if I step away from the desk, it gets worse. The whole circuit is unshielded, however, just sitting on the desk, with long lead wires posted into the breadboard. I'm not sure if I should bother looking into it or not.

Also I have the feeling that the bandwidth of 4 Mhz is a tad high. I'd rather reduce it to 200 kHz to avoid, or at least minimize, stability problems.
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Old 20th November 2008, 02:43 PM   #16
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- a pass transistor doesn't just let more current pass to the load, it also removes the output current from the voltage amplifier entirely. The main benefit is efficient RC filtering can be now be applied to the supply feeding the voltage amplifier.

- having a split supply to begin with, there are some advantages to running the voltage amplifiers from V+ to V-, rather than connecting one side to GND. First, you can have a small input voltage. A small input voltage means you use a high gain in the voltage amplifier to generate the output voltage ... increasing the gain is a good way to improve the stability. Second, the voltage amplifier can be configured inverting. The reference generated from the negative rail can be fed to the input of the op amp that provides the positive output voltage and vice versa. Inverting amplifiers are much less likely to cause problems, in my experience.

I tried it on the breadboard, it worked great. It was stable without an output capacitor, though with some HF noise. Adding 100uF brought the noise down to <0.2 mV.

The latest configuration, then, implements these latest ideas.
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Old 20th November 2008, 02:45 PM   #17
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Eagle files, for those following this at home...
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Old 20th November 2008, 03:16 PM   #18
Werner is offline Werner  Europe
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Looking more and more like mine now

Multiloop feedback can also be used to burn off
excess OL gain, at the expense of a super-low
output impedance. Which you IMO don't really
want anyway.

A series resistor between opamp and pass transistor
base promotes stability.

Running the opamp at high gain also amplifies its
own noise into the reg's output.
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Old 21st November 2008, 02:27 AM   #19
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Quote:
Originally posted by rjm
Eagle files, for those following this at home...
trying, but not succeeding.
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Old 21st November 2008, 05:09 AM   #20
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Quote:
Looking more and more like mine now
Please consider it a complement!

Quote:
A series resistor between opamp and pass transistor
base promotes stability.
Isolate the op amp from the capacitance of the transitor base, right? I was wondering about that... I'll give it a try on the bench to see if it makes a difference.

Quote:
Running the opamp at high gain also amplifies its
own noise into the reg's output.
This I knew, and actually you can see the effect of it on the scope trace. Still it seems for the present like a reasonable trade off for getting a sensible bandwidth.


@d to the g

You'll have to download and install Eagle www.cadsoftusa.com to view and manipulate the files. You may need the latest version, 5.3, I'm not sure.


I'm going to christen the circuit (inverting voltage amp + split supplies + crossed reference voltage + split output) the "Xreg". 'cause it sounds cool. A bit of dotting i's and crossing T's on the weekend and we will be about ready to proceed to the beta test stage.
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