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Old 11th July 2013, 08:11 AM   #11
Mooly is offline Mooly  United Kingdom
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This is the same technique applied to opamp circuitry,
A Different Opamp Compensation Technique.
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Old 11th July 2013, 09:40 PM   #12
teleman is offline teleman  Norway
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Thank you indeed for the the link. I've now tested the circuit with various values of R3 which gave the following figures;

Resistor value +V -V
3k3........+11.84 - 11.91
4k7........+12.00 - 12.06
10k........+12.04 - 12.06

Higher the R3, better the symmetry
Higher the R3, increase in output voltages!

None of the data sheets give Imax (sink/source currents) for L165.I wonder why?

Last edited by teleman; 11th July 2013 at 09:44 PM.
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Old 12th July 2013, 06:10 AM   #13
Mooly is offline Mooly  United Kingdom
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The source/sink current will be the max output current of the L165 which is around 3 amp.

The reason the output voltages vary slightly as the resistor is altered is partly because of slightly differing input bias currents (the tiny current that flows into or out of the two input pins) which cause a slight DC offset. That same problem affects ordinary opamps too. The other reason is that the L165 isn't a "precision" opamp, but neither is a cause of concern.
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Old 12th July 2013, 08:00 AM   #14
teleman is offline teleman  Norway
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No wonder that most C.F.B amplifier data sheets don't specify sink/source current! Then do you reckon using L165 to draw about 125mA is an overkill? The supply current for L165 is specified 40/60mA average/max @ +/-18v, which is quite high for a battry splitter IMO; My 3 prime concerns are heat,noise & battery drain.

I have a couple of LT1206 & BUF634 which are rated @ 250mA ....perhaps one of these is a better alternative? In any case I will try thes first with circuit & give you the results.

With regard to the circuit, I did move the output caps quite close to the pins, so perhaps this might have helped as well?
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Old 12th July 2013, 08:14 AM   #15
Mooly is offline Mooly  United Kingdom
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46 to 60 ma quiescent is way to high for battery power. I'd use the BUF634, no question about it. There's an example in the data sheet, hopefully that one is OK. Two resistors and two caps, job done.
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Old 12th July 2013, 09:09 AM   #16
teleman is offline teleman  Norway
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Thanks for your timely advice again! I will try the BUF634 as you've recommended. The only confusion is that on TI data... the splitter resistors are 10k, while "Tangent" design has used 220k! Why the difference & significance? Perhaps lesser current o/p with "Tangent's?

I actually started reading your link....very detailed, well over my head. Will read it a few times.

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Old 12th July 2013, 11:13 AM   #17
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I've never used the BUF634 but I see the data sheets mentions a low bandwidth mode that gives reduced current consumption. That's what you want to use.

The resistors, well 20K total is 1.5ma at 30 volts, comparable with the IC consumption. No reason not to go higher... its something you would have to try. Go to high and the input bias current will alter (pull down) the voltage at the junction of the two resistors. I'd try 100K or 22K and see how it perform.
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Old 16th July 2013, 10:33 AM   #18
teleman is offline teleman  Norway
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I tried the the BUF634 & found it to be more stable than L165 & was actually a doddle. However, I observed a few things which you may find interesting & perhaps give your take on.

1. Even with accurately matched resistors the - rail was a few mV higher on no load .
conditions.This increases with higher values & with a +/- voltages dropping
corrospondingly (10k,22k & 100k)

2. Choosing a slightly lower value for the + both rails gave exactly the
same voltages! Then after a few minutes on measuring the resistors I noticed the
lower value R too had increased to almost exactly the same value as the other R!

3. When connected to the load, the -rail increased by about -22mV compared to the
+ rail with the 10k R. This may have been due to imbalance in the load?

However, I didn't notice any increase in the dc offset as you had rightly assured. The BUF runs quite cool even on a very small heatsink.
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Old 16th July 2013, 11:03 AM   #19
Mooly is offline Mooly  United Kingdom
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Great it all seems to work OK.

It all sounds normal, the data sheet quotes an offset of typically -/+30 mv, 100mv worst case and variation with supply of 1mv/v

Higher resistor values mean that the current drawn by the input pin has a greater effect on the voltage at the resistor junction, and that current will almost certainly change with temperature. Just think of parallel resistors. If you used 100 ohms for the two then a variation in the current caused by a parallel 100K resistor makes little difference. Now imagaine if the resistors were 10K. It makes quite a difference now. The input current of the chip can be likened to the 10K except it will be a lot higher and variable in nature with temperature and supply
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