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Old 4th June 2012, 09:12 AM   #31
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Quote:
Originally Posted by SyncTronX View Post
So why don't you just use that TI part that the Spaniard discussed.
Use the LM833? It goes from +- 5v to +-18v. I ugess you
wouldn't have to worry about low voltage spikes. ; )

Sync
Original application note use a MAX4475 opamp, that has two problems: first it work only up to 6V, but I need an opamp tha work up to at least 30V; second it's an SMD components, and as you see about any other components in the schematics I avoid to use SMD components for this project.
I cannot use LM833 because it has a relatively high input bias current characteristic. As you can see in the application note: "... use an op amp with low input bias current..".
In my simulation that means to use an op amp with ultra low bias current, max 1-2 pA, to work properly, but LM833 has a typical 500 nA ib current.
The OPA637 in the HG schematic is not the cheapest op amp but surely an high performance op amp suitable for this implementation:
input bias current: 1 pA
input voltage noise: 4.5 nV/sqrt(Hz)
slew rate: 135 V/us
gain bandwidth: 80 MHz
AD820 used in the SG version, IMHO is the right price/performance compromise

Up to 5.5V OPA350, in my simulation, it's the best choise for its transconductance, perfect for this feedforward shunt regulator:
input bias current: 0.5 pA
input voltage noise: 5 nV/sqrt(Hz)
slew rate: 22 V/us
gain bandwidth: 38 MHz

Andrea
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Old 4th June 2012, 12:55 PM   #32
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LM833 is a dual so it's choice is rendered mute. getting to the theory, however, you don't want to use a comparatively slow opamp like the LM833 (16MHz) as the reduced bandwidth increases the effective impedance of the supply. use the fastest opamp you can keep stable. Better are the OPA353, LME49710, AD825 -- but you have to make sure that the regulator remains stable every time you change the opamp.
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Old 4th June 2012, 02:31 PM   #33
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Quote:
Originally Posted by jackinnj View Post
LM833 is a dual so it's choice is rendered mute. getting to the theory, however, you don't want to use a comparatively slow opamp like the LM833 (16MHz) as the reduced bandwidth increases the effective impedance of the supply. use the fastest opamp you can keep stable. Better are the OPA353, LME49710, AD825 -- but you have to make sure that the regulator remains stable every time you change the opamp.
As i said in the previous post, not all opamp are suitable for this application.
OPA353 is a low voltage opamp and for voltage up to 5.5V the best choise is OPA350, ideal for its transconductance.
LME49710 is a good opamp, but it has an high input bias current (70 nA), too high for this application.
AD825 is a good opamp, but is an SMD device.
BTW OPA637 is an high speed opamp (135V/us), very low noise and ultra low input bias current (1pA), so I thing ideal for this application, and my simulation confirm that. The only problem is the price, so I'll test AD820 in the SG version as a compromise.

Andrea
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Old 4th June 2012, 02:39 PM   #34
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hmm, so the opa637 is for higher voltage versions yes? its really not very happy with low gain in my experience it needs to be 3-4x or above, certainly not unity
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Old 4th June 2012, 03:23 PM   #35
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Originally Posted by qusp View Post
hmm, so the opa637 is for higher voltage versions yes? its really not very happy with low gain in my experience it needs to be 3-4x or above, certainly not unity
you're right, an unity gain stable opamp in not the ideal opamp, unfortunately OPA350 is a low voltage opamp and I cannot find any valid alternative. I have investigated several opamps in my simulations, but seems all I found suitable for this feedforward shunt regulator are low voltage type. Also AD825 (and I would like to avoid SMD device) does not work properly in this circuit, cause of its relatively high input bias current. Remember that the resistors network divider at the non-inverting input of the op amp must set 1-1.5 mV, so high values resistors are needed, then an input bias current greater than a few pA cause a voltage drop on the resistors.
So I decided to try OPA637 in the first prototype to see if it really has problems like oscillations or so.

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Old 4th June 2012, 04:43 PM   #36
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Ad825 was used by Waly Jung with SMD to DIP adapter - it works quite well.
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Old 4th June 2012, 04:46 PM   #37
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you could try opa827, its happy at unity gain and equal to 637 in every measure..ah SMD mate that requirement in particular is pretty limiting today. why? dont tell me you are SMD phobic or some other such 'audiophile' nonsense? or you wish to sell it to those who are?
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Old 4th June 2012, 10:14 PM   #38
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Quote:
Originally Posted by qusp View Post
you could try opa827, its happy at unity gain and equal to 637 in every measure..ah SMD mate that requirement in particular is pretty limiting today. why? dont tell me you are SMD phobic or some other such 'audiophile' nonsense? or you wish to sell it to those who are?
I have nothing against SMD devices, but my eyesight has declined in recent years and so I have a little difficult soldering them.
BTW OPA827 does not work properly in simulation, no problem about input bias current, but I think there are problems about its transconductance.
I'll investigate again among SMD devices.

Andrea
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Old 5th June 2012, 12:16 AM   #39
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You've got to choose what works for your design -- the AD825 tests "well" in the JDSR, not as close to perfection as the AD797, but it sounds really nice.

I used to be a power supply agnostic, but was knocked off my camel on the road to Damascus.
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Old 5th June 2012, 10:17 AM   #40
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It seems only OPA140 works properly in shunt circuit, not a bad opamp and 4.5 to 36V supply voltage.
I'll test both OPA140 and OPA637
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