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| TO220 rail splitter? |
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| David Hess:
Unlike the earlier and slower LM302 and LM310 voltage followers which internally use a symmetrical differential input stage for low offset voltage, the LT1010 power buffer relies on matching of an NPN and PNP pair and is not an operational amplifier and does not function like one. Widlar's design takes advantage of a "stored charge" PNP and "isolation base" NPNs to wring as much performance out of a junction isolated NPN process as possible. National also made some even faster buffers but they used hybrid construction to take advantage of complementary circuit designs and were much more expensive. Even National made the distinction between voltage followers and buffers. Grey beards might recognize these as the "Fast" and "Damn Fast" buffer amplifiers. |
| 0xdeadbeef:
You're right that the LT1010 is not a normal OpAmp but a unity gain buffer which is also why there is no negative input pin. So it's an OpAmp with a fixed negative feedback (and thus a gain of 1). Again, if you look for TO220-5 OpAmps on Mouser, there are not too many choices. Actually it boils down to the LM675, the LT1010 and the OPA544, where the OPA544 is the only one that doesn't have a rail splitter circuit as application example. Doesn't seem to be meant for unity gain circuits. Anyway, it seems that the TLE2426 has exceptionally high input bias currents which degrade its performance at higher voltages. So yeah, as the LM675 has much lower input bias currents, it might outperform the TLE2426 for input voltages higher than 15V even if only using 1% resistors. E.g. at 30V, the TLE2426's deviation from 50% is already >2%. |
| Zero999:
Have you done any calculations? If the total power supply voltage is 20V for a +/-10V rail, a 1% error is 100mV. Look at the the data sheets for half decent op-amps and you'll find that the offset voltage error will be an order of magnitude less than that. The effects of the bias offset currents can be catered for by not using stupidly high value resistors. The error due to the op-amp will be insignificant. The error due to the resistors will be under 1%, if they're in the same batch, they will be fairly well matched. An op-amp + potential divider can easily beat the TLE2426. |
| 0xdeadbeef:
Hm. I feel I will repeat myself at some points but in a nutshell: the idea was not to replace the TLE2426 with some OpAmp that can deliver even less current but with a beefy TO220 one. When looking for TO220-5 OpAmps on Mouser, it comes up only with the LM675, the LT1010 and the OPA544. The OPA544 doesn't seem to be unity gain stable and the LT1010 (which is actually a unity gain buffer) has a significant offset error (and can only deliver only +/-150mA anyway). Actually there is a 0.3V offset when you simulate the recommended application circuit in LTSpice with the official LT1010 model. So this leaves us with the LM675. I simulated the recommended application circuit in LTSpice and it doesn't show any significant error there but then again I can't tell how realistic the model is (taken from some opamp.lib with an author tag "rperez"). And of course there is no temperature model and no worst case model and what not. But yes, as stated in my last posting, it looks like the total error of the TLE2426 will exceed that of an LM675 with 1% resistors for input voltages >15V. So for a typical +/-12V or +/-15V scenario, the LM675 should give better results if the total error stays below 2% as the simulation etc. suggest(s). |
| David Hess:
--- Quote from: 0xdeadbeef on December 19, 2018, 08:58:13 am ---You're right that the LT1010 is not a normal OpAmp but a unity gain buffer which is also why there is no negative input pin. So it's an OpAmp with a fixed negative feedback (and thus a gain of 1). --- End quote --- No! The LT1010 is not an operational amplifier and does not function as one! The LM302 and LM310 voltage followers are operational amplifiers internally configured for unity gain. The LT1010 is a series connected complementary emitter follower. The LH0002 is a parallel pair of series connected complementary emitter followers operating in class-AB which some will recognize as a diamond buffer; these are easy to implement with 4 discrete transistors and form the input structure of many current feedback operational amplifiers and some high slew rate voltage feedback operational amplifiers. LM302 Voltage Follower 10V/us 50nA 20mV 0.9985V/V LM310 Voltage Follower 30V/us 10nA 10mV 0.999V/V LT1010 Power Buffer 75V/us 800uA 220mV 0.995 V/V LH0002 Diamond Buffer 200V/us 10uA 30mV 0.95V/V --- Quote ---Again, if you look for TO220-5 OpAmps on Mouser, there are not too many choices. Actually it boils down to the LM675, the LT1010 and the OPA544, where the OPA544 is the only one that doesn't have a rail splitter circuit as application example. Doesn't seem to be meant for unity gain circuits. --- End quote --- There used to be a few more. There are ways to wire dual and quad operational amplifiers in parallel for more output current but this does not help much with limited power dissipation. Integrated class-AB linear audio amplifiers were particularly useful for this application but have largely been replaced with class-D devices. Circuit stability is often a problem. A power splitter should have low AC output impedance which implies adding an output capacitor to either or both supplies but some devices will become very unhappy with this unless it is carefully done. The standard techniques for driving a capacitive load help but raise mid frequency impedance. Swamping the output with like a 10 microfarad solid tantalum or aluminum electrolytic capacitor often provides the best performance. |
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