When directly coupling a single supply non-inverting op amp buffer there are issues that seem to be impossible to fix.
If you simply hook up the source to the op amp without any coupling then any signal swings below V- of the op amp will be clipped. One can remedy this by "lifting" the source by biasing it's ground. This requires a resistor in series, at least in some cases(not sure if all). But doing this attenuates the signal.
Is there any way to direct couple an AC signal to a single supply op amp without having these issues(either clipping or attenuation)? I need to maximize headroom(hence bias it half way between the rails) and have no attenuation(as to reduce noise reduction. Simply amplifying the signal after the attenuation won't work).
Maybe there is some configuration of negative feedback that can compensate for the attenuation resistor in a way that doesn't increase noise? Or maybe there is another way to directly couple the source that avoids these issues?
Or worse. Some op amps (mostly older ones) suffer from an abrupt "jump all the way to the opposite rail!" problem if you push the inputs more than a short distance outside of the voltage rails... this can lead to a nasty latchup.
One way is to work back through the circuit and force the previous stage's common-mode reference point to be near the middle of the op amp's common-mode range. However, doing so simply pushes the whole problem/question back one stage in the circuit... it doesn't eliminate it.
You could stick a stack of diodes between the previous stage output (cathode end) and the op amp input (anode end) and use a pull-up resistor to V+ at this point. This will probably introduce some significant distortion, though.
So, just how much noise are you concerned about, and why? Can you quantify it for us?
If you stick (e.g.) a 1k resistor between the previous stage and the op amp input, and hang a 1k resistor from this input to V+ to bias it, you'd suffer a 50% (6 dB) loss of gain due to this biasing arrangement. Simply double the gain in the op amp stage to compensate. With modern low- noise low-distortion audio amplifiers, I really doubt that the added thermal noise from these resistors is going to be even vaguely significant... your incoming signal will have been through
*far* worse, earlier in its chain-of-processing.
The other (obvious) approach is simple - buy one high-quality audiophile-grade coupling capacitor, and AC couple the signal. For not a lot of money you should be able to buy a cap which doesn't introduce distortion anywhere near the threshold of audibility... once again, the signal you're reproducing will have suffered from much greater degradation before it ever saw your circuit!
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
This would work. The only thing I'm worried about is the noise/nonlinearity that now exists. The gain's depend on the source impedance. Using a large R will reduce it but then will add noise. My source impedance can vary a great deal from 10k to < 10M. I guess it's a trade off that is potentially better than the other methods but I'm not sure how much.
Or worse. Some op amps (mostly older ones) suffer from an abrupt "jump all the way to the opposite rail!" problem if you push the inputs more than a short distance outside of the voltage rails... this can lead to a nasty latchup.
One way is to work back through the circuit and force the previous stage's common-mode reference point to be near the middle of the op amp's common-mode range. However, doing so simply pushes the whole problem/question back one stage in the circuit... it doesn't eliminate it.
You could stick a stack of diodes between the previous stage output (cathode end) and the op amp input (anode end) and use a pull-up resistor to V+ at this point. This will probably introduce some significant distortion, though.
So, just how much noise are you concerned about, and why? Can you quantify it for us?
If you stick (e.g.) a 1k resistor between the previous stage and the op amp input, and hang a 1k resistor from this input to V+ to bias it, you'd suffer a 50% (6 dB) loss of gain due to this biasing arrangement. Simply double the gain in the op amp stage to compensate. With modern low- noise low-distortion audio amplifiers, I really doubt that the added thermal noise from these resistors is going to be even vaguely significant... your incoming signal will have been through
*far* worse, earlier in its chain-of-processing.
The other (obvious) approach is simple - buy one high-quality audiophile-grade coupling capacitor, and AC couple the signal. For not a lot of money you should be able to buy a cap which doesn't introduce distortion anywhere near the threshold of audibility... once again, the signal you're reproducing will have suffered from much greater degradation before it ever saw your circuit!
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I am trying to get away from using a capacitor. This is a VLF app(around
1Hz). The goal is to add as little distortion and noise as possible within reason and constraints. Since this is the first amplifier in the chain it is much more important to reduce the noise than further down the line. The source impedance has significant variation which can cause significant problems in biasing. Of course all this is expected to run on a battery with significant lifetime(not minutes or days but weeks to months).
The only problem I am having is the input biasing issue. Fred's idea of a summing amplifier works but I'm a bit concerned about some of the noise/non-linear distortion introduced which are inversely related(I can trade one off for the other). The Johnson noise will almost surely not be a problem but I've not done any noise analysis on the circuit to know for sure.
if you're using a battery for this device, why can't you use a virtual ground ? it's simple and would give you a basic +/- rail from a single battery, this is assuming that this battery is only operating your device here?
if you're using a battery for this device, why can't you use a virtual ground ? it's simple and would give you a basic +/- rail from a single battery, this is assuming that this battery is only operating your device here?
formatting link
Just an example.
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Battery or not it doesn't seem to work. I've tried using an op amp version to split the rails but it doesn't work. When connecting the source it draws a ton of current. I've also tried using a resistive divider to split and it doesn't work.
I don't know why I can just split the rails and treat the virtual ground as ground and not bias anything but it seems it's more complex than that? That is, V - 0 - -V is not the same as 2V - VV - 0 where VV is virtual ground?
Then again I may have had something hooked up wrong. If simply splitting the rails would let me treat the circuit as if I had a dual supply then life would be so much easier. I'll try again and see if I can get it to work. I may have left something connected as if it were single supply.
Yes, this basic idea should work, and should be pretty easy to get going.
Assuming you have something like a 9- or 10-volt source, the easiest way to split the rails would be to use an ordinary vanilla 7805 or
78L05 5-volt regulator, plus a few passive parts (e.g. the usual decoupling capacitors at the input and output pins of the regulator).
Use as follows:
- Connect the battery "-" to the 7805's "GND" terminal, and to the "V-" input of your op amp. This becomes your negative rail. Do *not* connect this to "GND" on any component other than the 7805!
- Connect the battery "+" to the 7805's "IN" terminal, to the "V+" of your op amp, and to the "V+" of your previous stage(s). This becomes your positive rail.
- Connect the 7805's "OUT" terminal to the "GND" bus for your circuit. This becomes your ground reference. Depending on the current consumption of your op amp, you may want/need to install a resistor between the 7805 "OUT" and "GND" pins, to ensure that the 7805 is seeing the minimum load it requires to regulate properly. A 7805 needs around 5 mA of load at a minimum; I think the
78L05 can be happy with as little as 1 mA.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Interesting. Does this have any stability problems? I was using something similar without the 0.33u cap. At AC the ap is shorted and it would seem to have some positive feedback that might cause stability issues?
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