How to Amplify Small Decay Variations in a 1.4V Low Impedance Signal Without Clipping

I should amplify a low impedance signal with the peak of maximum 1.4V.
The tricky part is that the signal itself is not important but the tail of the signal is important which might show some decay (falling edge could reach to zero soon or later than normal) and these signal decay variations are very small (a few microvolts)
The thing is if I amplify the whole signal by a typical opamp, it easily sticks to the supply rails and clipped. because the signal itself is not small but its decay is very small.

The application is the battery powered and the single supply design is preferred.

is there any way to just amplify the variations?

Sam I was prompted to reply to this question by my reply to another question just above yours here - you could use the same circuit.If you detect when the voltage is just under the 1.4V peak, and then when it is nearly zero (say a couple of hundred millivolts) then the length of that pulse will correspond to the decay time of the pulse.I don't know if this will help you, it will if your pulses are consistently of a max level of 1.4V and you are just interested in the decay time. As you say, amplifying the pulse beyond a factor of about 2 or 3 is problematic because your amplifier will be clipping at the maximum level.   But if the shape of your decay is what you are interested in you'd have to do something like that.  Maybe put a capacitor or a high pass filter in your signal so you only see the variations.  A bit more information on the type and shape of your pulses would help here.

Let me add a picture of the signal.The yellow rectangle shows the tail which might show the decay variation ( the tail here doesn't mean very close to the ground here).if I amplify the whole signal or even the yellow part (clipping it by a digital switch or something), the wave will stick to the rails, because of the high gain.if I select a low gain, then the variations will not be detectable.

Sam
Could you try a 2 part circuit? one with or without gain that will detect the trailing (positive going edge) edge of the signal a create a “valid window” pulse where the decay is valid. This is a digital function. There would also be a 2nd Circuit in parallel with high gain. You could then use the window pulse to enable this 2nd  Circuit or if it can recover from the saturation quick enough just use it as a trigger for your ‘scope. I am presuming that you simply want to measure this with a scope. Of course with some additional circuitry you could move on the electronic measurement.

Sam thanks for the scope trace, but it's still not clear exactly what you need to measure.  If it's the shape of the trailing edge of that pulse, Aubrey's suggestions will be the best.  If it's the slope or time of the part you've highlighted, then something with a window comparator to give you a pulse which will vary in length depending on the training edge slope would work if it's the decay time you are interested in.  In either case you'd need a reliable start point  to either start your display or to start a pulse length measurement - t's not clear whether that little positive going peak is reliable in time or amplitude?

Why it is not clear? that signal part might have decay, it means it could reach zero sooner or later which of course accompanies with some very small amplitude changes. that part itself is not low in amplitude, therefore clipping it before applying a high gain would not help.

Sam, it's not clear because you have not told us exactly what you need to know about this pulse.  The exact decay time?  the point at which it starts dropping off faster? the slope of that last decay to zero?With a good oscilloscope you should be able to set it to trigger on a negative going  voltage corresponding to the start of your window, then get a full screen trace of the part you're interested in.  Would this do what you want?  If not, why?Thanks / David

No, let me clear it up moreThis decay accompanies with some amplitude changes of course, isn't it?but these amplitude changes are very small. I must amplify and detect if there is the decay and if it is, how much. therefore I should amplify that first and then read it by an ADC. The decay time does not matter for this task.

OK, maybe the following will do what you want.Firstly find a way to generate a trigger when your signal gets to the part you are interested in.  This might be a voltage sensor as I've described above, or a timer that starts at the positive going edge of your waveform and generates a trigger at the right point.  You mention an ADC so you'd probably have a microcontroller, you could maybe do this in software.Once you've done that, you can then look at an amplified version of your waveform.  Sure it is going to clip in some parts, but as long as it is linear in the part you're interested in, you will be fine.  Once you generate your trigger pulse, then get your microcontroller to get the ADC to sample your waveform.  If you get a high resolution ADC (12 bits or more) and a fast one (as many samples / second as you're likely to need) you should be OK.  One thing you'd have to watch for is that when amplifiers are driven into clipping they may take some time to recover when they get back into the linear region.  You can use a thing called a clamp circuit to minimise this if it will be a problem.This is much the same as Aubrey suggested above.

Sam,This explains why you were asking about a "no jitter" ADC trigger...The only thing I have to add to what Aubrey and David said is that for what you want to measure you have to be careful that you don't introduce noise into your decay when amplifying the signal. So you might need a "low-noise" amplifier. You will also want the highest precision ADC that you can afford.

Field and energy gain factors, and loss factors are ratios. Gain is usually defined as the mean ratio of the signal output of a system to the signal input of the same system prepaidgiftbalance.

The tricky part is that the signal itself is not important but the tail of the signal is important which might show some decay (falling edge could reach to zero sooner or later than normal) and these signal decay variations are very small krogerfeedback

Only a person who did the same project can realize the complexity of the task.  no part of the signal is weak and easily goes to the rails when I use a high gain. therefore small changes are hidden. the interesting part which you mention is NOT weak itself, but its decay is weak.

Sam I am not belittling your task here, you are obviously after some specific information from this signal and need to extract that as accurately as possible. But you say "but (the signal's) decay is weak".  Now a signal can be weak but the decay cannot be described as "weak".  It may be slow or fast, it may be exponential, it may have artefacts in it, its rate may change due to something external....  Decay can be expressed as a time, or as a rate (volts per time) so I presume one of these is what you are interested in.It seems your task is to look at a small part of the waveform, so you need to- Amplify it enough so that the part you are interested in covers your ADC input range  as best as possible, for maximum precision, without introducing noise or other distortion. You may need no amplification at all.- reliably trigger your ADC (or the MCU driving it) to start monitoring the signal so it covers exactly the part of the signal you are interested in.  This may entail triggering on a level or an edge and may or may not need a delay before you start acquiring your data.Once you have fulfilled these requirements you should be able to amass enough data to make whatever conclusions you need to.Your task seems akin to the data gathering needed to prove the existence of the Higgs Boson at the Large Hadron Collider - only very tiny changes in a signal were indicative of its presence.  But that was done, so I have no doubt that there is a solution somewhere to your problem.

You could then use the window pulse to enable this second circuit or if it can recover from the saturation quick enough just use it as a trigger for your scope official survey.

Thus, much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal.

So much research has gone into both limiting the attenuation and maximizing the amplification of the optical signal.