fast rise time, slow fall time pulse generator

Hi all,

I have been trying to create a circuit that has a very fast rise time (in the order of 10ns) but with a relatively slow fall time (prefereably as high as possible in the range 500ns to 4ms).

The reason for this is so that I can drive an RLC circuit with a positive going edge only (which needs to be fast) and for it to ignore the negative going edge (which can be achieved by making it very slow).

I have simulated a circuit that appears to achieve this very simply - a button drives a schmitt inverter. The output of the schmitt drives a fast opamp buffer and is tied to ground with a 100mH inductor. The opamp is running from a single supply.

In the simulation when the button is pressed it creates a low on the schmitt input which generates a fast rising edge (10ns) on the inductor. The voltage accross the inductor decays to zero in about 1ms. When the button is release the voltage accross the inductor goes negative very quickly and then decays to zero slowly. The opamp buffer clips out the negative going aspect so I get a nice fast rising edge and slow falling edge to drive my RLC circuit with. (See attached file for a schematic and waveform output - sorry for the proo quality but I had to redraw itin paint as i cannot get it from the my laptop to the internet computer at work!)

However when I build it I find that I get a fast rising edge but it decays to about 2V the stays at that level.

Does anyone know why the output never drops to 0V??? Or can anyone suggest another way to achieve what I want?

You don't say what opamp you are using, but that could be the problem. Some more details of what you built would allow a better answer.

Keith.

It's a bad idea for any OP type, to short the output with an inductor. At worst case, the OP will be permanently damaged by a current that exceeds it's maximum ratings. Fast OPs usually don't have short circuit protection!

Apparently, you simulated the circuit with an ideal inductor but the real inductor has a high DC resistance, that causes the voltage drop. You should better use an RC highpass between the OPs, but you still have to care for the OP maximum input and output ratings.

Is it possible that your opamp can't set the output to zero? Make sure you have a rail-to-rail opamp or at least an opamp which can go all the way to ground.

I haven't actually used the opamp yet so that will not be the problem. It is only in the design to buffer the signal for driving the follow on circuit so it will not pose a problem. At the moment I am just scoping the signal accross the inductor.

FvM - Are you sure you mean current? An inductor does not generate any more current than is supplied to it so it can never exceed the maximum rating of the schmitt output as this is the limiting factor in the amount of current in the circuit. The VOLTAGE across the inductor will spike when the current is removed, but that is the point of the circuit! It raises a good point that I should put clamping diodes on the signal to keep them within a few hundred millivolts of the supply rails, but that is by the by.

However you do raise a good point about the DC resistance, I had not thought of that. I will have to investigate this one further.

I had already considered a high pass CR. The problem is that to acheive the very fast rising edge you need a very low C (a few picofarads), but to get the slow discharge you need an extremely high R (100s of megaohms to gigaohms). This would be fine until you factor in the opamp buffer whos input impedance is only going to be a few megaohms (for an opamp fast enough to maintain the fidelity of the fast edge), so the capacitor discharges too quickly.

I will look into the DC resistance problem, if you have any other ideas please let me know.

Thanks

Andy