Output pulse shape

[elki]

I am sorry but I have a very newbie question. I have some circuit from which I get the two types of outputs, depending on how I look at it with an oscilloscope. In the first case it is just directly connecting to the output through a very big resistor without any load. In the second case, I use 1k to ground. Obviously, the first case is problematic, because I am basically capacitively read the output without any load. However, this is the shape that I want. Could you please suggest a proper way of arranging the output stage to get the pulse shape as shown in the first case?

[jwet]

Can you put some numbers on these waveforms, especially time and maybe give other details.  I'll try to answer with what's given...

The first case looks like a high impedance source, something like a 10k output impedance.  Its not affected by 100k (likely 100k with 10 meg in series from the scope).  You should be able to just use a 10x scope probe- the 100k isn't doing much.  It might be isolating the direct cap load of the probe...

The second case is pretty confounding- it doesn't look possible.  The first pulse is what you might get with a capacitively coupled output, the width of the pulse is a few RC's but the second pulse is somewhat impossible from a linear circuit, it rises before the output rises then follows it down.  This is non causal and suggests some kind of triggered delay from the rising edge - not linear.  A classic under cap coupled output would have the the first edge create a pos pulse and the second create a neg pulse.

[elki]

Thank you. Here is an example of the problem I am dealing with.

I have a high impedance input signal on R1. I am trying to convert it to a low impedance output on R2. Obviously, if I further reduce the input current by increasing R3, my output voltage on R2 becomes zero. Naively, I thought that the proper way of dealing with it would be to have an op amp buffer placed in-between R1 and R2, but somehow it does not work. Does anyone have a suggestion? The input signal is about 10-20mV.

[Andree Henkel]

What is Purpose of R2?

If you want to drive coaxial cable with 50Ohms WaveImpedance with signals of high enough frequenzy or short enough rise / fall times
and you want to correctly source terminate?

Then R2 needs to be in series to the output, not to ground

[xvr]

I have a high impedance input signal on R1. I am trying to convert it to a low impedance output on R2.

It's impossible. No any passive network can convert high impedance input to low impedance without loosing signal power a much.

I thought that the proper way of dealing with it would be to have an op amp buffer placed in-between R1 and R2

Exactly. And may be even without R1 & R2