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Why is the falling edge slower than the rising edge?

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Mar 21, 2023
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I have a radiation sensor connected to an amplifier and then to an oscilloscope (50 Ω). This is the circuit:

The radiation sensor (an LGAD) is connected straight into the "In" terminal, few millimeters away from the circuit. The amplifier is basically two stages based on the PSA4-5043+ with a 3 dB attenuator in between. The output is connected to the oscilloscope. For some reason the falling edge is about half as fast as I would expect it to be. This is how a typical signal looks like:

I expect the falling edge to have the same (or similar) slope as the rising edge. I know the amplifier is not saturated because the shape of the signals is the same for any deposited energy, i.e. I see the same shape for amplitudes ranging from 20 mV up to 120 mV.

This is a typical signal from the same radiation detector but connected to a different amplifier which behaves as expected:

This other amplifier is a single transistor common emitter configuration built with a BFP840FESD followed by a single stage SPF5189Z amplifier. Don't pay much attention to the amplitude as this it is random depending on the energy deposited by the radiation in the sensor. But look at the falling edge, here it is similar to the rising edge.

What are possible reasons for this?

There may be many reasons to affect this response such as cables, oscilloscope probes, PCB layout, capacitor types, etc.
Without all those knowledge, it's hard to say .

Make R24 resitor 0 ohms, and make L13 a 10 ohms resistor (could be even higher a bit) and this may solve your problem.

Signal form distortion is a non-linear effect, probably caused by interaction of detector with amplifier input impedance or amplifier saturation.

For a profound discussion, we would need to see direct detector output and output of first amplifier stage for comparison. Due to the high cut-off frequency of LC circuit, saturation of first amplifier might happen without showing clearly at the amplifier output.

It's the difference in driving impedance, "detector" source
vs passive-match "sink". An ionized charge track can present
a pretty low impedance (though you may find this dependent
on species dE/dx in material, and access resistance to the
"business end" of that material).

It's not uncommon for amplifiers to exhibit asymmetric slew
rates, especially if they have no, or loose, full power HF
distortion specs.

Perhaps the common emitter transistor presents a lower
impedance in both directions, or lower Cin to the point that
the track impulse is closer to its "in physics" shape. It seems
to be "soaking up less signal" going by the plot axes. Which
alone makes it a better choice.

But why not go for some common-emitter gain while you're
at it, if the transistor is that good (or, if you can find one
even better)?

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