# opamp shaping amplifiers

#### pretty_fly

Joined Feb 2, 2015
21
Hello everybody, nowadays I am working on shaping amlifiers and need guidiance about how to design one and do its simulations before pcb design and manufacturing. When googling about shaping amplifiers i found documents mostly about filters (active and passive), which as i know works for rejecting some frequencies. So pleae give me some advices to understand the shaping amplification concempt. I mean how i can convert a filter to a shaping amplifier? For example designing a CR-(RC)^4 filter which seems a gaussian output shaper.

Looking forward to see your feedback...

#### MikeML

Joined Oct 2, 2009
5,444
Is "shaping amplifier" = Gaussian filter?

#### pretty_fly

Joined Feb 2, 2015
21
Is "shaping amplifier" = Gaussian filter?
I think it is a general name, comes from pulse shaping. But the expected output is gaussian shaped and i want to learn how to design unipolar one.

#### MikeML

Joined Oct 2, 2009
5,444
Specify the transfer function of your "shaping amplifier".

#### pretty_fly

Joined Feb 2, 2015
21
Actually I know that I need a transfer function but dont know how to get it. Because of this any suggestion about app notes, books websites are very welcome.

Btw I am a physics student and working on a gamma ray detector, an app. note of maxim integrated.

#### MikeML

Joined Oct 2, 2009
5,444
We then, maybe you could describe (in detail) what the input pulse looks like (amplitude, duration, repetition rate, rise time, fall time, etc)?

Now describe what you want it to look like after it is "shaped"?

#### pretty_fly

Joined Feb 2, 2015
21
Hi MikeML thank you for your interest

The first picture is the output of my preamplifier. I can see this on the scope when I apply a 10 mV/(dont remember pulse width now) pulse signal over a 1 picofarad capacitor to the input of preamplifier.

Second picture is the shaping amp output signal which I want to get when the preamplifier output was shaped.

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#### crutschow

Joined Mar 14, 2008
28,214
How accurately do you need a Gaussian shape?
The more accurate the shape the more filter stages that are needed.

#### MikeML

Joined Oct 2, 2009
5,444
Here is a hack at it: The opamps would have to have a GBW =>~20MHz, layout would require short leads.

Stuff inside the dashed box simulates your source. Two stages are required to get the gain of 100 to get 1V output.

#### pretty_fly

Joined Feb 2, 2015
21
Which simulation software you used MikeML?? I want to try it on my self. Simulating the circuits is another issue i consider.

crutschow, I want to have it in symmetrical manner.

#### crutschow

Joined Mar 14, 2008
28,214
Mike uses the LTspice program from Linear Technology (as he stated in the fine print at the bottom of his post), as do several on these forums including myself. It's one of the best free Spice programs available.

Mike will likely post his .asc simulation file if you want to experiment with it.

#### pretty_fly

Joined Feb 2, 2015
21
I saw it now

Can you share details of your simulation. I mean probably there must be some mathematical side. Then I can compare the one I built ( hopefully ).

#### MikeML

Joined Oct 2, 2009
5,444
Can you share details of your simulation. I mean probably there must be some mathematical side. Then I can compare the one I built ( hopefully ).
It is on another computer at my lab. I'll upload it next time I get over there...

#### MikeML

Joined Oct 2, 2009
5,444
Here 'tis.

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#### crutschow

Joined Mar 14, 2008
28,214
The mathematics of filters is somewhat complex as you may know.
There are many types of filter responses such as Butterworth, Bessel, Chebyshev, linear phase, etc.
The Bessel and linear phase low-pass filters can give a reasonable approximation of a Gaussian filter pulse with proper parameters.
A common active-filter type to achieve that is the Sallen-Key design.
The design of those is greatly simplified using an on-line tool or a filter-design program such as the free FilterPro from Texas Instruments.

Note that for your pulse requirements a high-speed op amp will be required. Mike used a generic op amp with a high gain-bandwidth (10MHz) for his simulation.

#### pretty_fly

Joined Feb 2, 2015
21

Currently i got some other advices too and need to read, think and work on all of those. Hope to see your further advices soon )

But before concentrating on all those notes, MikeML can you share the calculations or formulas if you did specificly? I am curious about how to find out a resistor value e.g. 59.2k or 90.9k. Or I will look for online tools as crutcrutschow adviced.

#### MikeML

Joined Oct 2, 2009
5,444
I cheated: