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Old 26th November 2012, 04:11 AM   #1
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Default Homebrew active probe

I want to add an active probe to my test gears. Downloaded D. Feucht "Buffer Amplifier Design" (http://www.analogzone.com/col_01302004.pdf). Is the JFET circuit shown in this article adequate? LTspice AC analysis shows f_3dB at about 225 MHz. At 100 MHz, gain is about -1 dB.
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Old 26th November 2012, 08:53 AM   #2
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Location: Zürich
How should we know if a certain circuit is adequate if we don't know your requirements? Key parameters are:

* Input resistance and capacitance
* attenuation (perhaps even gain?)
* bandwidth
* AC coupling
* input signal range
* offset control

Some links:

http://welecw2000a.sourceforge.net/d...mit_OPA659.pdf (German only)

BTW, Tektronix P6201 are now getting so cheap that, besides the fun and learning experience, there's probably few reasons to DIY.

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Old 26th November 2012, 11:18 AM   #3
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I need the probe be able to measure down to 10 Hz. Upper frequency cut-off should be at least 100 MHz. 500 MHz would be better to match with the scope BW.

Input resistance 1 MOhm, capacitance less than or equal to 1 pF.
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Old 26th November 2012, 01:09 PM   #4
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That low input C at this bandwidth is surely pretty demanding. Your best bet is probably a 10x design. Layout will be very critical--there's quickly a few 100 fF sneaking in.

At low frequencies you can use bootstrapping (see http://www.ti.com/lit/an/snoa620/snoa620.pdf, page2), but I'm not sure if this can be extended to such high bandwidths using ordinary PCB level techniques.

Also see Pease, Troubleshooting Analog Circuits, page 16/figure 2.3 (available at Google Books).

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Old 15th September 2013, 12:33 PM   #5
jcx is offline jcx  United States
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N2795A Active Probe, 1 GHz | Agilent

so the questions are how is it done and how much does making your own cost - and how much you're wiling to give up in performance, usability, reliability, accuracy...

Last edited by jcx; 15th September 2013 at 12:36 PM.
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Old 29th September 2013, 09:12 AM   #6
alayn91 is offline alayn91  France
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May be this one ??

It's from Elektuur 04/2004.
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Old 16th June 2017, 12:08 PM   #7
EUVL is offline EUVL  Europe
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> Also see Pease, Troubleshooting Analog Circuits, page 16/figure 2.3 (available at Google Books).

After looking around for all options, I settled on the Peace circuit.
Do not need very high bandwidth, but need higher input voltage (+/-30V AC+DC) and Zin > 1Gohm.
The Peace suits my requirements best, using 2SK223E matched pair, 2SC2240BL & 2SA970BL.
BJTs have 10k bias, JFETs 10R source resistor and 100R gate stoppers.
Can work up to +/-40V rails.

Bandwidth -0.5dB at 2MHz (my func gen's limit).
Gain = 0.997 measured, also as simulated in Spice.
Cin = 2.5pF.

Really nice tool to have.
Took 2 hours to build. Highky recommended.
(Alternative FETs 2SK373 @ +/-50V, 2SK117 @ +/-25V.)


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File Type: jpg 170616 Active Probe K223 Top.JPG (226.9 KB, 71 views)
File Type: jpg 170616 Active Probe K223 Btm.JPG (219.3 KB, 69 views)
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