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Bob Pease's High Impedance Active Scope Probe simulations

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The jfet input active oscilloscope probe described in "Troubleshooting Analog Circuits" by Bob Pease; Part 2 "The right equipment is essential for effective troubleshooting", Section 3, Figure 3.

Published in EDN January 5 1989.

A copy can be found here:

The original design can be found in:

There is one piece of information missing in Bob Pease's article and that is how to connect the signal output of the probe to the scope input and that is not as simple as it may seem because when probing a low impedance source, the probe bandwidth is high enough for the inevitable impedance mismatch due to driving a coax cable from the relatively low probe output into the complex x1 scope input impedance (1MOhms//15pF) to cause serious problems with reflections.

A higher performance design of output buffer is required for high frequency use so that it can drive either a lossy coax interconnecting cable or a higher performance 50 Ohm coax cable. This project contains the simulations for a simple DIY low frequency high input impedance active scope probe adapter.


Simulation schematic for Bob Pease's High Impedance Active Scope Probe


ID Name Designator Footprint Quantity Manufacturer Part Manufacturer Supplier Supplier Part Price LCSC Assembly Mounted
1 mmbfj309lt1 J1,J2 SOT23 2
2 Qmmbt3904lt1 Q1 SOT23 1
3 mmbt3906lt1 Q2 SOT23 1
4 51 R1 R0805 1
5 100 R2 TRIM_POT_PTH 1
6 3.3k R3,R4 R0805 2
7 10Meg R5,R6,R7 R3 3
8 10n C1,C2 C0805 2
9 15 V1,V2 BATTERY 2
10 1Meg R9 R3 1
11 15p C3 1206 1
12 SIN(0 1 50) AC 1 0 V3 2P-5.0 1
13 1m R10 R3 1
14 SPST_OFF S1 1
15 0 VAMMETER1 2P-5.0 1
16 620 RDAMP1 R0805 1
17 47 R8 R0805 1


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