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Old 15th Jun 2020, 4:48 pm   #1
Skywave
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Question Hi-Z input probe by Bob Pease.

Hi-Z input probe by Bob Pease, the well-known and highly accomplished design engineer at National Semiconductor who sadly passed away in 1969. (Info. from Wikipedia).

The cct. diagram of this probe is attached. Note that there are two versions: the original, referred to here as 'V1' and a revised version, 'V2', which uses SOT FETs.

There are three things about this probe that I do not understand, so enlightenment, please!

1. The claimed Rin is 1*e11 Ohms, which to me is 10*10 Ohms, i.e. 10^4 Meg Ohms. I cannot see how the Rin can be that high.
2. The claimed Cin is 0.23 pF. But the Cgs alone of Q1 will be approx. 5 pF.
3. What is the purpose of the 620 Ohm R at the gate of Q1?

Al.
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Old 15th Jun 2020, 5:03 pm   #2
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Default Re: Hi-Z input probe by Bob Pease.

I haven't seen that version before, I've only seen the original V1.

I'm nervous to comment on the design because the last time I reviewed the original version (elsewhere on the internet) I was slated for daring to point out any limitations of a Bob Pease design

However, one major issue with the original was that it was an obvious generator of negative resistance (at HF and VHF) at the gate of the input JFET. I think the 620R series resistor has been added to try and combat this.

I think the original version didn't use the shunt 30M resistance and this was optional. Without these resistors the input impedance at low frequencies will be very high indeed.

With the Q2 JFET acting as a current source the voltage at the source of Q1 will track very closely with the signal voltage at the gate of Q1 and this means the effect of the Cgs capacitance will be effectively nulled as it has virtually no differential voltage across it.
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Old 15th Jun 2020, 5:08 pm   #3
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Default Re: Hi-Z input probe by Bob Pease.

His Beetle was from'69; Bob died in 2011. Very sad, especially given the event he was driving home from.

While I've not read Bob's write-up of this, I'd say that the quoted input impedance must be that of the JFET itself. The input capacitance of the JFET follower will be bootstrapped, and the 620 ohm resistor is a "stopper" to prevent HF oscillation from low impedance sources.

I've got a couple of his books - I'll take a look to see if this appears in any of them...
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Old 15th Jun 2020, 5:12 pm   #4
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Default Re: Hi-Z input probe by Bob Pease.

I just spotted that this version uses a BFJ309 rather than the original 2N548x JFETs. I'm not sure the input capacitance will be as low as the original design.

I think the original design also relied on using a probe tip that was very skinny and it was arranged to give very low stray capacitance. I'm not sure what the input capacitance will be for the V2 version but I doubt it will be as low as 0.29pF.
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Old 15th Jun 2020, 5:18 pm   #5
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
and the 620 ohm resistor is a "stopper" to prevent HF oscillation from low impedance sources.
I suspect that the best way to make the original circuit go unstable up at HF/VHF would be to probe a high Q inductor of a few uH inductance. In other words, connect a handwound airspaced inductor across the input terminals of the probe.
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Old 15th Jun 2020, 5:23 pm   #6
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Default Re: Hi-Z input probe by Bob Pease.

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Originally Posted by mhennessy View Post
I've got a couple of his books - I'll take a look to see if this appears in any of them...
Yes, found V1 in "Troubleshooting Analog Circuits". Page 16.

Available here: http://preview.kingborn.net/27000/4c...20212e7a4b.pdf
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Old 15th Jun 2020, 5:31 pm   #7
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Arrow Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by G0HZU_JMR View Post
I just spotted that this version uses a BFJ309 rather than the original 2N548x JFETs. I'm not sure the input capacitance will be as low as the original design.
An Internet search plus recourse to data books gives Cgs = 5pF for 2N5456 and same for BFJ309. For comparison, the 2N4416 is 4 pF; min. value of 2.2 pF. And there is always the possibility of using the dual N-FET U440: Cgs = 3 pF.

Al.
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Old 15th Jun 2020, 5:37 pm   #8
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by G0HZU_JMR View Post
I'm nervous to comment on the design because the last time I reviewed the original version (elsewhere on the internet) I was slated for daring to point out any limitations of a Bob Pease design
Clearly understood. However, if you'd care to send me a PM with your thoughts, I'll appreciate that & read them with interest. Anything you do so write will remain strictly confidential.

Al.
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Old 15th Jun 2020, 5:50 pm   #9
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by G0HZU_JMR View Post
I'm nervous to comment on the design because the last time I reviewed the original version (elsewhere on the internet) I was slated for daring to point out any limitations of a Bob Pease design
Clearly understood. However, if you'd care to send me a PM with your thoughts, I'll appreciate that & read them with interest.

Al.
Well, I for one, find this an interesting topic. Surely, this forum has not reached a point where reasonable discussions need to go on via Private Messages .

B
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Old 15th Jun 2020, 5:56 pm   #10
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by Skywave View Post
Quote:
Originally Posted by G0HZU_JMR View Post
I just spotted that this version uses a BFJ309 rather than the original 2N548x JFETs. I'm not sure the input capacitance will be as low as the original design.
An Internet search plus recourse to data books gives Cgs = 5pF for 2N5456 and same for BFJ309. For comparison, the 2N4416 is 4 pF; min. value of 2.2 pF. And there is always the possibility of using the dual N-FET U440: Cgs = 3 pF.

Al.
I'm a bit rusty on this stuff but I think a J309 is a process 92 JFET and the 2N5485 is process 50. This means the internal die will be quite different. I'd expect the J309 to have slightly higher input capacitance and the Idss will be slightly higher too. It would generally be a bit unusual to group these two JFETs as equivalents because of the different process used to manufacture them.

However, someone must have used the J309 in this probe without any obvious issues.
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Old 15th Jun 2020, 6:01 pm   #11
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Default Re: Hi-Z input probe by Bob Pease.

I'm interested, too- and I promise not to throw any brickbats! I find it very odd that someone would have got shirty about constructive criticism on any subject, but the 'net can be a funny place. Bob Pease may have been a benign sort of person to be a craven disciple of, but folk being unable to countenance infallibility of an idol has been a Very Bad Thing indeed throughout history
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Old 15th Jun 2020, 6:10 pm   #12
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Default Re: Hi-Z input probe by Bob Pease.

Quote:
Well, I for one, find this an interesting topic.
It certainly is. I just hope no-one gets miffed if I comment on the design. I did look at it a few years ago and my memory of it is slowly returning...

One thing to be aware of will be the spread in Idss between JFETs. If it is assumed that Vg and Vs of Q1 are always at the same potential then I'd expect the JFET to run at Idss all the time. If the drain of Q1 is held at 15V and the gate of Q1 can be probed down to maybe -10V then this puts 25V across the JFET. If the Idss of Q1 is about 15mA then I'd expect to see this JFET get very hot when probing negative dc voltages. These voltages are also very near the limits on the datasheet.

Therefore, I'd recommend using the 2N5485 rather than the 2N5486 and it might be worth an initial check of the Idss of any BFJ309 that gets fitted. It might get very hot when probing negative voltages if the FET has an Idss up near the max on the datasheet.

At the minute all this is guesswork. If I get time this evening I'll have a go at simulating the probe.
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Old 15th Jun 2020, 7:32 pm   #13
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Arrow Re: Hi-Z input probe by Bob Pease.

Quote:
Originally Posted by G0HZU_JMR View Post
I'm nervous to comment on the design because the last time I reviewed the original version (elsewhere on the internet) I was slated for daring to point out any limitations of a Bob Pease design
Quote:
Originally Posted by Skywave View Post
Clearly understood. However, if you'd care to send me a PM with your thoughts, I'll appreciate that & read them with interest.
Quote:
Originally Posted by Bazz4CQJ View Post
Well, I for one, find this an interesting topic. Surely, this forum has not reached a point where reasonable discussions need to go on via Private Messages .
Bazz - Point taken - but don't overlook the fact that anything stated in this forum is readable by anyone who can access the Internet. Membership of this Forum is not a prerequisite to read posts. Moreover, a non-member might proceed to become a member simply to respond to a post that has upset him / her. And if that post is clearly unpleasant, it will cause additional work for the moderators. Plus, there is then the risk of premature thread closure. All-in-all, no-one 'benefits'.

Having said all that, let's now not drift off-topic, please.

Al.

Last edited by Skywave; 15th Jun 2020 at 7:40 pm.
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Old 15th Jun 2020, 7:48 pm   #14
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Default Re: Hi-Z input probe by Bob Pease.

The other area I recall being a bit confused about was the output connection that 'feeds to a scope'. I'm not sure what format this interface took in the original design. Was it by a very short piece of (RG174?) coax or was it with short flying wires?

Has anyone built the probe and experimented with different methods to interface to a scope?
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Old 15th Jun 2020, 9:49 pm   #15
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Default Re: Hi-Z input probe by Bob Pease.

Reading over the link in post #6, it's almost as if the active probe is being presented as "outline sketch", i.e. an illustration of topology with potential for development for specific roles- again, I see no problem in refinement and discussion around improvement in various aspects. It's not as if it's holy writ, more wise guidance.

Colin
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Old 15th Jun 2020, 10:42 pm   #16
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Default Re: Hi-Z input probe by Bob Pease.

Remember that Bob Pease was principally concerned with low frequency electronics, not RFery. Those designs are definitely not RF beasties and I've got doubts about those input capacitor figures.

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Old 15th Jun 2020, 11:09 pm   #17
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Default Re: Hi-Z input probe by Bob Pease.

Agreed. I think the 0.29pF figure for the original circuit may be a bit optimistic.

The original version based on the 2N5485/6 will have some contribution from Cdg inside the JFET and also from the free space capacitance of the lengthy probe tips. I suspect that the capacitance will also vary slightly depending on the dc offset presented at the input at the gate of Q1.

I think the J309 version (developed by someone else?) will have higher input capacitance because the process 92 JFETs have a higher Cdg capacitance.

I suspect that this probe was developed for looking at high impedance 'instrumentation' electronics so it would rarely encounter any inductors in a circuit. I do think it will be prone to instability when probing inductors.
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Old 15th Jun 2020, 11:19 pm   #18
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Default Re: Hi-Z input probe by Bob Pease.

It keeps reminding me of a problem I never found a good solution to, producing a 1.8k//1.5pF termination for a crystal filter I was using a BF904 dual gate mosfet in the following stage and that's about 3pF, so I was using 3.3 pf series capacitor and taking a hit on the voltage gain. It still didn't work because of pad track and stray C. It was a losing game. The other approach was a tuned circuit to get some negative capacitance in there, but component tolerances meant it needed adjusters and adjustments were most strongly not wanted. Bit of a dilemma.

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Old 16th Jun 2020, 12:06 am   #19
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Default Re: Hi-Z input probe by Bob Pease.

Yes, the claimed BW is 90MHz, which is fast in my book, but nowhere near the dizzy heights of RF wizardry that you and Jeremy seem to enjoy

But no mention of BW requirements have been made by the OP in this thread yet, so I don't know if this is a follow-on from previous threads or just a general enquiry about the circuit as presented?

One thing that isn't clear is how the input capacitance was determined. I'd like to think that Bob didn't just pluck that number out of thin air, or just loosely calculated it based on the known value of Cgs and the "gain" of the first transistor (a bit like PA speakers that quote their maximum SPL based on sensitivity and quoted power handling, which ignores a whole host of practicalities - not least thermal compression). So I would guess that he measured it in some empirical way - perhaps with added known series capacitance at the input, or perhaps a high value of series resistance instead. Or both, like a normaliser used for 'scope attenuator adjustments. He was also able to state the capacitance added by the FR4 probes, so he must have had some method for doing this. He was famously quite good at responding to correspondence, but sadly we're too late now.
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Old 16th Jun 2020, 12:42 am   #20
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Default Re: Hi-Z input probe by Bob Pease.

At a guess I would say that if the original 2N5485 JFETs were used then the input capacitance would/could be just under 1pF. This is based on the numbers in the datasheet and also on a basic simulation in Genesys. There may be some spread in this across a sample of 2N5485 JFETs but that really is just a guess. Even if a typical homebrew 'copy' only achieves 0.8 to 1pF it is still very low.

My greater concern would be the significant levels of negative resistance it can generate. The simulation predicts this occurs from the middle of the HF band up to about 250MHz. This won't matter much when probing high impedance circuits comprising opamps, resistors, capacitors and transistors but it can matter if there are inductors in the circuit.
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