Groups > sci.electronics.design > #518817

Re: really fast buffers

From	Jeroen Belleman <jeroen@nospam.please>
Newsgroups	sci.electronics.design
Subject	Re: really fast buffers
Date	2018-10-09 15:17 +0200
Organization	Aioe.org NNTP Server
Message-ID	<ppi9pg$12hj$1@gioia.aioe.org> (permalink)
References	(7 earlier) <787c7a2b-c1a9-4330-830b-8ab9a58d30d0@googlegroups.com> <ApmdnWelI5JdnyTGnZ2dnUU7-V3NnZ2d@supernews.com> <vb8ordth51oge27cjcdu0dtidte8qbkuje@4ax.com> <g22rjhFnledU1@mid.individual.net> <x21vD.6457$vf.4197@fx05.am4>

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Chris Jones wrote:
> On 09/10/2018 16:11, Gerhard Hoffmann wrote:
>> Am 09.10.2018 um 06:00 schrieb John Larkin:
>>> On Sat, 6 Oct 2018 14:45:19 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>
>>>> If you put a resistor in series with the emitter, the reflected
>>>> impedance at the base is a negative resistance.
>>>
>>> I don't understand that. At low frequencies, as the base voltage goes
>>> up, the base current goes up. The base looks ohmic.
>>
>> That does not work at low frequencies. It takes takes transistor-
>> internal parasitic capacitance to have an effect. The effect gets
>> more pronounced for high emitter resistance and high emitter-GND
>> capacitance. In the pages from Rhea that I have referenced, there is
>> a transformation that converts such a loaded follower to the usual
>> feedback loop by re-assigning GND to the emitter itself and redrawing
>> the circuit.
>>
>> If you measure into base-gnd at RF, you see some pF in series
>> with, say -50 Ohms. That is used in virtually all VHF/UHF
>> VCOs from Z-Comm and others. I have once tried to pull down
>> a Z-Comm VCXO by adding more varicaps since I had a few of them
>> and needed lower frequencies. I didn't get very far because of
>> the few pF that are in series to the base at RF. Disappointing,
>> but finally understood.
>>
>> In the case of my baseband amplifier with the 2 IF3602 pairs,
>> there are nF capacitances, and the amplifier goes negative
>> impedance below 100 KHz. Ferrite chokes have quite a good Q
>> at these frequencies, and one can tune the oscillation frequency
>> with them :-(
>>
>> At higher frequencies where they work like resistors (as intended)
>> they also generate voltage noise. You can't have your Kate, and
>> Edith, too.
>>
>> regards,
>> Gerhard
>>
>>
> 
> I found that a unity gain buffer built from an AD8066 opamp also seems 
> to have a negative resistance at its input. If you decide that you want 
> to measure e.g. the DC offset, and short the input to ground with a 40cm 
> jumper lead, it will oscillate at about 40MHz, and with shorter wires it 
> will oscillate up to about 116MHz. Putting a 560R resistor in series 
> with the input connector on the board, and a 3.3pF capacitor from the 
> non-inverting input pin of the opamp to ground fixed it, but made it 
> otherwise worse. I recall that there is nothing much in the datasheet to 
> warn about this sort of problem.
> 

I use discrete amplifiers with feedback to set the input impedance
to 50 Ohms, because that's quieter than using an actual terminating
resistor. These also have a negative input impedance in a certain
frequency range, which causes instability with reactive sources.
Fortunately --well no, by design-- this happens outside of my signal
bandwidth, so I can muffle that with some filtering at the input.

Of course, like Gerhard says, the muffler produces thermal noise,
but since that's outside my signal bandwidth, it doesn't matter.

Jeroen Belleman

Thread

really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-03 15:03 -0700
  Re: really fast buffers Lasse Langwadt Christensen <langwadt@fonz.dk> - 2018-10-03 15:23 -0700
    Re: really fast buffers Andy Bennet <andyb@andy.com> - 2018-10-04 08:20 +0100
      Re: really fast buffers Lasse Langwadt Christensen <langwadt@fonz.dk> - 2018-10-04 08:45 -0700
  Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-03 15:52 -0700
    Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-03 18:08 -0700
      Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-03 20:02 -0700
  Re: really fast buffers bitrex <user@example.net> - 2018-10-03 21:54 -0400
    Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-03 20:10 -0700
      Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-03 20:40 -0700
        Re: really fast buffers pcdhobbs@gmail.com - 2018-10-04 05:00 -0700
          Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-04 08:39 -0700
            Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 12:38 -0400
              Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-04 14:35 -0700
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 18:27 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:21 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:38 -0400
            Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 11:50 -0700
              Re: really fast buffers Clifford Heath <no.spam@please.net> - 2018-10-05 09:26 +1000
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 17:09 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:12 +0000
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-05 14:18 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 23:42 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:41 -0400
              Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:09 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:40 -0400
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 11:06 -0400
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-05 19:16 -0700
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-05 14:26 -0700
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-05 18:36 -0700
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-05 19:22 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 02:28 +0000
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-06 06:03 +0000
                Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-05 23:41 -0700
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-06 08:12 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:46 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 19:03 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 15:32 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 20:08 +0000
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-06 22:19 +0200
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 21:02 +0000
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-06 16:57 -0700
                Re: really fast buffers krw@notreal.com - 2018-10-06 22:30 -0400
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-07 04:30 +0200
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-07 07:15 +0000
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-07 08:24 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-07 18:51 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-08 08:16 -0700
                Re: really fast buffers Winfield Hill <hill@rowland.harvard.edu> - 2018-10-07 07:19 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-10 00:38 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-10 03:02 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-10 04:07 -0700
                Re: really fast buffers DecadentLinuxUserNumeroUno@decadence.org - 2018-10-10 11:28 +0000
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-11 22:06 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-12 00:08 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-13 22:29 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-14 00:30 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:38 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-15 20:55 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-14 04:31 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-14 07:29 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:42 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-15 20:57 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-16 08:41 -0700
                Re: really fast buffers krw@notreal.com - 2018-10-16 17:38 -0400
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-16 19:37 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-16 19:40 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:40 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-11 22:04 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-12 08:02 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-10 14:06 +0200
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-10 08:31 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-11 01:49 +0200
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-10 17:41 -0700
                Re: really fast buffers "John Miles, KE5FX" <jmiles@gmail.com> - 2018-10-10 20:53 -0700
                Re: really fast buffers Jasen Betts <jasen@xnet.co.nz> - 2018-10-11 05:47 +0000
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-11 12:35 +0200
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-06 12:39 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 20:11 +0000
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 20:26 -0700
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:45 -0400
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-06 12:46 -0700
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 21:00 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-09 07:11 +0200
                Re: really fast buffers Chris Jones <lugnut808@spam.yahoo.com> - 2018-10-09 23:20 +1100
                Re: really fast buffers Jeroen Belleman <jeroen@nospam.please> - 2018-10-09 15:17 +0200
          Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-04 09:18 -0700
            Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 12:39 -0400
              Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 11:51 -0700
        Re: really fast buffers Piotr Wyderski <peter.pan@neverland.mil> - 2018-10-08 13:19 +0200
          Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 08:52 -0700
  Re: really fast buffers whit3rd <whit3rd@gmail.com> - 2018-10-04 11:57 -0700
  Re: really fast buffers David Nadlinger <david@klickverbot.at> - 2018-10-04 23:05 +0100
    Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 17:23 -0700
      Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-05 11:26 +0200
        Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:44 -0400

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