George Herold <gherold@teachspin.com> wrote:> My first (or second) cap multiplier oscillated and I added a F-bead and > 100 ohm resistor on the base lead, through hole hair-pin. > No problems since then.Beads are bad. They can crack or break. The 100 ohm resistor is all that is needed.> George H.
really fast buffers
Started by ●October 3, 2018
Reply by ●October 6, 20182018-10-06
Reply by ●October 6, 20182018-10-06
Am 06.10.2018 um 21:03 schrieb Steve Wilson:> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 10/5/18 10:28 PM, Steve Wilson wrote: >>> George Herold <gherold@teachspin.com> wrote: > >>>> It oscillates because of a slow edge and no hysteresis? > >>> It oscillates because of inductance in the base lead. Along with the >>> base- emitter capacitance and capacitance from the emitter to ground, >>> this forms a Colpitts oscillator, which is one of the most vigorous >>> oscillators known. It can accept an extremely wide range of parameters >>> and still oscillate. > >>> The purpose of the resistor in the base lead is to dampen the Q of the >>> tank so oscillations are no longer possible. > >>>> Is it similar to cap. multiplier oscillations? > >>> Could be. Any place where you have some inductance in the base and >>> capacitance from the emitter to ground could form a Colpitts, >>> especially where you have some inductance in the emitter lead. > >> The key ingredient is negative input resistance. > > That term always bothered me. Negative input resistance to what? If you > look a the base input impedance, it is always positive. When the > oscillations get going, the transistor is cut off most of the time. Where > is the negative input impedance then?Negative input impedance from base/gate to GND. It is not positive and it sustains the oscillation. And it can be measured. < https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682404684680/ > This is S11 of a baseband amplifier with absurdly high gm to enforce low voltage noise with IF3602 JFETs. Everything that is outside the circle that goes through X=0 has a negative real part of the input impedance. The exact values at the 3 cursors are given in the picture. The green line is outside of that circle for most of the intended frequency range, until the amplifier runs out of steam. BTW I have found no way to cure that without compensating it with a positive resistance in the input or cutting the feedback. None of this is attractive in a low noise measurement amplifier. :-( :-( :-( Those network analyzers are so unforgiving. If you just ignore it, you can happily measure around in circuits that have enough source resistance. Just sometimes, it won't work. There are lots of low noise amplifiers that share its fate. BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from AOE3. The non-differential version, since my source is never centered around 0 Vdc. Works as promised. :-) Yes, it could use even more 'lytics, but that was a test balloon for a chopper amplifier. At 500 KHz the caps are no more that critical. < https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662535945536/ > cheers, Gerhard
Reply by ●October 6, 20182018-10-06
Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote:> Am 06.10.2018 um 21:03 schrieb Steve Wilson: >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>>> On 10/5/18 10:28 PM, Steve Wilson wrote: >>>> George Herold <gherold@teachspin.com> wrote:>>>>> It oscillates because of a slow edge and no hysteresis?>>>> It oscillates because of inductance in the base lead. Along with the >>>> base- emitter capacitance and capacitance from the emitter to ground, >>>> this forms a Colpitts oscillator, which is one of the most vigorous >>>> oscillators known. It can accept an extremely wide range of >>>> parameters and still oscillate.>>>> The purpose of the resistor in the base lead is to dampen the Q of >>>> the tank so oscillations are no longer possible.>>>>> Is it similar to cap. multiplier oscillations?>>>> Could be. Any place where you have some inductance in the base and >>>> capacitance from the emitter to ground could form a Colpitts, >>>> especially where you have some inductance in the emitter lead.>>> The key ingredient is negative input resistance.>> That term always bothered me. Negative input resistance to what? If you >> look a the base input impedance, it is always positive. When the >> oscillations get going, the transistor is cut off most of the time. >> Where is the negative input impedance then?> Negative input impedance from base/gate to GND. It is not positive and > it sustains the oscillation. And it can be measured.> https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682 > 404684680/> This is S11 of a baseband amplifier with absurdly high gm to enforce > low voltage noise with IF3602 JFETs. Everything that is outside the > circle that goes through X=0 has a negative real part of the input > impedance. The exact values at the 3 cursors are given in the picture.> The green line is outside of that circle for most of the intended > frequency range, until the amplifier runs out of steam.> BTW I have found no way to cure that without compensating it with a > positive resistance in the input or cutting the feedback. None of this > is attractive in a low noise measurement amplifier. :-( :-( :-(> Those network analyzers are so unforgiving. If you just ignore it, > you can happily measure around in circuits that have enough source > resistance. Just sometimes, it won't work. There are lots of > low noise amplifiers that share its fate.> BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from > AOE3. The non-differential version, since my source is never centered > around 0 Vdc. Works as promised. :-)> Yes, it could use even more 'lytics, but that was a test balloon for > a chopper amplifier. At 500 KHz the caps are no more that critical.> https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662 > 535945536/> cheers, > GerhardGerhard, I have the utmost appreciation for your accomplishements and your skill in electronics. However, a simple Colpitts oscillator in LTspice shows the input impedance at the base is positive. The emitter current is also in phase with the base current. Another example is a simple 74HC04 Pierce oscillator with an LC tank as feedback. The input impedance of the 74HC04 is capacitive. The impedance is Xc = 1 / (2piFC) There is no negative input resistance. A further example is "A General Theory of Phase Noise in Electrical Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, Member, IEEE" https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf "Consider as one example the Colpitts oscillator of Fig. 5(a). The collector voltage and the collector current of the transistor are shown in Fig. 13. Note that the collector current consists of a short period of large current followed by a quiet interval. The surge of current occurs at the minimum of the voltage across the tank where the ISF is small." Clearly the collector current is a function of the base current, and is in phase with the base current. The input impedance at the base is positive, therefore there is no negative resistance term available. Oscillators work on positive feedback. If the loop gain is positive and in phase, oscillations ensue.
Reply by ●October 6, 20182018-10-06
On Saturday, 6 October 2018 22:02:14 UTC+1, Steve Wilson wrote:> Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote: > > > Am 06.10.2018 um 21:03 schrieb Steve Wilson: > >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >>> On 10/5/18 10:28 PM, Steve Wilson wrote: > >>>> George Herold <gherold@teachspin.com> wrote: > > >>>>> It oscillates because of a slow edge and no hysteresis? > > >>>> It oscillates because of inductance in the base lead. Along with the > >>>> base- emitter capacitance and capacitance from the emitter to ground, > >>>> this forms a Colpitts oscillator, which is one of the most vigorous > >>>> oscillators known. It can accept an extremely wide range of > >>>> parameters and still oscillate. > > >>>> The purpose of the resistor in the base lead is to dampen the Q of > >>>> the tank so oscillations are no longer possible. > > >>>>> Is it similar to cap. multiplier oscillations? > > >>>> Could be. Any place where you have some inductance in the base and > >>>> capacitance from the emitter to ground could form a Colpitts, > >>>> especially where you have some inductance in the emitter lead. > > >>> The key ingredient is negative input resistance. > > >> That term always bothered me. Negative input resistance to what? If you > >> look a the base input impedance, it is always positive. When the > >> oscillations get going, the transistor is cut off most of the time. > >> Where is the negative input impedance then? > > > Negative input impedance from base/gate to GND. It is not positive and > > it sustains the oscillation. And it can be measured. > > > https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682 > > 404684680/ > > > This is S11 of a baseband amplifier with absurdly high gm to enforce > > low voltage noise with IF3602 JFETs. Everything that is outside the > > circle that goes through X=0 has a negative real part of the input > > impedance. The exact values at the 3 cursors are given in the picture. > > > The green line is outside of that circle for most of the intended > > frequency range, until the amplifier runs out of steam. > > > > BTW I have found no way to cure that without compensating it with a > > positive resistance in the input or cutting the feedback. None of this > > is attractive in a low noise measurement amplifier. :-( :-( :-( > > > Those network analyzers are so unforgiving. If you just ignore it, > > you can happily measure around in circuits that have enough source > > resistance. Just sometimes, it won't work. There are lots of > > low noise amplifiers that share its fate. > > > BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from > > AOE3. The non-differential version, since my source is never centered > > around 0 Vdc. Works as promised. :-) > > > Yes, it could use even more 'lytics, but that was a test balloon for > > a chopper amplifier. At 500 KHz the caps are no more that critical. > > > https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662 > > 535945536/ > > > cheers, > > Gerhard > > Gerhard, I have the utmost appreciation for your accomplishements and your > skill in electronics. > > However, a simple Colpitts oscillator in LTspice shows the input impedance > at the base is positive. The emitter current is also in phase with the base > current. > > Another example is a simple 74HC04 Pierce oscillator with an LC tank as > feedback. > > The input impedance of the 74HC04 is capacitive. The impedance is > > Xc = 1 / (2piFC) > > There is no negative input resistance. > > A further example is "A General Theory of Phase Noise in Electrical > Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, > Member, IEEE" > > https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf > > "Consider as one example the Colpitts oscillator of Fig. 5(a). > The collector voltage and the collector current of the transistor > are shown in Fig. 13. Note that the collector current consists > of a short period of large current followed by a quiet interval. > The surge of current occurs at the minimum of the voltage > across the tank where the ISF is small." > > Clearly the collector current is a function of the base current, and is in > phase with the base current. The input impedance at the base is positive, > therefore there is no negative resistance term available. > > Oscillators work on positive feedback. If the loop gain is positive and in > phase, oscillations ensue.FWIW it's not hard to demonstrate the existence of oscillators that rely purely on negative resistance. L, C, carbon arc. The v/i plot of the carbon arc shows positive large signal dc resistance, negative small signal ac resistance over a limited v swing. NT
Reply by ●October 6, 20182018-10-06
On Sat, 6 Oct 2018 16:57:16 -0700 (PDT), tabbypurr@gmail.com wrote:>On Saturday, 6 October 2018 22:02:14 UTC+1, Steve Wilson wrote: >> Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote: >> >> > Am 06.10.2018 um 21:03 schrieb Steve Wilson: >> >> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >>> On 10/5/18 10:28 PM, Steve Wilson wrote: >> >>>> George Herold <gherold@teachspin.com> wrote: >> >> >>>>> It oscillates because of a slow edge and no hysteresis? >> >> >>>> It oscillates because of inductance in the base lead. Along with the >> >>>> base- emitter capacitance and capacitance from the emitter to ground, >> >>>> this forms a Colpitts oscillator, which is one of the most vigorous >> >>>> oscillators known. It can accept an extremely wide range of >> >>>> parameters and still oscillate. >> >> >>>> The purpose of the resistor in the base lead is to dampen the Q of >> >>>> the tank so oscillations are no longer possible. >> >> >>>>> Is it similar to cap. multiplier oscillations? >> >> >>>> Could be. Any place where you have some inductance in the base and >> >>>> capacitance from the emitter to ground could form a Colpitts, >> >>>> especially where you have some inductance in the emitter lead. >> >> >>> The key ingredient is negative input resistance. >> >> >> That term always bothered me. Negative input resistance to what? If you >> >> look a the base input impedance, it is always positive. When the >> >> oscillations get going, the transistor is cut off most of the time. >> >> Where is the negative input impedance then? >> >> > Negative input impedance from base/gate to GND. It is not positive and >> > it sustains the oscillation. And it can be measured. >> >> > https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682 >> > 404684680/ >> >> > This is S11 of a baseband amplifier with absurdly high gm to enforce >> > low voltage noise with IF3602 JFETs. Everything that is outside the >> > circle that goes through X=0 has a negative real part of the input >> > impedance. The exact values at the 3 cursors are given in the picture. >> >> > The green line is outside of that circle for most of the intended >> > frequency range, until the amplifier runs out of steam. >> >> >> > BTW I have found no way to cure that without compensating it with a >> > positive resistance in the input or cutting the feedback. None of this >> > is attractive in a low noise measurement amplifier. :-( :-( :-( >> >> > Those network analyzers are so unforgiving. If you just ignore it, >> > you can happily measure around in circuits that have enough source >> > resistance. Just sometimes, it won't work. There are lots of >> > low noise amplifiers that share its fate. >> >> > BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from >> > AOE3. The non-differential version, since my source is never centered >> > around 0 Vdc. Works as promised. :-) >> >> > Yes, it could use even more 'lytics, but that was a test balloon for >> > a chopper amplifier. At 500 KHz the caps are no more that critical. >> >> > https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662 >> > 535945536/ >> >> > cheers, >> > Gerhard >> >> Gerhard, I have the utmost appreciation for your accomplishements and your >> skill in electronics. >> >> However, a simple Colpitts oscillator in LTspice shows the input impedance >> at the base is positive. The emitter current is also in phase with the base >> current. >> >> Another example is a simple 74HC04 Pierce oscillator with an LC tank as >> feedback. >> >> The input impedance of the 74HC04 is capacitive. The impedance is >> >> Xc = 1 / (2piFC) >> >> There is no negative input resistance. >> >> A further example is "A General Theory of Phase Noise in Electrical >> Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, >> Member, IEEE" >> >> https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf >> >> "Consider as one example the Colpitts oscillator of Fig. 5(a). >> The collector voltage and the collector current of the transistor >> are shown in Fig. 13. Note that the collector current consists >> of a short period of large current followed by a quiet interval. >> The surge of current occurs at the minimum of the voltage >> across the tank where the ISF is small." >> >> Clearly the collector current is a function of the base current, and is in >> phase with the base current. The input impedance at the base is positive, >> therefore there is no negative resistance term available. >> >> Oscillators work on positive feedback. If the loop gain is positive and in >> phase, oscillations ensue. > > >FWIW it's not hard to demonstrate the existence of oscillators that rely purely on negative resistance. L, C, carbon arc. The v/i plot of the carbon arc shows positive large signal dc resistance, negative small signal ac resistance over a limited v swing. >Tunnel diodes
Reply by ●October 6, 20182018-10-06
Am 06.10.2018 um 23:02 schrieb Steve Wilson:> Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote:Some pages of Randall W. Rhea: Discrete oscillator Design. That won't stay there for long, but for the discussion, methinks it is fair use. The book is worth every penny. < https://www.flickr.com/photos/137684711@N07/44424328234/in/album-72157672050645837/ > Sorry for the quick & dirty repro. 04:30 am here. Have a good night. Gerhard
Reply by ●October 7, 20182018-10-07
Steve Wilson>Another example is a simple 74HC04 Pierce oscillator with an LC tank as >feedback. > >The input impedance of the 74HC04 is capacitive. The impedance is > >Xc = 1 / (2piFC) > >There is no negative input resistance. > >A further example is "A General Theory of Phase Noise in Electrical >Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, >Member, IEEE" > >https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf > >"Consider as one example the Colpitts oscillator of Fig. 5(a). >The collector voltage and the collector current of the transistor >are shown in Fig. 13. Note that the collector current consists >of a short period of large current followed by a quiet interval. >The surge of current occurs at the minimum of the voltage >across the tank where the ISF is small." > >Clearly the collector current is a function of the base current, and is in >phase with the base current. The input impedance at the base is positive, >therefore there is no negative resistance term available. > >Oscillators work on positive feedback. If the loop gain is positive and in >phase, oscillations ensue.Steve I completely agree with your viewpoint. Just saying 'input has negative impedance' does not explain a thing, and then adding a resistor to make it positive again is fake close to string theory. Better is to understand the Cbe - CeGND step up, and that can be compensated for in many ways and is of no significance when driving the base with some circuits. I have had exactly one case of an oscillating emitter follower in my life, and that was in a missile tracking system, replacing the transistor fixed it, not all trannies are the same ... I do not use base resistors in my RF amps, that just decreases performance. And no those do not oscillate. You oscillators are doing something basically wrong, layout? ;-) hehe ;-)
Reply by ●October 7, 20182018-10-07
Gerhard Hoffmann wrote...> > BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier > from AoE3. The non-differential version, since my source is > never centered around 0 Vdc. Works as promised. :-) > > Yes, it could use even more 'lytics, but that was a test balloon for > a chopper amplifier. At 500 kHz the caps are no more that critical. > >https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662535945536/Nice! Please post your complete schematic. We only show a common-emitter amplifying stage, with millions of paralleled transistors, and our measurements, pages 506-509. -- Thanks, - Win
Reply by ●October 7, 20182018-10-07
On Sat, 06 Oct 2018 21:02:09 GMT, Steve Wilson <no@spam.com> wrote:>Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote: > >> Am 06.10.2018 um 21:03 schrieb Steve Wilson: >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >>>> On 10/5/18 10:28 PM, Steve Wilson wrote: >>>>> George Herold <gherold@teachspin.com> wrote: > >>>>>> It oscillates because of a slow edge and no hysteresis? > >>>>> It oscillates because of inductance in the base lead. Along with the >>>>> base- emitter capacitance and capacitance from the emitter to ground, >>>>> this forms a Colpitts oscillator, which is one of the most vigorous >>>>> oscillators known. It can accept an extremely wide range of >>>>> parameters and still oscillate. > >>>>> The purpose of the resistor in the base lead is to dampen the Q of >>>>> the tank so oscillations are no longer possible. > >>>>>> Is it similar to cap. multiplier oscillations? > >>>>> Could be. Any place where you have some inductance in the base and >>>>> capacitance from the emitter to ground could form a Colpitts, >>>>> especially where you have some inductance in the emitter lead. > >>>> The key ingredient is negative input resistance. > >>> That term always bothered me. Negative input resistance to what? If you >>> look a the base input impedance, it is always positive. When the >>> oscillations get going, the transistor is cut off most of the time. >>> Where is the negative input impedance then? > >> Negative input impedance from base/gate to GND. It is not positive and >> it sustains the oscillation. And it can be measured. > >> https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682 >> 404684680/ > >> This is S11 of a baseband amplifier with absurdly high gm to enforce >> low voltage noise with IF3602 JFETs. Everything that is outside the >> circle that goes through X=0 has a negative real part of the input >> impedance. The exact values at the 3 cursors are given in the picture. > >> The green line is outside of that circle for most of the intended >> frequency range, until the amplifier runs out of steam. > > >> BTW I have found no way to cure that without compensating it with a >> positive resistance in the input or cutting the feedback. None of this >> is attractive in a low noise measurement amplifier. :-( :-( :-( > >> Those network analyzers are so unforgiving. If you just ignore it, >> you can happily measure around in circuits that have enough source >> resistance. Just sometimes, it won't work. There are lots of >> low noise amplifiers that share its fate. > >> BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from >> AOE3. The non-differential version, since my source is never centered >> around 0 Vdc. Works as promised. :-) > >> Yes, it could use even more 'lytics, but that was a test balloon for >> a chopper amplifier. At 500 KHz the caps are no more that critical. > >> https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662 >> 535945536/ > >> cheers, >> Gerhard > >Gerhard, I have the utmost appreciation for your accomplishements and your >skill in electronics. > >However, a simple Colpitts oscillator in LTspice shows the input impedance >at the base is positive. The emitter current is also in phase with the base >current. > >Another example is a simple 74HC04 Pierce oscillator with an LC tank as >feedback. > >The input impedance of the 74HC04 is capacitive. The impedance is > >Xc = 1 / (2piFC) > >There is no negative input resistance. > >A further example is "A General Theory of Phase Noise in Electrical >Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, >Member, IEEE" > >https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf > >"Consider as one example the Colpitts oscillator of Fig. 5(a). >The collector voltage and the collector current of the transistor >are shown in Fig. 13. Note that the collector current consists >of a short period of large current followed by a quiet interval. >The surge of current occurs at the minimum of the voltage >across the tank where the ISF is small." > >Clearly the collector current is a function of the base current, and is in >phase with the base current. The input impedance at the base is positive, >therefore there is no negative resistance term available. > >Oscillators work on positive feedback. If the loop gain is positive and in >phase, oscillations ensue.Not always. Most positive feedback loops just latch in one state. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 7, 20182018-10-07
On Monday, October 8, 2018 at 2:24:45 AM UTC+11, John Larkin wrote:> On Sat, 06 Oct 2018 21:02:09 GMT, Steve Wilson <no@spam.com> wrote: > > >Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> wrote: > > > >> Am 06.10.2018 um 21:03 schrieb Steve Wilson: > >>> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > > > >>>> On 10/5/18 10:28 PM, Steve Wilson wrote: > >>>>> George Herold <gherold@teachspin.com> wrote: > > > >>>>>> It oscillates because of a slow edge and no hysteresis? > > > >>>>> It oscillates because of inductance in the base lead. Along with the > >>>>> base- emitter capacitance and capacitance from the emitter to ground, > >>>>> this forms a Colpitts oscillator, which is one of the most vigorous > >>>>> oscillators known. It can accept an extremely wide range of > >>>>> parameters and still oscillate. > > > >>>>> The purpose of the resistor in the base lead is to dampen the Q of > >>>>> the tank so oscillations are no longer possible. > > > >>>>>> Is it similar to cap. multiplier oscillations? > > > >>>>> Could be. Any place where you have some inductance in the base and > >>>>> capacitance from the emitter to ground could form a Colpitts, > >>>>> especially where you have some inductance in the emitter lead. > > > >>>> The key ingredient is negative input resistance. > > > >>> That term always bothered me. Negative input resistance to what? If you > >>> look a the base input impedance, it is always positive. When the > >>> oscillations get going, the transistor is cut off most of the time. > >>> Where is the negative input impedance then? > > > >> Negative input impedance from base/gate to GND. It is not positive and > >> it sustains the oscillation. And it can be measured. > > > >> https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682 > >> 404684680/ > > > >> This is S11 of a baseband amplifier with absurdly high gm to enforce > >> low voltage noise with IF3602 JFETs. Everything that is outside the > >> circle that goes through X=0 has a negative real part of the input > >> impedance. The exact values at the 3 cursors are given in the picture. > > > >> The green line is outside of that circle for most of the intended > >> frequency range, until the amplifier runs out of steam. > > > > > >> BTW I have found no way to cure that without compensating it with a > >> positive resistance in the input or cutting the feedback. None of this > >> is attractive in a low noise measurement amplifier. :-( :-( :-( > > > >> Those network analyzers are so unforgiving. If you just ignore it, > >> you can happily measure around in circuits that have enough source > >> resistance. Just sometimes, it won't work. There are lots of > >> low noise amplifiers that share its fate. > > > >> BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from > >> AOE3. The non-differential version, since my source is never centered > >> around 0 Vdc. Works as promised. :-) > > > >> Yes, it could use even more 'lytics, but that was a test balloon for > >> a chopper amplifier. At 500 KHz the caps are no more that critical. > > > >> https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662 > >> 535945536/ > > > >> cheers, > >> Gerhard > > > >Gerhard, I have the utmost appreciation for your accomplishements and your > >skill in electronics. > > > >However, a simple Colpitts oscillator in LTspice shows the input impedance > >at the base is positive. The emitter current is also in phase with the base > >current. > > > >Another example is a simple 74HC04 Pierce oscillator with an LC tank as > >feedback. > > > >The input impedance of the 74HC04 is capacitive. The impedance is > > > >Xc = 1 / (2piFC) > > > >There is no negative input resistance. > > > >A further example is "A General Theory of Phase Noise in Electrical > >Oscillators, by Ali Hajimiri, Student Member, IEEE, and Thomas H. Lee, > >Member, IEEE" > > > >https://authors.library.caltech.edu/4917/1/HAJieeejssc98.pdf > > > >"Consider as one example the Colpitts oscillator of Fig. 5(a). > >The collector voltage and the collector current of the transistor > >are shown in Fig. 13. Note that the collector current consists > >of a short period of large current followed by a quiet interval. > >The surge of current occurs at the minimum of the voltage > >across the tank where the ISF is small." > > > >Clearly the collector current is a function of the base current, and is in > >phase with the base current. The input impedance at the base is positive, > >therefore there is no negative resistance term available. > > > >Oscillators work on positive feedback. If the loop gain is positive and in > >phase, oscillations ensue. > > Not always. Most positive feedback loops just latch in one state.An over-general claim. If the positive gain is between 1.0 and 2.0 it isn't even unstable and won't latch. Back in 1979, I put a positive gain of 1.003 into a circuit to raise the current through a platinum resistive sensor as it got hotter, to compensate for the decline in the temperature coefficient of resistance with increasing temperature. The clown that took over the circuit claimed that this would make the circuit "unstable", and replaced it with a diode-break linear approximation. Someone at Honeywell seems to have invented the same circuit as I had at much the same time, and it worked fine in the field. Positive feedback loops can be designed for all sorts of jobs, and - if they are designed by people who know what they are doing - they won't latch up. -- Bill Sloman, Sydney
