John Larkin's LC oscillator
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From: Jim Thompson on 14 Jun 2010 17:45 On Mon, 14 Jun 2010 14:41:08 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 14 Jun 2010 12:27:57 -0700 (PDT), dagmargoodboat(a)yahoo.com >wrote: > >>> Q1 doesn't conduct in reverse with your values, but no >>> Schottky... the current is always out of the emitter... though it does >>> get awfully close to zero: -376uA and -12.5mA peak. >> >>Huh. LTSpice says Q1 does conduct in reverse, a nasty little 5mA >>spike's worth. The schottky feedback prevents that by cutting the >>base bias enough to make sure the collector never gets that low. > >Right; the schottly is a more pure AGC mode. The reverse emitter >conduction depends on the inverse beta of the transistor. If the model >includes inverse beta, when the collector dips down to close to >ground, and the c-b junction forward biases, it essentially flips >ends: collector becomes emitter, emitter becomes collector, emitter >current flows upwards. > >ftp://jjlarkin.lmi.net/LC_YDx.gif > >Either way, the base cap gets discharged. > >You can force this to work class-C if you jack around with the >transformer ratio, and use a small base cap, but that's not the way I >originally described the circuit. > >Maybe I'll try moving the feedback into the base circuit, one of these >days. > >Cute circuit. > >John > If Q1 conducts in reverse what _class_ is the oscillator ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: dagmargoodboat on 14 Jun 2010 18:27 On Jun 14, 4:41 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Mon, 14 Jun 2010 12:27:57 -0700 (PDT), dagmargoodb...(a)yahoo.com > wrote: > > >> Q1 doesn't conduct in reverse with your values, but no > >> Schottky... the current is always out of the emitter... though it does > >> get awfully close to zero: -376uA and -12.5mA peak. > > >Huh. LTSpice says Q1 does conduct in reverse, a nasty little 5mA > >spike's worth. The schottky feedback prevents that by cutting the > >base bias enough to make sure the collector never gets that low. > > Right; the schottly is a more pure AGC mode. The reverse emitter > conduction depends on the inverse beta of the transistor. If the model > includes inverse beta, when the collector dips down to close to > ground, and the c-b junction forward biases, it essentially flips > ends: collector becomes emitter, emitter becomes collector, emitter > current flows upwards. > > ftp://jjlarkin.lmi.net/LC_YDx.gif > > Either way, the base cap gets discharged. It's interesting that Jim's PSpice doesn't show the same spike. I put the schottky clamp in on theoretical grounds though--without it the collector _has_ to swing a diode-drop below the base to discharge the base cap, which puts Q1 at least on the verge if not frankly in reverse mode conduction. With the schottky, damping (via the bias) kicks in as the collector swing approaches 0.4v below Vb, preventing Vce ever going negative. > You can force this to work class-C if you jack around with the > transformer ratio, and use a small base cap, but that's not the way I > originally described the circuit. I did that first, with the savage versions. Theoretically I think that's more stable, letting the LC ring as it will, but with the effect of the reverse spike...I'm still not sure. Leaving the transistor on all the time gives a cleaner waveform, but manhandling the tank full-time slightly out-of-phase pulls the tank off its natural frequency. Hmmm. I guess it doesn't really matter--I was thinking RF oscillator here, but the feedback phase delays at audio are trivial. > Maybe I'll try moving the feedback into the base circuit, one of these > days. I did that too. Not much different really. > Cute circuit. I think so too. -- Cheers, James Arthur
From: John Larkin on 14 Jun 2010 19:01 On Mon, 14 Jun 2010 15:27:30 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: >On Jun 14, 4:41�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Mon, 14 Jun 2010 12:27:57 -0700 (PDT), dagmargoodb...(a)yahoo.com >> wrote: >> >> >> Q1 doesn't conduct in reverse with your values, but no >> >> Schottky... the current is always out of the emitter... though it does >> >> get awfully close to zero: -376uA and -12.5mA peak. >> >> >Huh. �LTSpice says Q1 does conduct in reverse, a nasty little 5mA >> >spike's worth. �The schottky feedback prevents that by cutting the >> >base bias enough to make sure the collector never gets that low. >> >> Right; the schottly is a more pure AGC mode. The reverse emitter >> conduction depends on the inverse beta of the transistor. If the model >> includes inverse beta, when the collector dips down to close to >> ground, and the c-b junction forward biases, it essentially flips >> ends: collector becomes emitter, emitter becomes collector, emitter >> current flows upwards. >> >> ftp://jjlarkin.lmi.net/LC_YDx.gif >> >> Either way, the base cap gets discharged. > >It's interesting that Jim's PSpice doesn't show the same spike. He blames it on defects in LT Spice. That's crazy. I suspect it's actually different transistor models. Note that in my sim, the emitter current reverses exactly at the points in time when Vc swings through Ve. John
From: Jim Thompson on 14 Jun 2010 19:15 On Mon, 14 Jun 2010 16:01:55 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 14 Jun 2010 15:27:30 -0700 (PDT), dagmargoodboat(a)yahoo.com >wrote: > >>On Jun 14, 4:41�pm, John Larkin >><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> On Mon, 14 Jun 2010 12:27:57 -0700 (PDT), dagmargoodb...(a)yahoo.com >>> wrote: >>> >>> >> Q1 doesn't conduct in reverse with your values, but no >>> >> Schottky... the current is always out of the emitter... though it does >>> >> get awfully close to zero: -376uA and -12.5mA peak. >>> >>> >Huh. �LTSpice says Q1 does conduct in reverse, a nasty little 5mA >>> >spike's worth. �The schottky feedback prevents that by cutting the >>> >base bias enough to make sure the collector never gets that low. >>> >>> Right; the schottly is a more pure AGC mode. The reverse emitter >>> conduction depends on the inverse beta of the transistor. If the model >>> includes inverse beta, when the collector dips down to close to >>> ground, and the c-b junction forward biases, it essentially flips >>> ends: collector becomes emitter, emitter becomes collector, emitter >>> current flows upwards. >>> >>> ftp://jjlarkin.lmi.net/LC_YDx.gif >>> >>> Either way, the base cap gets discharged. >> >>It's interesting that Jim's PSpice doesn't show the same spike. > >He blames it on defects in LT Spice. That's crazy. I suspect it's >actually different transistor models. Note that in my sim, the emitter >current reverses exactly at the points in time when Vc swings through >Ve. > >John > I didn't say that at all. I said I don't see it in PSpice, AND it's an advertised feature of LTspice that models are tweaked for speed. However I suspect it's that I assigned a resistance to the feedback winding proportionate to the Q assigned to the primary. You two did not. PSpice does model BR, so it's not that. It could also be that I'm looking way out at 1 second, where the loop is steady, and loop "replenishment current" is very small. It's quite possible that, during loop closure, you have some inverse transistor action. It's certainly not there at 1 second... and the spectral analysis does not show it either. However, it is dead clear, there is no AGC action controlling TRANSCONDUCTANCE :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 14 Jun 2010 19:36
On Mon, 14 Jun 2010 16:15:09 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Mon, 14 Jun 2010 16:01:55 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Mon, 14 Jun 2010 15:27:30 -0700 (PDT), dagmargoodboat(a)yahoo.com >>wrote: >> >>>On Jun 14, 4:41�pm, John Larkin >>><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Mon, 14 Jun 2010 12:27:57 -0700 (PDT), dagmargoodb...(a)yahoo.com >>>> wrote: >>>> >>>> >> Q1 doesn't conduct in reverse with your values, but no >>>> >> Schottky... the current is always out of the emitter... though it does >>>> >> get awfully close to zero: -376uA and -12.5mA peak. >>>> >>>> >Huh. �LTSpice says Q1 does conduct in reverse, a nasty little 5mA >>>> >spike's worth. �The schottky feedback prevents that by cutting the >>>> >base bias enough to make sure the collector never gets that low. >>>> >>>> Right; the schottly is a more pure AGC mode. The reverse emitter >>>> conduction depends on the inverse beta of the transistor. If the model >>>> includes inverse beta, when the collector dips down to close to >>>> ground, and the c-b junction forward biases, it essentially flips >>>> ends: collector becomes emitter, emitter becomes collector, emitter >>>> current flows upwards. >>>> >>>> ftp://jjlarkin.lmi.net/LC_YDx.gif >>>> >>>> Either way, the base cap gets discharged. >>> >>>It's interesting that Jim's PSpice doesn't show the same spike. >> >>He blames it on defects in LT Spice. That's crazy. I suspect it's >>actually different transistor models. Note that in my sim, the emitter >>current reverses exactly at the points in time when Vc swings through >>Ve. >> >>John >> > >I didn't say that at all. I said I don't see it in PSpice, AND it's >an advertised feature of LTspice that models are tweaked for speed. > >However I suspect it's that I assigned a resistance to the feedback >winding proportionate to the Q assigned to the primary. You two did >not. > >PSpice does model BR, so it's not that. It could also be that I'm >looking way out at 1 second, where the loop is steady, and loop >"replenishment current" is very small. It's quite possible that, >during loop closure, you have some inverse transistor action. It's >certainly not there at 1 second... and the spectral analysis does not >show it either. > >However, it is dead clear, there is no AGC action controlling >TRANSCONDUCTANCE :-) > > ...Jim Thompson I re-measured: IE is _never_ less than 376uA OUT of the emitter... no reverse transistor action. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy |