Maintaining a Vbe Multiplier's bias value
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From: Spehro Pefhany on 11 Feb 2010 04:54 On Thu, 11 Feb 2010 12:07:49 +1100, the renowned "Phil Allison" <phil_a(a)tpg.com.au> wrote: > >"Tim Williams" >> "George Herold" >>> Say can you make a push-pull stage run class A? (Or is that just a >>> silly idea?) > >** Push-pull class A is the MOST COMMON method use in tube and transistor >audio power amplifiers. > >With transistor amps, it is only necessary to set to standing bias current >to a high value like an amp or two. > >> You can, but it stops being class A for large signal swings or low load >> impedances. > > >** No need for either thing to happen. > >One picks the load to suit the amplifier and the peak ( class A) current is >double the bias setting current. > >Eg: > >With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts >into 8 ohms. > >Class A power is then 64 watts rms. "watts rms"? >The DC rails need to be about +/- 35 volts and the supply current a steady 2 >amps. > > >..... Phil > > > > > Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Bitrex on 11 Feb 2010 07:44 Jim Thompson wrote: > On Wed, 10 Feb 2010 07:55:09 -0700, Jim Thompson > <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >> On Wed, 10 Feb 2010 03:41:23 -0500, Bitrex >> <bitrex(a)de.lete.earthlink.net> wrote: >> >>> Jim Thompson wrote: >>>> On Tue, 09 Feb 2010 14:50:50 -0800, Jon Kirwan >>>> <jonk(a)infinitefactors.org> wrote: >>>> >>>>> On Tue, 09 Feb 2010 15:42:48 -0700, Jim Thompson >>>>> <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>>>> >>>>>> What in the world ?:-) >>>>> View in fixed-spaced font. And it's a rendition of the >>>>> schematic that Bob Monson had posted, earlier, from EDN. He >>>>> wrote, "On a related note, there was an article in a recent >>>>> EDN about a self biasing preamp which was kinda cool. Instead >>>>> of trying to track the difference using diodes or a >>>>> multiplier, it used a couple of transistors and an opamp to >>>>> set the correct values at the bases of the pass transistors. >>>>> It was so novel (at least to me) that I typed it into >>>>> LTSpice." >>>>> >>>>> I merely re-arranged it in LTspice to be a little more to my >>>>> taste and then passed it through a program that generates >>>>> ASCII from that. >>>>> >>>>> Jon >>>> Burr-Brown was famous for using bias compensation like that in the >>>> front ends of some of their operational amplifiers, but I doubt its >>>> efficacy in power output stages. >>>> >>>> The Burr-Brown scheme is similar to a discussion here a few (seven :-) >>>> years ago... >>>> >>>> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf >>>> >>>> ...Jim Thompson >>> >>> Could you give a quick explanation of how the circuit in the link works? >>> I'm having a bit of trouble following what's going on... >> Start with IB coming out (*) of IN+ and IN- of EACH section of the >> LM324. >> >> (*) LM324 has as inputs PNP diff pairs. >> >> Then simply compose loop and node equations. >> >> If any of you are struggling with this kind of analysis, I'll mark it >> up and show how to solve it. >> > > Examine this mark-up... > > http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps_MarkedUp.pdf > > ...Jim Thompson Thanks for the hints, I do need more practice in nodal analysis and I'll make a try at solving it myself. If I understand correctly the purpose of the circuit is to minimize the voltage difference between the two inputs. It's funny that it uses the LM324, as I was playing around with one the other day and for some reason attempted to make a 10Khz square wave oscillator with it, not remembering that its slew rate is pretty limited.
From: Jim Thompson on 11 Feb 2010 09:32 On Wed, 10 Feb 2010 20:55:12 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 10 Feb 2010 20:47:33 -0800 (PST), "miso(a)sushi.com" ><miso(a)sushi.com> wrote: > >>On Feb 10, 8:35�am, George Herold <ggher...(a)gmail.com> wrote: >>> On Feb 9, 4:05�pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: >>> >>> >>> >>> > On Tue, 9 Feb 2010 11:35:36 -0800 (PST), George Herold >>> >>> > <ggher...(a)gmail.com> wrote: >>> > >On Feb 9, 5:39�am, Jon Kirwan <j...(a)infinitefactors.org> wrote: >>> > >> On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold >>> >>> > >> <ggher...(a)gmail.com> wrote: >>> > >> >><snip> >>> > >> >"I'm wondering about additional topology changes to improve >>> > >> >the performance still more." >>> >>> > >> >Hi Jon, �I've been 'sorta' following your thread on s.e.basics. �I >>> > >> >wonder if you abandoned class �A operation too early? �Why not keep >>> > >> >things linear evreywhere and avoid the �dead band�? �So what if you >>> > >> >need a bigger heat sink. �It�s certainly a lot simpler. >>> >>> > >> >George H. >>> >>> > >> Well, George... No, I've not abandoned it. �Actually, it's my >>> > >> hope to wind up building the amplifier and then operating it >>> > >> (by hopefully choosing a design where that is possible) in >>> > >> different modes for the learning experience of it. �I hope >>> > >> that is in the cards. �I really do. >>> >>> > >> But to make a sharp point on it, although it's probably just >>> > >> an extreme case, I remember reading about a 10W amplifier, >>> > >> single channel, dissipating 120W! �Creeps me out. �So I >>> > >> definitely _want_ to consider other classes of operation. And >>> > >> cripes, I want to learn, anyway. �So why not keep my options >>> > >> open? >>> >>> > >> Jon >>> >>> > >" I remember reading about a 10W amplifier, >>> > >> single channel, dissipating 120W! " >>> >>> > >It might have been here, >>> > >http://www.passdiy.com/default.html >>> > >I got to reading about amplifiers on the above site... Do in part to >>> > >your interest. >>> >>> > >George H. >>> >>> > Egads. �Loads of PDF files. �Now I have to create a >>> > directory, download them one by one, and then call them up >>> > with my slow machine to look. �Any particular page or file >>> > where you saw it? �(No, that isn't where I saw the comment.) >>> >>> > But thanks for the link. �I'll add it to those I read, also. >>> >>> > Jon- Hide quoted text - >>> >>> > - Show quoted text - >>> >>> Ohh sorry about that... Don't bother reading them... He (Nelson Pass) >>> has designs for Class A amps using a FET. �(named Zen) �One of his >>> variations (son of Zen?) was made with no NFB and I think wasted your >>> stated 120 Watts of power for 10W into the speaker. �But this seemed >>> pretty pointless to me... some audio guys wanted an amp with out >>> NFB.... Don't aks me why! >>> >>> Say can you make a push-pull stage run class A? �(Or is that just a >>> silly idea?) >>> >>> George H. >> >>Pass designs are interesting reading. Nobody says you have to build >>them. ;-) Nelson is the opposite of Randy Slone, who is Mr. Practical. >>Doug Self is somewhere in the middle. > >How about an amp using complementary voltage regulators as the output >stage, like LM317 and LM337? Nice current and thermal protections. > >John > > And a 2.5V "dead-band", but it _is_ precisely known, and temperature stable. Interesting thought if you have high enough power supplies. ...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 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 11 Feb 2010 09:34 On Thu, 11 Feb 2010 01:07:52 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Wed, 10 Feb 2010 23:24:04 -0700, "bg" <bg(a)nospam.com> >wrote: > >>This is the model that came with circuitmaker - - >>*2N3904 >>*Si 310mW 40V 200mA 300MHz pkg:TO-92B 1,2,3 >>.MODEL 2N3904 NPN(IS=1.4E-14 BF=300 VAF=100 IKF=0.025 ISE=3E-13 >>+ BR=7.5 RC=2.4 CJE=4.5E-12 TF=4E-10 CJC=3.5E-12 TR=2.1E-8 XTB=1.5 >>KF=9E-16 ) > >Thanks!! LTspice's model is: > >.model 2N3904 NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 >+ BR=4 CJC=4E-12 CJE=8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9 >+ TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m >+ mfg=Philips) > >Note that it does NOT include a figure for ISE, which your >model does. LTspice is setting it to zero, instead. Your >model may very well be right. If so, it affects the beta >value (degrades it due to some recombination cause) and that >affects my calculations. In fact, that value of ISE cuts it >down to around 100, or so. Which about doubles the >contribution to the final term in the equation I gave where a >term is divided by beta, so that adds back a few tenths of a >volt to the prediction! Great! > >Now, with your model, LTspice shows the same results you got. > >>I don't use LT so I'm not sure if the commented (*) text needs to be >>removed. > >No, that is pretty much the universal spice comment >character. It's fine. > >>These spice model files might be of interest but possibly obsolete - google >>for them >>OnSemiconductorAllModels.zip >>OnSemiconductorDiscreteModels.zip >>OnSemiconductorIntegratedFunctionsModels.zip > >I already had the first one laying about. I downloaded the >second one just to have it and extracted 2N3904.LIB from it, >using that model, as well. It works much more like the >LTspice one and perhaps even more different than the one you >were using. However, it does produce about the same _beta_ >as your model does. > >Very interesting. I need to consider the reasons carefully. > >Thanks, >Jon > >P.S. Just for completeness, the OnSemi model is this: > >.MODEL 2N3904 NPN( >+IS=1.26532e-10 BF=206.302 NF=1.5 VAF=1000 >+IKF=0.0272221 ISE=2.30771e-09 NE=3.31052 BR=20.6302 >+NR=2.89609 VAR=9.39809 IKR=0.272221 ISC=2.30771e-09 >+NC=1.9876 RB=5.8376 IRB=50.3624 RBM=0.634251 >+RE=0.0001 RC=2.65711 XTB=0.1 XTI=1 >+EG=1.05 CJE=4.64214e-12 VJE=0.4 MJE=0.256227 >+TF=4.19578e-10 XTF=0.906167 VTF=8.75418 ITF=0.0105823 >+CJC=3.76961e-12 VJC=0.4 MJC=0.238109 XCJC=0.8 >+FC=0.512134 CJS=0 VJS=0.75 MJS=0.5 >+TR=6.82023e-08 PTF=0 KF=0 AF=1) Go with the OnSemi model. ...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 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 11 Feb 2010 09:41
On Thu, 11 Feb 2010 07:44:42 -0500, Bitrex <bitrex(a)de.lete.earthlink.net> wrote: >Jim Thompson wrote: >> On Wed, 10 Feb 2010 07:55:09 -0700, Jim Thompson >> <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>> On Wed, 10 Feb 2010 03:41:23 -0500, Bitrex >>> <bitrex(a)de.lete.earthlink.net> wrote: >>> >>>> Jim Thompson wrote: >>>>> On Tue, 09 Feb 2010 14:50:50 -0800, Jon Kirwan >>>>> <jonk(a)infinitefactors.org> wrote: >>>>> >>>>>> On Tue, 09 Feb 2010 15:42:48 -0700, Jim Thompson >>>>>> <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>>>>> >>>>>>> What in the world ?:-) >>>>>> View in fixed-spaced font. And it's a rendition of the >>>>>> schematic that Bob Monson had posted, earlier, from EDN. He >>>>>> wrote, "On a related note, there was an article in a recent >>>>>> EDN about a self biasing preamp which was kinda cool. Instead >>>>>> of trying to track the difference using diodes or a >>>>>> multiplier, it used a couple of transistors and an opamp to >>>>>> set the correct values at the bases of the pass transistors. >>>>>> It was so novel (at least to me) that I typed it into >>>>>> LTSpice." >>>>>> >>>>>> I merely re-arranged it in LTspice to be a little more to my >>>>>> taste and then passed it through a program that generates >>>>>> ASCII from that. >>>>>> >>>>>> Jon >>>>> Burr-Brown was famous for using bias compensation like that in the >>>>> front ends of some of their operational amplifiers, but I doubt its >>>>> efficacy in power output stages. >>>>> >>>>> The Burr-Brown scheme is similar to a discussion here a few (seven :-) >>>>> years ago... >>>>> >>>>> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf >>>>> >>>>> ...Jim Thompson >>>> >>>> Could you give a quick explanation of how the circuit in the link works? >>>> I'm having a bit of trouble following what's going on... >>> Start with IB coming out (*) of IN+ and IN- of EACH section of the >>> LM324. >>> >>> (*) LM324 has as inputs PNP diff pairs. >>> >>> Then simply compose loop and node equations. >>> >>> If any of you are struggling with this kind of analysis, I'll mark it >>> up and show how to solve it. >>> >> >> Examine this mark-up... >> >> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps_MarkedUp.pdf >> >> ...Jim Thompson > > >Thanks for the hints, I do need more practice in nodal analysis and I'll >make a try at solving it myself. If I understand correctly the purpose >of the circuit is to minimize the voltage difference between the two inputs. No. The object of the _overall_ circuit is to amplify the input signal (bottom graph of Page 2). The differential inputs _are_ servo'd to zero when the amplifier is in its linear region... that may be what you are thinking of. The second OpAmp and resistors are to null out at least a portion of the bias current needs (top graph) at the _circuit_ input... result is middle graph. > >It's funny that it uses the LM324, as I was playing around with one the >other day and for some reason attempted to make a 10Khz square wave >oscillator with it, not remembering that its slew rate is pretty limited. In the '70's LM324 and LM339 were my jelly-bean parts, for general purpose stuff, but TL084 OpAmp for fancy work ;-) ...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 | I love to cook with wine. Sometimes I even put it in the food. |