Maintaining a Vbe Multiplier's bias value
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From: Jim Thompson on 9 Feb 2010 18:08 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 -- | 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: Jon Kirwan on 9 Feb 2010 18:17 On Tue, 09 Feb 2010 16:08:22 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> 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. I am still struggling to understand it. There are very obvious parts that I completely understand. For example, the divider for a "center" voltage followed by a unity gain buffer for drive current compliance. I would guess that the gain is determined by the NFB resistor divided by the input impedance, which is mostly R4 in this case... so 10. I see an opamp looking like an integrator, but I'm frankly unfamiliar with the 4-BJT arrangement structure and need to think about that one. >The Burr-Brown scheme is similar to a discussion here a few (seven :-) >years ago... > > http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf I'll download it now and look when I get a moment to engage a little thought. Thanks, Jon
From: Jon Kirwan on 9 Feb 2010 18:20 On Tue, 09 Feb 2010 15:17:24 -0800, I wrote: ><snip> >I'm frankly >unfamiliar with the 4-BJT arrangement structure and need to >think about that one. ><snip> I do "see" the emitter followers of Q3/Q5 on the schematic, of course. It's the R1/Q1/Q4/C1 parts that I'm assuming is the bias compensation and is the part I don't follow. The C5 looks like a very lightly applied integrator cap, which I take is needed to avoid oscillation. And that's about where I'm stuck. Jon
From: Jim Thompson on 9 Feb 2010 18:21 On Tue, 09 Feb 2010 15:17:24 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 09 Feb 2010 16:08:22 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> 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. > >I am still struggling to understand it. There are very >obvious parts that I completely understand. For example, the >divider for a "center" voltage followed by a unity gain >buffer for drive current compliance. I would guess that the >gain is determined by the NFB resistor divided by the input >impedance, which is mostly R4 in this case... so 10. I see >an opamp looking like an integrator, but I'm frankly >unfamiliar with the 4-BJT arrangement structure and need to >think about that one. > >>The Burr-Brown scheme is similar to a discussion here a few (seven :-) >>years ago... >> >> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf > >I'll download it now and look when I get a moment to engage a >little thought. > >Thanks, >Jon Study up on writing loop and nodal equations and either solving by simultaneous equations or matrix manipulation. ...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 9 Feb 2010 18:24
On Tue, 09 Feb 2010 14:30:54 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 9 Feb 2010 15:48:42 -0600, "Tim Williams" ><tmoranwms(a)charter.net> wrote: > >>"Bob Monsen" <rcmonsen(a)gmail.com> wrote in message >>news:1265746051.657782(a)sj-nntpcache-3.cisco.com... >>> 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. Here it is: >> >>Could you take a screenshot of the schematic? > >I'll include an ASCII version here: > >>: R2 >>: +V = 12V ,------/\/\---------------------, >>: | 1k | >>: | +V | >>: | | | >>: | \ | >>: | / R1 | >>: | \ 1k5 | >>: | / | >>: | | +V | >>: C2 | ,----+ | | >>: || 10uF R4 | | | 2N3904| | >>: ,------||------/\/\---------+ | | | | >>: I| || 100 | | Q4 e>| |/c Q3 | >>: N| | | |-------| | >>: | | | c/| |>e | >>: | | C1 --- | | | >>: --- | 10uF--- |2N3906 | | >>: - V2 | | | | | >>: --- SINE(0 .2 1k) | | | | | C3 >>: - | C5 | | | | || 470uF >>: | | || 10p| | +----+-||----,O >>: | +V +---||----+ | | || |U >>: | | | || | | | |T >>: gnd | | | | | \ >>: | | | |2N3904 | / R5 >>: \ ,-------, | | | | \ 8 >>: / R3 | | | +V | Q1 c\| |<e Q5 / >>: \ 1k | +V | | | 2N| |-------| 2N3906 | >>: / | | | | |\| | e<| |\c | >>: | | |\| | '-|-\ | | | | >>: | '--|-\ | | >----+----' | gnd >>: | | >-+-----|+/ | >>: +-------|+/ |/| LT6234 | >>: ,-----+ |/| LT6234 | gnd >>: | | | gnd >>: --- C4 \ gnd >>: --- 1uF/ R9 >>: | \ 1k >>: | / >>: | | >>: gnd gnd > >(This was auto-generated from my LTspice to ASCII program.) > >Jon Did something get lost in the ASCII? Otherwise collector-to-collector as in Q1-Q4 is a no-no... one of those devices will saturate. ...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. |