Op Amp Calculations
in [Design]
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From: Jim Thompson on 6 Oct 2005 10:35 On Thu, 06 Oct 2005 17:45:05 +1300, Terry Given <my_name(a)ieee.org> wrote: >Jim Thompson wrote: >> On Wed, 05 Oct 2005 13:03:55 GMT, Fred Bloggs <nospam(a)nospam.com> >> wrote: >>>Fred Bartoli wrote: >>>>"Terry Given" <my_name(a)ieee.org> a ?crit dans le message de >>>>news:1128463369.256572(a)ftpsrv1... >>>>>Fred Bloggs wrote: >>>>>>Terry Given wrote: >>>>>>>What about the typo? the 2nd transistor shorts out the +5V supply.... >>>>>shouldnt there be something to limit current though? +5V...Vce...Vbe >>>>>with nary a resistor in sight. ultimately the base current could be as >>>>>high as the opamp output current (assuming negligible contribution from >>>>>the summing junction).... [snip] >>>Not quite, essentially all the incremental base drive voltage is >>>developed across hie of the first transistor. Your reasoning is wrong >>>and your estimate is high. "hie"... what BS! Fred must be of the "Kevin School" of transistor operation ;-) >> >> >> And it STILL will go up in flames during power-up. Sheeesh! Such >> DESIGN ;-) >> >> [snip] >> >> ...Jim Thompson > >Hi Jim, > >can you please elaborate on why? I presume because of the 2nd transistor.... > >Cheers >Terry No limit on IB of 1st transistor except output capability of OpAmp IC of 1st transistor IS limited somewhat by FB resistors, but substantial IB is introduced into 2nd transistor Absolutely nothing to limit IC of 2nd transistor except current crowding My best guess is that all three transistors will flame or be seriously damaged when Murphy chooses ;-) (And the bandwidth will be nothing like claimed.) What was the original intent (OP)? I didn't get in on this thread at the beginning. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Fred Bloggs on 6 Oct 2005 11:39 Jim Thompson wrote: > On Thu, 06 Oct 2005 17:45:05 +1300, Terry Given <my_name(a)ieee.org> > wrote: > > >>Jim Thompson wrote: >> >>>On Wed, 05 Oct 2005 13:03:55 GMT, Fred Bloggs <nospam(a)nospam.com> >>>wrote: >>> >>>>Fred Bartoli wrote: >>>> >>>>>"Terry Given" <my_name(a)ieee.org> a ?crit dans le message de >>>>>news:1128463369.256572(a)ftpsrv1... >>>>> >>>>>>Fred Bloggs wrote: >>>>>> >>>>>>>Terry Given wrote: >>>>>>> >>>>>>>>What about the typo? the 2nd transistor shorts out the +5V supply.... >>>>>>> >>>>>>shouldnt there be something to limit current though? +5V...Vce...Vbe >>>>>>with nary a resistor in sight. ultimately the base current could be as >>>>>>high as the opamp output current (assuming negligible contribution from >>>>>>the summing junction).... >>>>> > [snip] > >>>>Not quite, essentially all the incremental base drive voltage is >>>>developed across hie of the first transistor. Your reasoning is wrong >>>>and your estimate is high. >>> > > "hie"... what BS! Fred must be of the "Kevin School" of transistor > operation ;-) > > >>> >>>And it STILL will go up in flames during power-up. Sheeesh! Such >>>DESIGN ;-) >>> >>>[snip] >>> >>> ...Jim Thompson >> >>Hi Jim, >> >>can you please elaborate on why? I presume because of the 2nd transistor.... >> >>Cheers >>Terry > > > No limit on IB of 1st transistor except output capability of OpAmp > > IC of 1st transistor IS limited somewhat by FB resistors, but > substantial IB is introduced into 2nd transistor > > Absolutely nothing to limit IC of 2nd transistor except current > crowding > > My best guess is that all three transistors will flame or be seriously > damaged when Murphy chooses ;-) > > (And the bandwidth will be nothing like claimed.) > > What was the original intent (OP)? I didn't get in on this thread at > the beginning. > > ...Jim Thompson This is not rocket science- we already said that the second transistor C should be tied to the CE collector. If you're worried by the OA dumping excessive current into the base then put a 560 in series with its output.
From: Jim Thompson on 6 Oct 2005 11:56 On Thu, 06 Oct 2005 15:39:08 GMT, Fred Bloggs <nospam(a)nospam.com> wrote: > > >Jim Thompson wrote: >> On Thu, 06 Oct 2005 17:45:05 +1300, Terry Given <my_name(a)ieee.org> >> wrote: >> >> >>>Jim Thompson wrote: >>> >>>>On Wed, 05 Oct 2005 13:03:55 GMT, Fred Bloggs <nospam(a)nospam.com> >>>>wrote: >>>> >>>>>Fred Bartoli wrote: >>>>> >>>>>>"Terry Given" <my_name(a)ieee.org> a ?crit dans le message de >>>>>>news:1128463369.256572(a)ftpsrv1... >>>>>> >>>>>>>Fred Bloggs wrote: >>>>>>> >>>>>>>>Terry Given wrote: >>>>>>>> >>>>>>>>>What about the typo? the 2nd transistor shorts out the +5V supply.... >>>>>>>> >>>>>>>shouldnt there be something to limit current though? +5V...Vce...Vbe >>>>>>>with nary a resistor in sight. ultimately the base current could be as >>>>>>>high as the opamp output current (assuming negligible contribution from >>>>>>>the summing junction).... >>>>>> >> [snip] >> >>>>>Not quite, essentially all the incremental base drive voltage is >>>>>developed across hie of the first transistor. Your reasoning is wrong >>>>>and your estimate is high. >>>> >> >> "hie"... what BS! Fred must be of the "Kevin School" of transistor >> operation ;-) >> >> >>>> >>>>And it STILL will go up in flames during power-up. Sheeesh! Such >>>>DESIGN ;-) >>>> >>>>[snip] >>>> >>>> ...Jim Thompson >>> >>>Hi Jim, >>> >>>can you please elaborate on why? I presume because of the 2nd transistor.... >>> >>>Cheers >>>Terry >> >> >> No limit on IB of 1st transistor except output capability of OpAmp >> >> IC of 1st transistor IS limited somewhat by FB resistors, but >> substantial IB is introduced into 2nd transistor >> >> Absolutely nothing to limit IC of 2nd transistor except current >> crowding >> >> My best guess is that all three transistors will flame or be seriously >> damaged when Murphy chooses ;-) >> >> (And the bandwidth will be nothing like claimed.) >> >> What was the original intent (OP)? I didn't get in on this thread at >> the beginning. >> >> ...Jim Thompson > >This is not rocket science- we already said that the second transistor C >should be tied to the CE collector. If you're worried by the OA dumping >excessive current into the base then put a 560 in series with its output. That's still insufficient. Good engineering practice would limit base drive from the OpAmp AND also the collector current of the 2nd transistor. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Rich Grise on 6 Oct 2005 14:58 On Thu, 06 Oct 2005 01:12:02 -0700, Winfield Hill wrote: > John Woodgate wrote... >> Jim Thompson wrote ... >>> Winfield Hill wrote: >>>> Jim Thompson wrote... >>>>> >>>>> ... Sheeesh! ... >>>> >>>> Hey, that's my word! >>> >>> Will you share? It's a GOOD word! >> >> Offer him a half-share in 'Snooooort!' > > Plus a half share of Bwaahahahaha! Dibs on a half share of ROFLMAOPIMP! ;-)
From: Winfield Hill on 6 Oct 2005 15:16
Rich Grise wrote... > > Dibs on a half share of ROFLMAOPIMP! PIMP? -- Thanks, - Win |