Forums

Current source circuit fail.

Started by George Herold November 14, 2017
I made this nice VCCS from a differential amp (Ina134), 
resistor and opamp (opa134) feedback.  It was good to 
40 or 50 mA.  Bad ass-ci.

gnd---RR---+--RR-----+
           ||\       |
           +|-\      |
    Ina134  |  >-(A)-+--Rset-+---output to gnd.
           +|+/              |
           ||/        /|     |  
  Vin--RR--+----RR---< +-----+
                      \| 
                    Buffer opa134    
It's in AoE3.  
Rset sets the current/voltage ratio and the other R's are 
in the ina134.  
I was thinking about more current and added a power opamp 
in the (A) spot.  Didn't work, the power opamp OPA544 railed 
at the wrong side.. or something.  The ina134 spec sheet 
shows a fast buffer BUF634? Would that be better.  
(maybe I need to limit the voltage to the ina134?)

This bipolar current source has nice return to ground.  
It's got an output impedance of Rset. (less nice.) 

George H.       
On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>I made this nice VCCS from a differential amp (Ina134), >resistor and opamp (opa134) feedback. It was good to >40 or 50 mA. Bad ass-ci. > >gnd---RR---+--RR-----+ > ||\ | > +|-\ | > Ina134 | >-(A)-+--Rset-+---output to gnd. > +|+/ | > ||/ /| | > Vin--RR--+----RR---< +-----+ > \| > Buffer opa134 >It's in AoE3. >Rset sets the current/voltage ratio and the other R's are >in the ina134. >I was thinking about more current and added a power opamp >in the (A) spot. Didn't work, the power opamp OPA544 railed >at the wrong side.. or something. The ina134 spec sheet >shows a fast buffer BUF634? Would that be better. >(maybe I need to limit the voltage to the ina134?) > >This bipolar current source has nice return to ground. >It's got an output impedance of Rset. (less nice.) > >George H.
Zout should be theoretically infinite! It's a current source. The '544 should have worked, with maybe a little frequency compensation. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote:
> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >I made this nice VCCS from a differential amp (Ina134), > >resistor and opamp (opa134) feedback. It was good to > >40 or 50 mA. Bad ass-ci. > > > >gnd---RR---+--RR-----+ > > ||\ | > > +|-\ | > > Ina134 | >-(A)-+--Rset-+---output to gnd. > > +|+/ | > > ||/ /| | > > Vin--RR--+----RR---< +-----+ > > \| > > Buffer opa134 > >It's in AoE3. > >Rset sets the current/voltage ratio and the other R's are > >in the ina134. > >I was thinking about more current and added a power opamp > >in the (A) spot. Didn't work, the power opamp OPA544 railed > >at the wrong side.. or something. The ina134 spec sheet > >shows a fast buffer BUF634? Would that be better. > >(maybe I need to limit the voltage to the ina134?) > > > >This bipolar current source has nice return to ground. > >It's got an output impedance of Rset. (less nice.) > > > >George H. > > Zout should be theoretically infinite! It's a current source.
Right, head slap, Vout compliance is limited by opamp supply and Rset. (It wasn't very happy driving a capacitor... it'd go to one one rail or the other.)
> > The '544 should have worked, with maybe a little frequency > compensation.
I tried some C (100 pF) from feed back (pin 5) to ground, I probably put the C in the wrong branch. The input and output of the 544 were backwards, not sure why. (I had no R's in the 544 circuit, just a 5 pin to-220 in space, 0.1uF C's on the power leads... word on the street is that the second amp needs to be faster. (I've never quite understood that.) George H.
> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: >> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold >> <gherold@teachspin.com> wrote: >> >> >I made this nice VCCS from a differential amp (Ina134), >> >resistor and opamp (opa134) feedback. It was good to >> >40 or 50 mA. Bad ass-ci. >> > >> >gnd---RR---+--RR-----+ >> > ||\ | >> > +|-\ | >> > Ina134 | >-(A)-+--Rset-+---output to gnd. >> > +|+/ | >> > ||/ /| | >> > Vin--RR--+----RR---< +-----+ >> > \| >> > Buffer opa134 >> >It's in AoE3. >> >Rset sets the current/voltage ratio and the other R's are >> >in the ina134. >> >I was thinking about more current and added a power opamp >> >in the (A) spot. Didn't work, the power opamp OPA544 railed >> >at the wrong side.. or something. The ina134 spec sheet >> >shows a fast buffer BUF634? Would that be better. >> >(maybe I need to limit the voltage to the ina134?) >> > >> >This bipolar current source has nice return to ground. >> >It's got an output impedance of Rset. (less nice.) >> > >> >George H. >> >> Zout should be theoretically infinite! It's a current source. >Right, head slap, Vout compliance is limited by opamp supply and >Rset. >(It wasn't very happy driving a capacitor... it'd go to one >one rail or the other.)
Wait...wait..... it's a current source!
>> >> The '544 should have worked, with maybe a little frequency >> compensation. >I tried some C (100 pF) from feed back (pin 5) to ground, >I probably put the C in the wrong branch. > >The input and output of the 544 were backwards, not sure why. >(I had no R's in the 544 circuit, just a 5 pin to-220 in space, >0.1uF C's on the power leads... word on the street is that the >second amp needs to be faster. (I've never quite understood that.)
If one is fast and one is slow, it's likely to be stable. Either one. If they are near the same speed, some theory may be required. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Tuesday, November 14, 2017 at 10:23:13 PM UTC-5, John Larkin wrote:
> On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: > >> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold > >> <gherold@teachspin.com> wrote: > >> > >> >I made this nice VCCS from a differential amp (Ina134), > >> >resistor and opamp (opa134) feedback. It was good to > >> >40 or 50 mA. Bad ass-ci. > >> > > >> >gnd---RR---+--RR-----+ > >> > ||\ | > >> > +|-\ | > >> > Ina134 | >-(A)-+--Rset-+---output to gnd. > >> > +|+/ | > >> > ||/ /| | > >> > Vin--RR--+----RR---< +-----+ > >> > \| > >> > Buffer opa134 > >> >It's in AoE3. > >> >Rset sets the current/voltage ratio and the other R's are > >> >in the ina134. > >> >I was thinking about more current and added a power opamp > >> >in the (A) spot. Didn't work, the power opamp OPA544 railed > >> >at the wrong side.. or something. The ina134 spec sheet > >> >shows a fast buffer BUF634? Would that be better. > >> >(maybe I need to limit the voltage to the ina134?) > >> > > >> >This bipolar current source has nice return to ground. > >> >It's got an output impedance of Rset. (less nice.) > >> > > >> >George H. > >> > >> Zout should be theoretically infinite! It's a current source. > >Right, head slap, Vout compliance is limited by opamp supply and > >Rset. > >(It wasn't very happy driving a capacitor... it'd go to one > >one rail or the other.) > > Wait...wait..... it's a current source! >
Grin. Well I was hitting it with square wave and getting a triangle out. (but it drifted up or down.. as I tweaked the dc offset. Right that's expected.) Hey hitting it with a triangle wave gives an output that looks a little like a sine wave but isn't. (A parabola?)
> > > >> > >> The '544 should have worked, with maybe a little frequency > >> compensation. > >I tried some C (100 pF) from feed back (pin 5) to ground, > >I probably put the C in the wrong branch. > > > >The input and output of the 544 were backwards, not sure why. > >(I had no R's in the 544 circuit, just a 5 pin to-220 in space, > >0.1uF C's on the power leads... word on the street is that the > >second amp needs to be faster. (I've never quite understood that.) > > If one is fast and one is slow, it's likely to be stable. Either one. > If they are near the same speed, some theory may be required.
OK. Or maybe I'll try the BUF634 from the spec. sheet. ~100mA would be enough... +/-1 amp would be cool though. George H.
> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
On Wed, 15 Nov 2017 06:37:07 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, November 14, 2017 at 10:23:13 PM UTC-5, John Larkin wrote: >> On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold >> <gherold@teachspin.com> wrote: >> >> >On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: >> >> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold >> >> <gherold@teachspin.com> wrote: >> >> >> >> >I made this nice VCCS from a differential amp (Ina134), >> >> >resistor and opamp (opa134) feedback. It was good to >> >> >40 or 50 mA. Bad ass-ci. >> >> > >> >> >gnd---RR---+--RR-----+ >> >> > ||\ | >> >> > +|-\ | >> >> > Ina134 | >-(A)-+--Rset-+---output to gnd. >> >> > +|+/ | >> >> > ||/ /| | >> >> > Vin--RR--+----RR---< +-----+ >> >> > \| >> >> > Buffer opa134 >> >> >It's in AoE3. >> >> >Rset sets the current/voltage ratio and the other R's are >> >> >in the ina134. >> >> >I was thinking about more current and added a power opamp >> >> >in the (A) spot. Didn't work, the power opamp OPA544 railed >> >> >at the wrong side.. or something. The ina134 spec sheet >> >> >shows a fast buffer BUF634? Would that be better. >> >> >(maybe I need to limit the voltage to the ina134?) >> >> > >> >> >This bipolar current source has nice return to ground. >> >> >It's got an output impedance of Rset. (less nice.) >> >> > >> >> >George H. >> >> >> >> Zout should be theoretically infinite! It's a current source. >> >Right, head slap, Vout compliance is limited by opamp supply and >> >Rset. >> >(It wasn't very happy driving a capacitor... it'd go to one >> >one rail or the other.) >> >> Wait...wait..... it's a current source! >> >Grin. Well I was hitting it with square wave and getting a triangle out. >(but it drifted up or down.. as I tweaked the dc offset. Right that's expected.) Hey hitting it with a triangle wave gives an output that looks >a little like a sine wave but isn't. (A parabola?) > >> >> >> >> >> >> The '544 should have worked, with maybe a little frequency >> >> compensation. >> >I tried some C (100 pF) from feed back (pin 5) to ground, >> >I probably put the C in the wrong branch. >> > >> >The input and output of the 544 were backwards, not sure why. >> >(I had no R's in the 544 circuit, just a 5 pin to-220 in space, >> >0.1uF C's on the power leads... word on the street is that the >> >second amp needs to be faster. (I've never quite understood that.) >> >> If one is fast and one is slow, it's likely to be stable. Either one. >> If they are near the same speed, some theory may be required. > >OK. Or maybe I'll try the BUF634 from the spec. sheet. ~100mA would be enough... +/-1 amp would be cool though. > >George H.
You could use a discrete follower pair, nfet+pfet or NPN+PNP. TCA0372 is a dual 1-amp opamp for about 50 cents. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Wednesday, November 15, 2017 at 10:32:55 AM UTC-5, John Larkin wrote:
> On Wed, 15 Nov 2017 06:37:07 -0800 (PST), George Herold > <gherold@teachspin.com> wrote: > > >On Tuesday, November 14, 2017 at 10:23:13 PM UTC-5, John Larkin wrote: > >> On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold > >> <gherold@teachspin.com> wrote: > >> > >> >On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: > >> >> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold > >> >> <gherold@teachspin.com> wrote: > >> >> > >> >> >I made this nice VCCS from a differential amp (Ina134), > >> >> >resistor and opamp (opa134) feedback. It was good to > >> >> >40 or 50 mA. Bad ass-ci. > >> >> > > >> >> >gnd---RR---+--RR-----+ > >> >> > ||\ | > >> >> > +|-\ | > >> >> > Ina134 | >-(A)-+--Rset-+---output to gnd. > >> >> > +|+/ | > >> >> > ||/ /| | > >> >> > Vin--RR--+----RR---< +-----+ > >> >> > \| > >> >> > Buffer opa134 > >> >> >It's in AoE3. > >> >> >Rset sets the current/voltage ratio and the other R's are > >> >> >in the ina134. > >> >> >I was thinking about more current and added a power opamp > >> >> >in the (A) spot. Didn't work, the power opamp OPA544 railed > >> >> >at the wrong side.. or something. The ina134 spec sheet > >> >> >shows a fast buffer BUF634? Would that be better. > >> >> >(maybe I need to limit the voltage to the ina134?) > >> >> > > >> >> >This bipolar current source has nice return to ground. > >> >> >It's got an output impedance of Rset. (less nice.) > >> >> > > >> >> >George H. > >> >> > >> >> Zout should be theoretically infinite! It's a current source. > >> >Right, head slap, Vout compliance is limited by opamp supply and > >> >Rset. > >> >(It wasn't very happy driving a capacitor... it'd go to one > >> >one rail or the other.) > >> > >> Wait...wait..... it's a current source! > >> > >Grin. Well I was hitting it with square wave and getting a triangle out. > >(but it drifted up or down.. as I tweaked the dc offset. Right that's expected.) Hey hitting it with a triangle wave gives an output that looks > >a little like a sine wave but isn't. (A parabola?) > > > >> > >> > >> >> > >> >> The '544 should have worked, with maybe a little frequency > >> >> compensation. > >> >I tried some C (100 pF) from feed back (pin 5) to ground, > >> >I probably put the C in the wrong branch. > >> > > >> >The input and output of the 544 were backwards, not sure why. > >> >(I had no R's in the 544 circuit, just a 5 pin to-220 in space, > >> >0.1uF C's on the power leads... word on the street is that the > >> >second amp needs to be faster. (I've never quite understood that.) > >> > >> If one is fast and one is slow, it's likely to be stable. Either one. > >> If they are near the same speed, some theory may be required. > > > >OK. Or maybe I'll try the BUF634 from the spec. sheet. ~100mA would be enough... +/-1 amp would be cool though. > > > >George H. > > You could use a discrete follower pair, nfet+pfet or NPN+PNP. > > TCA0372 is a dual 1-amp opamp for about 50 cents.
Hey it worked OK when I had the power opamp inside only one loop. gnd---RR---+--RR--+ ||\ | +|-\ | Ina134 | >--+-(A)-Rset-+---output to gnd. +|+/ | ||/ /| | Vin--RR--+----RR---< +-----+ \| Buffer opa134 I moved my (A) power opamp. Though the opa544 doesn't seem to like having it's output driven near the negative rail. https://www.dropbox.com/s/c4qmibb7gcfv0mb/VCCS.BMP?dl=0 Chan. 1 is voltage across 200 ohm resistor, chan 2. is input. (100 ohm 'set' resistor. 1 V_in= 10mA George H.
> > > -- > > John Larkin Highland Technology, Inc > > lunatic fringe electronics
George Herold wrote on 11/15/2017 11:15 AM:
> On Wednesday, November 15, 2017 at 10:32:55 AM UTC-5, John Larkin wrote: >> On Wed, 15 Nov 2017 06:37:07 -0800 (PST), George Herold >> <gherold@teachspin.com> wrote: >> >>> On Tuesday, November 14, 2017 at 10:23:13 PM UTC-5, John Larkin wrote: >>>> On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold >>>> <gherold@teachspin.com> wrote: >>>> >>>>> On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: >>>>>> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold >>>>>> <gherold@teachspin.com> wrote: >>>>>> >>>>>>> I made this nice VCCS from a differential amp (Ina134), >>>>>>> resistor and opamp (opa134) feedback. It was good to >>>>>>> 40 or 50 mA. Bad ass-ci. >>>>>>> >>>>>>> gnd---RR---+--RR-----+ >>>>>>> ||\ | >>>>>>> +|-\ | >>>>>>> Ina134 | >-(A)-+--Rset-+---output to gnd. >>>>>>> +|+/ | >>>>>>> ||/ /| | >>>>>>> Vin--RR--+----RR---< +-----+ >>>>>>> \| >>>>>>> Buffer opa134 >>>>>>> It's in AoE3. >>>>>>> Rset sets the current/voltage ratio and the other R's are >>>>>>> in the ina134. >>>>>>> I was thinking about more current and added a power opamp >>>>>>> in the (A) spot. Didn't work, the power opamp OPA544 railed >>>>>>> at the wrong side.. or something. The ina134 spec sheet >>>>>>> shows a fast buffer BUF634? Would that be better. >>>>>>> (maybe I need to limit the voltage to the ina134?) >>>>>>> >>>>>>> This bipolar current source has nice return to ground. >>>>>>> It's got an output impedance of Rset. (less nice.) >>>>>>> >>>>>>> George H. >>>>>> >>>>>> Zout should be theoretically infinite! It's a current source. >>>>> Right, head slap, Vout compliance is limited by opamp supply and >>>>> Rset. >>>>> (It wasn't very happy driving a capacitor... it'd go to one >>>>> one rail or the other.) >>>> >>>> Wait...wait..... it's a current source! >>>> >>> Grin. Well I was hitting it with square wave and getting a triangle out. >>> (but it drifted up or down.. as I tweaked the dc offset. Right that's expected.) Hey hitting it with a triangle wave gives an output that looks >>> a little like a sine wave but isn't. (A parabola?) >>> >>>> >>>> >>>>>> >>>>>> The '544 should have worked, with maybe a little frequency >>>>>> compensation. >>>>> I tried some C (100 pF) from feed back (pin 5) to ground, >>>>> I probably put the C in the wrong branch. >>>>> >>>>> The input and output of the 544 were backwards, not sure why. >>>>> (I had no R's in the 544 circuit, just a 5 pin to-220 in space, >>>>> 0.1uF C's on the power leads... word on the street is that the >>>>> second amp needs to be faster. (I've never quite understood that.) >>>> >>>> If one is fast and one is slow, it's likely to be stable. Either one. >>>> If they are near the same speed, some theory may be required. >>> >>> OK. Or maybe I'll try the BUF634 from the spec. sheet. ~100mA would be enough... +/-1 amp would be cool though. >>> >>> George H. >> >> You could use a discrete follower pair, nfet+pfet or NPN+PNP. >> >> TCA0372 is a dual 1-amp opamp for about 50 cents. > > Hey it worked OK when I had the power opamp inside only one loop. > > gnd---RR---+--RR--+ > ||\ | > +|-\ | > Ina134 | >--+-(A)-Rset-+---output to gnd. > +|+/ | > ||/ /| | > Vin--RR--+----RR---< +-----+ > \| > Buffer opa134 > > I moved my (A) power opamp. Though the opa544 doesn't seem to like > having it's output driven near the negative rail. > https://www.dropbox.com/s/c4qmibb7gcfv0mb/VCCS.BMP?dl=0 > > Chan. 1 is voltage across 200 ohm resistor, chan 2. is input. > (100 ohm 'set' resistor. 1 V_in= 10mA
What is your load? Is the output connected to ground? If you leave the output open the current has nowhere to go and the circuit won't work correctly maxing the output because of the positive feedback. If you ground the output the output of the buffer amp is ground and the op amp is a unity gain buffer into Rset (assuming RR is the same value everywhere). What are your power rails? Your scope traces pretty clearly show the amp is being maxed out. Is your load 200 ohms? Then with -5 volts in, the current in the load is 50 mA and the voltage on a 200 ohm load is 10 volts. The voltage at (A) is -15 volts which likely is your rail. As the input exceeds -5 volts input the output can't drive harder and a large difference develops across the inputs of the op amp. I guess you are just over-driving the circuit. -- Rick C Viewed the eclipse at Wintercrest Farms, on the centerline of totality since 1998
On Wed, 15 Nov 2017 08:15:02 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

[snip]
> >gnd---RR---+--RR--+ > ||\ | > +|-\ | > Ina134 | >--+-(A)-Rset-+---output to gnd. > +|+/ | > ||/ /| | > Vin--RR--+----RR---< +-----+ > \| > Buffer opa134 > >I moved my (A) power opamp. Though the opa544 doesn't seem to like >having it's output driven near the negative rail. >https://www.dropbox.com/s/c4qmibb7gcfv0mb/VCCS.BMP?dl=0 > >Chan. 1 is voltage across 200 ohm resistor, chan 2. is input. >(100 ohm 'set' resistor. 1 V_in= 10mA > > >George H. >>
Why all that complexity? Plus the load behavior is inside your loop. A dual R-R OpAmp plus either a PMOS or PNP for the high current will easily solve your problem... and be stable. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | To save time, Let's just assume I know everything.
On Wednesday, November 15, 2017 at 12:47:10 PM UTC-5, rickman wrote:
> George Herold wrote on 11/15/2017 11:15 AM: > > On Wednesday, November 15, 2017 at 10:32:55 AM UTC-5, John Larkin wrote: > >> On Wed, 15 Nov 2017 06:37:07 -0800 (PST), George Herold > >> <gherold@teachspin.com> wrote: > >> > >>> On Tuesday, November 14, 2017 at 10:23:13 PM UTC-5, John Larkin wrote: > >>>> On Tue, 14 Nov 2017 17:49:53 -0800 (PST), George Herold > >>>> <gherold@teachspin.com> wrote: > >>>> > >>>>> On Tuesday, November 14, 2017 at 8:13:41 PM UTC-5, John Larkin wrote: > >>>>>> On Tue, 14 Nov 2017 16:43:10 -0800 (PST), George Herold > >>>>>> <gherold@teachspin.com> wrote: > >>>>>> > >>>>>>> I made this nice VCCS from a differential amp (Ina134), > >>>>>>> resistor and opamp (opa134) feedback. It was good to > >>>>>>> 40 or 50 mA. Bad ass-ci. > >>>>>>> > >>>>>>> gnd---RR---+--RR-----+ > >>>>>>> ||\ | > >>>>>>> +|-\ | > >>>>>>> Ina134 | >-(A)-+--Rset-+---output to gnd. > >>>>>>> +|+/ | > >>>>>>> ||/ /| | > >>>>>>> Vin--RR--+----RR---< +-----+ > >>>>>>> \| > >>>>>>> Buffer opa134 > >>>>>>> It's in AoE3. > >>>>>>> Rset sets the current/voltage ratio and the other R's are > >>>>>>> in the ina134. > >>>>>>> I was thinking about more current and added a power opamp > >>>>>>> in the (A) spot. Didn't work, the power opamp OPA544 railed > >>>>>>> at the wrong side.. or something. The ina134 spec sheet > >>>>>>> shows a fast buffer BUF634? Would that be better. > >>>>>>> (maybe I need to limit the voltage to the ina134?) > >>>>>>> > >>>>>>> This bipolar current source has nice return to ground. > >>>>>>> It's got an output impedance of Rset. (less nice.) > >>>>>>> > >>>>>>> George H. > >>>>>> > >>>>>> Zout should be theoretically infinite! It's a current source. > >>>>> Right, head slap, Vout compliance is limited by opamp supply and > >>>>> Rset. > >>>>> (It wasn't very happy driving a capacitor... it'd go to one > >>>>> one rail or the other.) > >>>> > >>>> Wait...wait..... it's a current source! > >>>> > >>> Grin. Well I was hitting it with square wave and getting a triangle out. > >>> (but it drifted up or down.. as I tweaked the dc offset. Right that's expected.) Hey hitting it with a triangle wave gives an output that looks > >>> a little like a sine wave but isn't. (A parabola?) > >>> > >>>> > >>>> > >>>>>> > >>>>>> The '544 should have worked, with maybe a little frequency > >>>>>> compensation. > >>>>> I tried some C (100 pF) from feed back (pin 5) to ground, > >>>>> I probably put the C in the wrong branch. > >>>>> > >>>>> The input and output of the 544 were backwards, not sure why. > >>>>> (I had no R's in the 544 circuit, just a 5 pin to-220 in space, > >>>>> 0.1uF C's on the power leads... word on the street is that the > >>>>> second amp needs to be faster. (I've never quite understood that.) > >>>> > >>>> If one is fast and one is slow, it's likely to be stable. Either one. > >>>> If they are near the same speed, some theory may be required. > >>> > >>> OK. Or maybe I'll try the BUF634 from the spec. sheet. ~100mA would be enough... +/-1 amp would be cool though. > >>> > >>> George H. > >> > >> You could use a discrete follower pair, nfet+pfet or NPN+PNP. > >> > >> TCA0372 is a dual 1-amp opamp for about 50 cents. > > > > Hey it worked OK when I had the power opamp inside only one loop. > > > > gnd---RR---+--RR--+ > > ||\ | > > +|-\ | > > Ina134 | >--+-(A)-Rset-+---output to gnd. > > +|+/ | > > ||/ /| | > > Vin--RR--+----RR---< +-----+ > > \| > > Buffer opa134 > > > > I moved my (A) power opamp. Though the opa544 doesn't seem to like > > having it's output driven near the negative rail. > > https://www.dropbox.com/s/c4qmibb7gcfv0mb/VCCS.BMP?dl=0 > > > > Chan. 1 is voltage across 200 ohm resistor, chan 2. is input. > > (100 ohm 'set' resistor. 1 V_in= 10mA > > What is your load? Is the output connected to ground? If you leave the > output open the current has nowhere to go and the circuit won't work > correctly maxing the output because of the positive feedback. > > If you ground the output the output of the buffer amp is ground and the op > amp is a unity gain buffer into Rset (assuming RR is the same value > everywhere). What are your power rails? Your scope traces pretty clearly > show the amp is being maxed out. > > Is your load 200 ohms? Then with -5 volts in, the current in the load is 50 > mA and the voltage on a 200 ohm load is 10 volts. The voltage at (A) is -15 > volts which likely is your rail. As the input exceeds -5 volts input the > output can't drive harder and a large difference develops across the inputs > of the op amp.
Right 200 ohm load, +/- 15V rails. (I might have bumped 'em up a bit.)
> > I guess you are just over-driving the circuit.
Exactly. I wanted to see how it behaves when over driven. George H.
> > -- > > Rick C > > Viewed the eclipse at Wintercrest Farms, > on the centerline of totality since 1998