Re: Interesting sensor design problem

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 02/13/05 

Date: Sun, 13 Feb 2005 22:21:22 +0000 (UTC)

In article <ebxwf24j2ue2.dlg@ID-222894.news.individual.net>,
Active8 <reply2group@ndbbm.net> wrote:
>On Sun, 13 Feb 2005 17:25:07 +0000 (UTC), Ken Smith wrote:
>
>> In article <luc04q468arn$.dlg@ID-222894.news.individual.net>,
>> Active8 <reply2group@ndbbm.net> wrote:
>> [...]
>>>I don't recall seeing any circuits using transistors to make a
>>>better r-r, like you suggest doing with the TIPs. Again, we don't
>>>know what accuracy the OP needs at the endpoints, but how perfect
>>>can we get?
>>
>> How much money do you have?
>>
>> I haven't actually tried the TIP35 but I suspect that you could easily get
>> to within 1mV of the rail at a few mA of load current. Using a transistor
>> inverted like that gives a quite low saturation voltage.
>
>Where can I find an example of this circuit?

Here's an ascii art example of the output portion of a bipolar one:

                   Vcc Vcc
                    ! !
                    / !
                    \ !
                    / !
                    \ !
                    ! !/ PNP
                    +----!
             NPN ! !\e
                  !/ Vcc !
                --! ! !
               ! !\e / !
               ! ! \ !
               ! ! / !
         A ! ! ! ! B
          -----+ +--+ +-------
               ! ! ! !
               ! ! \ !
               ! ! / !
               ! ! \ !
               ! ! / !
               ! ! ! !
               ! !/e GND !
                --! !
                  !\ !
             PNP ! !/ e
                    +----!
                    ! !\ NPN
                    \ !
                    / !
                    \ !
                    / !
                    ! !
                   GND GND

There usually has to be a series RC from the point A to B and some local
feedback at the driving op-map to prevent oscillations. The resistor fed
emitters of the driver stage set the bias current of the output devices
when we are in saturation.

The CMOS buffer version is much simpler but I don't think we can use it at
only 3V:

             -------/\/\/-------+--------
            ! !
   ----/\/\-+-!+\ !\ !
              ! >---+-----! >O-
   ----/\/\-+-!-/ ! !/CMOS unbuffered inverter
            ! !
             --!!----

The local feedback on the op-amp is needed if the op-amp is anything but
the slowest type. The extra delay in the CMOS section will take you out
of stability. 

-- 
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