Strategies for temperature-compensating differential pairs
in [Design]
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From: Bitrex on 8 May 2010 20:19 If you put a reference current into the collector of one half of a differential pair, and then take an output current from the collector of the other transistor of a differential pair, the output current as a fraction of the reference current will be compensated against variations of Is. However, because of the dependence of VT on temperature Vbe will now have a positive tempco, and the output current as a fraction of the reference current will decrease with increasing temperature of the pair. I'm wondering what strategies the analog designers here have used to compensate for this second-order temperature dependence? TIA!
From: miso on 8 May 2010 20:57 On May 8, 5:19 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote: > If you put a reference current into the collector of one half of a > differential pair, and then take an output current from the collector of > the other transistor of a differential pair, the output current as a > fraction of the reference current will be compensated against variations > of Is. However, because of the dependence of VT on temperature Vbe will > now have a positive tempco, and the output current as a fraction of > the reference current will decrease with increasing temperature of the > pair. I'm wondering what strategies the analog designers here have used > to compensate for this second-order temperature dependence? TIA! Your really need to explain your circuit better. I have a gut feeling you don't mean a differential pair since the collectors would be high impedance and just form gain stages if fed a reference current.
From: John Larkin on 8 May 2010 21:53 On Sat, 08 May 2010 20:19:15 -0400, Bitrex <bitrex(a)de.lete.earthlink.net> wrote: > > >If you put a reference current into the collector of one half of a >differential pair, You've lost me already. What do you mean by putting a current into the collector of a diff pair? Normally one stuffs a current into the paralleled emitters. Can you post a schematic? John and then take an output current from the collector of >the other transistor of a differential pair, the output current as a >fraction of the reference current will be compensated against variations >of Is. However, because of the dependence of VT on temperature Vbe will > now have a positive tempco, and the output current as a fraction of >the reference current will decrease with increasing temperature of the >pair. I'm wondering what strategies the analog designers here have used >to compensate for this second-order temperature dependence? TIA!
From: Bitrex on 8 May 2010 23:01 John Larkin wrote: > On Sat, 08 May 2010 20:19:15 -0400, Bitrex > <bitrex(a)de.lete.earthlink.net> wrote: > >> >> If you put a reference current into the collector of one half of a >> differential pair, > > You've lost me already. What do you mean by putting a current into the > collector of a diff pair? Normally one stuffs a current into the > paralleled emitters. > > Can you post a schematic? > > John > > Sorry for not being more clear - in the meantime I was able to find a website that has an example of the circuit configuration I am talking about here, figure 3: http://www.uni-bonn.de/~uzs159/ It appears from the text the most common way to compensate the 1/T temperature dependence of the circuit is to use a resistor with a positive temperature coefficient in the circuit that provides the control voltage. To make it work well I guess you would want to bind the differential pair to the tempco resistor with some kind of thermally conductive adhesive. > > and then take an output current from the collector of >> the other transistor of a differential pair, the output current as a >> fraction of the reference current will be compensated against variations >> of Is. However, because of the dependence of VT on temperature Vbe will >> now have a positive tempco, and the output current as a fraction of >> the reference current will decrease with increasing temperature of the >> pair. I'm wondering what strategies the analog designers here have used >> to compensate for this second-order temperature dependence? TIA! >
From: miso on 8 May 2010 23:36
On May 8, 8:01 pm, Bitrex <bit...(a)de.lete.earthlink.net> wrote: > John Larkin wrote: > > On Sat, 08 May 2010 20:19:15 -0400, Bitrex > > <bit...(a)de.lete.earthlink.net> wrote: > > >> If you put a reference current into the collector of one half of a > >> differential pair, > > > You've lost me already. What do you mean by putting a current into the > > collector of a diff pair? Normally one stuffs a current into the > > paralleled emitters. > > > Can you post a schematic? > > > John > > Sorry for not being more clear - in the meantime I was able to find a > website that has an example of the circuit configuration I am talking > about here, figure 3: > > http://www.uni-bonn.de/~uzs159/ > > It appears from the text the most common way to compensate the 1/T > temperature dependence of the circuit is to use a resistor with a > positive temperature coefficient in the circuit that provides the > control voltage. To make it work well I guess you would want to bind > the differential pair to the tempco resistor with some kind of thermally > conductive adhesive. > > > > > and then take an output current from the collector of > >> the other transistor of a differential pair, the output current as a > >> fraction of the reference current will be compensated against variations > >> of Is. However, because of the dependence of VT on temperature Vbe will > >> now have a positive tempco, and the output current as a fraction of > >> the reference current will decrease with increasing temperature of the > >> pair. I'm wondering what strategies the analog designers here have used > >> to compensate for this second-order temperature dependence? TIA! Looks like frames did you a disservice. It this the page in question? http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html If not, then right click on the link in question and open it in another window to get the correct URL. |