d123
Advanced Member level 5
Hi,
To make a bandgap reference using discrete components is easy, I see.
To make it independent of temperature is extremely difficult, as well.
Using matched beta transistors (+/- 98% for all four), and 50ppm/ºC resistors is obviously a waste of time, even as a fun hobby. Extrapolating, the version I put together would have about 150mV change from 1.23V over 25 degrees in both temperature directions, from 25ºC as the zero TC.
I saw that you can use e.g. LM35 to compensate for the drop in Vout, however, that would require an enormous circuit to a) level shift, and b) drop LM35 output/ºC to approx 2mV/ºC - not even for the sake of science.
Related: In the (TI) LM35 datasheet, the transistors in the simplified schematic have no "hats" across the C and E - what sort of BJT are they (like super beta have a wide "hat" on datasheet schematics)?
Is there some other method of compensating for the 2mV/ºC drop?
For people who don't want to use a reference IC and prefer to build them, is it more common practice to use the Brokaw cell that the Widlar 4 transistor bandgap because of this "problem"?
To make a bandgap reference using discrete components is easy, I see.
To make it independent of temperature is extremely difficult, as well.
Using matched beta transistors (+/- 98% for all four), and 50ppm/ºC resistors is obviously a waste of time, even as a fun hobby. Extrapolating, the version I put together would have about 150mV change from 1.23V over 25 degrees in both temperature directions, from 25ºC as the zero TC.
I saw that you can use e.g. LM35 to compensate for the drop in Vout, however, that would require an enormous circuit to a) level shift, and b) drop LM35 output/ºC to approx 2mV/ºC - not even for the sake of science.
Related: In the (TI) LM35 datasheet, the transistors in the simplified schematic have no "hats" across the C and E - what sort of BJT are they (like super beta have a wide "hat" on datasheet schematics)?
Is there some other method of compensating for the 2mV/ºC drop?
For people who don't want to use a reference IC and prefer to build them, is it more common practice to use the Brokaw cell that the Widlar 4 transistor bandgap because of this "problem"?
