Zero temco?

KSTR

Member
Paid Member
2007-07-17 2:35 am
Central Berlin, Germany
It means that the parameter in question -- typically drain current -- does not change with temperature, zero temperature coefficient. With many FETs (not all) there is one single useful operating point, strongly specific to the actual device, where this happens.

Example:
http://www.nxp.com/acrobat_download/datasheets/BF245A-B-C_2.pdf
Look ak fig.9. At Vgs=-1V there is little variation with temperature, the zero tempco point (flat straight curve) seems to be at -1.1V or so.

Example, different way of plotting the relationship:
http://www.fairchildsemi.com/ds/2N/2N5457.pdf
See first two figures, there is a point where the three curves meet, at that point the drain current does not vary with temperature (but transconductance, normally corresponding to gain, does). If they don't meet, then there is no operating point with stable characteristics over temperature, then this is to be compensated for externally with additional circuitry (always true for bipolar transistors).

From the plots you directly derive how it is measured.

- Klaus
 

roender

Member
2006-09-11 1:38 pm
maxpou said:
Hi,
It is what can cause thermal distortion and it's why the vertical mosfet have a thermal stability problem?

But if i don't have a good figure for found the zero tempco how can i do this? For 2SK170 2SK246, 2SJ74 and 2SJ109

Thank you! Maxpou

2sk170 has zero TC at 13-13.5mA, only the V version are suitable for ZTC.
2sk246 has ZTC at 0.32 ... 0.35mV.
 

KSTR

Member
Paid Member
2007-07-17 2:35 am
Central Berlin, Germany
maxpou said:
Hi,
It is what can cause thermal distortion and it's why the vertical mosfet have a thermal stability problem?
There a two things going on. While with many devices you can find an operating point where the device current has zero tempco, the gain of the device (transconductance) will still vary. This change also can give thermal distortion, albeit in rather less important amounts, compared to shifted bias problems in class A/B outputs.

With VMOS, these devices often have a zero tempco current point, but unfortunately at very high currents that one cannot make practical use of it. Therefore one needs external bias stability circuitry to keep standing current under control, same problem as with bipolar transistors

Klaus