Fabio Eboli wrote:
Il 2012-12-23 11:36 Bruce Griffiths ha scritto:
The simulation indicates that the TAC capacitor charging current is
far from constant whilst charging.
This is due to the use of saturated switches rather than current
steering switches.
The capacitor charging current is poorly controlled.
So in this respect your circuit fails miserably.
So if I'm understanding correctly, the non linearity is due
to the uncontrolled current flowing from the collectors to
the bases of saturated transistors, that changes the magnitude
of charging current in uncontrolled way.
I was trying to use a sungle supply, but now I'm starting to
see the limits of my implementations.
Fabio.
Corrections:
R6 = 250 ohm
R14 = 125 ohm
R10 = 250 ohm
R5 = 250 ohm
R2 = 2k ohm
The current source tempco is around 2000ppm/C
This can be reduced to about 100ppm/C (limited by hfe tempco of the
transistors) by replacing the LEDS with emitter followers the base of
which is driven by an opamp.
Bruce
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