voltage not sensitive to temperature

timescope

Joined Dec 14, 2011
298
To obtain a voltage of 0.7v across 0.33 ohms implies a current of 0.7 / 0.33 = 2.12 Amps.

No load is shown on the schematic and you will not get 0.7v without one.

This circuit is a constant current source

Timescope
 

ErnieM

Joined Apr 24, 2011
8,007
You are depending on The B-E drop of Q4 to regulate that voltage, and that drop will vary with temperature... 2mv per degree C as I remember.
 

crutschow

Joined Mar 14, 2008
23,545
You can build a (relatively) temperature stable constant-current source using a low offset op amp and a temperature stable voltage reference. The circuit would be similar to that shown here, but substitute you P-MOSFET for the PNP transistor and generate the reference voltage with the reference connected between the +28V and the Vref point (remove R1 and R2).

Note that Vref must be referenced to the V+ (+28V) rail, not ground. Low voltage references usually go no lower that 1.2V, so you can use a voltage divider between the voltage reference and +28V to reduce the voltage to your desired 0.7V below +28V.
 
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