Hi,
From what i can tell about your schematic and your notes it seems
that your bias point is changing with temperature, and that it is
going down as temperature goes up.
The traditional way of fixing this provided you dont need super super
accuracy is to use an NTC thermistor. An NTC thermistors resistance
goes down as the temperature goes up.
Since your bias point drifts down, you would want to connect the
thermistor from the positive supply to the bias point possibly with
another fixed resistor in series with it. As the temperature rises and
the natural bias point starts to drift down, the NTC thermistor resistance
starts to drift down which increases the bias again. The circuit is
temperature compensated when the thermistor exactly compensates
for the natural tendency of the bias to drift down and so the bias
point stays close to the same value over temperature.
To select the thermistor and possibly a fixed resistor in series, you
need to calculate what value resistor you need at a given temperature
where the bias drifts down in order to keep it stable. Since the
thermistor value changes the bias even at room temperature, you
probably also need to change some of the resistor values to compensate
for the addition of the thermistor, then recalculate to make sure
the thermistor will still work at the selected drift temperature. If not,
the thermistor value may have to change or perhaps the fixed resistor
or both.
One way to test your calculations (or just to wing it) is to calculate
the value of the thermistor at two temperatures such as room temperature
(20 deg C) and say ten degrees above that (30 deg C). Now that you
have these two resistance values (we'll call them R20 and R30) you
would force the circuit to run at 20 deg C and insert the R20 resistor
(and possibly another fixed resistor) and readjust the other resistors to
get the bias point right again. Then, force the circuit up to 30 deg C
and insert the R30 resistor and check that the circuit is again biased
correctly. In other words, the two resistors R20 and R30 take the
place of the thermistor at two distinct temperatures of 20 and 30 deg C.
Once you get this working you can then go out and purchase a
thermistor that works the way the two resistors did.
I dont know if you have ever worked with thermistors before and if not
you will need to look into the calculations for these devices before
you start or you could be trying this for the next 10 years
Just in case you havent there is data on the web about these things.
Let us know what you need to do this.
Alternately, if your design can accommodate an op amp or two, you
can build in a little bias compensation circuit that works with quite a bit
of accuracy, better than a thermistor. Let us know about this too.