Thermal drift in Darlington connected transistor pair
Thermal drift in Darlington connected transistor pair
(OP)
I have a power amplifier circuit that has a darlington connected bjt pair. The voltage drop of the transistor goes from 1.5 volts to around 1.2 volts after the circuit has been turned on and is running for a few minutes. I was wondering if there is anyway to make the bias voltage to the transistor stiffer, so that the thermal voltage drift does not result in higher base currents.





RE: Thermal drift in Darlington connected transistor pair
TTFN
RE: Thermal drift in Darlington connected transistor pair
However, this new problem I am encountering is that the output for a given drive signal is not what is expected due to the thermal drift. I cannot keep changing what I expect, to be based on the different temperature variations that the transistor may experience. However, if I can make the bias voltage somehow stiffer, so that it does not change very much based on temperature, I can keep everything else according to design.
I wish I knew how to upload a schematic of the circuit, so you would see what I am talking about. A little difficult to explain in words. Sorry for any confusion
Thanks
RE: Thermal drift in Darlington connected transistor pair
You'd almost be better off with a current mirror design. Current mirrors can be designed to be quite insensitive to temperature.
Alternately, you might want to consider some sort of emitter resistor and using the voltage developed there for your current sense feedback
TTFN
RE: Thermal drift in Darlington connected transistor pair
RE: Thermal drift in Darlington connected transistor pair
As cbarn says, you can't keep stable bias conditions without feedback of some form or other.
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RE: Thermal drift in Darlington connected transistor pair
Thanks again