As diodes are non-linear, it helps simplify things by figuring out approximately the voltage and current you're expecting to be passing through the diode, the Q-point. Once you have the Q-point, then you can assume that any variations around that point are linear, making your life easier. This can be found in a variety of ways but it needs to be understood that these Q-points are approximations around which we assume things are linear.
In summary, the way the Q-Point of a diode can be found is one of the following techniques:
- Load line analysis (need real data to do this and then basically graphing)
- Mathematical modeling (iterative techniques)
- Ideal Diode Model (assume 0V drop across a forward biased diode and no impedance, calculate the current)
- Constant Voltage Drop Model (assume typically .7V drop across a forward biased diode, calculate the current)
The designation Q-point is derived from the word quiescent, which means “still or unvarying.”
Electronic Devices and Circuit Theory, 11th Edition by Robert L. Boylestad & Louis Nashelsky