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Diode Q-Point

Our Take

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:

  1. Load line analysis (need real data to do this and then basically graphing)
  2. Mathematical modeling (iterative techniques)
  3. Ideal Diode Model (assume 0V drop across a forward biased diode and no impedance, calculate the current)
  4. Constant Voltage Drop Model (assume typically .7V drop across a forward biased diode, calculate the current)

Book Definition

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

The Q-point of a diode consists of the dc current and voltage (ID, VD) that define the point of operation on the diode’s i-v characteristic.

Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock

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