# Diodes, transistors (including BJT) and current mirrors

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1. One simplified model of a forward conducting Silicon diode's behaviour is to consider the diode to be equivalent to a series combination of a low resistance, a 0.7V fixed voltage source, and an ideal diode. Explain how this model is a reasonable approximation in terms of the true I-V relationship for a practical diode. What impact does the operating environment have on the true I-V relationship, and how can we account for this effect in the model? Describe the practical application of this environmental effect.

2. What effect do internal capacitances in the transistor model have on practical performance of a real system? How would you use these model values to predict system performance?

3. How are mutual transconductance, Î² and Î± of a BJT transistor related to each other? What role do these parameters play in circuit design?

4. Describe the operation of a current mirror and illustrate how one might be used.

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Answers:

1. The diode's model of a series resistor, a 0.7V fixed voltage source and an ideal diode is the closest approximation for the true operating behavior of a diode. The diode is practically non-conductive at reverse bias because the ideal diode will always be open at any point. In forward bias, the 0.7V fixed voltage serves as the potential barrier for the diode to start conducting, so the supply voltage to the diode must exceed 0.7V ...

#### Solution Summary

The solution discusses diodes, transistors (including BJT) and current mirrors.