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Special Diodes
8 questions By Tony R. Kuphaldt
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Question 7 of 8
There is a special type of diode called a varactor, which is used to create a voltage-dependent capacitance. This function is often used in electronic radio tuner circuits:
The voltage-dependent capacitance of this diode is given by the following equation:
$$C_j = \frac{C_o}{\sqrt{2V+1}}$$ Where,
CJ = Junction capacitance
Co = Junction capacitance with no applied voltage
V = Applied reverse junction voltage
Based on this equation, would you say that capacitance is directly or inversely related to the applied reverse-bias voltage of a varactor diode? Based on what you know of diode theory, explain why this makes sense.
Reveal answerCj is inversely related to V for a varactor diode.
Follow-up question: substitute the varactor diode capacitance equation into the standard resonant frequency equation to arrive at one equation solving for frequency in terms of L and diode voltage V.
Notes:This question reinforces students’ understanding of the mathematical terms direct and inverse, as well as review basic PN junction theory and capacitor theory.
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Question 8 of 8
What type of diode is always packaged in a clear glass or plastic body (as opposed to an opaque plastic body)? Explain how the appearance of the component is helpful in determining its identity.
Reveal answerThis is a nasty, trick question! The physical appearance of any semiconductor component is a poor indication of its identity. Many small zener diodes are packaged in clear bodies, but this does not mean all clear-bodied diodes are zeners, nor that all zeners are found in clear packages! The part number of a diode is the only reliable indicator of its identity.
Follow-up question: the physical size of a component is often an indication of what performance parameter?
Notes:I usually do not ask “trick” questions such as this, but occasionally they work really well to get the point across.
