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Conventional Transistor Overview and Special Transistors
13 questions By Tony R. Kuphaldt
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Question 13 of 13
In this system, the voltage output of a digital timing circuit controls the charging and discharging of a resistor-capacitor network. The inner workings of the digital timing circuit are hidden for simplicity’s sake, but we may model it as a two-position switch, outputting either a “high” voltage signal (full supply voltage) or a “low” voltage signal (ground potential) at regular intervals:
First, identify what signal level from the digital circuit (“high” or “low”) causes the capacitor to charge, and what level causes it to discharge. Then, replace the BJT with a suitable MOSFET to accomplish the exact same timing function:
Reveal answerIn the BJT version of the circuit, a “low” signal output by the digital circuit causes the capacitor to charge. The same thing happens in this MOSFET version of the circuit:
Follow-up question: explain why no resistor is required in series with the MOSFET gate as there was with the BJT base in the original circuit version.
Notes:This circuit could be used as an introduction to the 555 timer, since that IC uses the same scheme for capacitor discharge.
