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NMOS FET and BJT analysis

Show all logic/work/sources leading up to the conclusions drawn for each problem.

1. In an NMOS FET, why does the drain current reach a maximum value, independent of increasing drain-to-source voltage?

2. Quite frequently, one needs to control a light emitting diode with a microprocessor output pin. A typical LED requires about 15 mA to fully illuminate at which time it will have a voltage drop of about 1.7V, while the microprocessor may only be capable of generating an output current of about 250 μA, while switching between 0 and 5 V. Describe how an NPN transistor like the 2N2222 or 2N3904 could be connected to the microprocessor, the LED, one or more resistors (specify the value(s)), and a 5V power source to turn the LED on and off.

3. Coupling capacitors are often used in the input and output stages of an amplifier. Why? What impact do these components have on the amplifier operation? Why? In a bipolar junction transistor amplifier with a base resistor of about 5 k and a collector resistor of about 500 , roughly estimate these capacitors for an audio amplifier that has a frequency response from 100 Hz to 10 kHz?

4. A standard silicon NPN BJT is to be used as an amplifier. The emitter voltage is biased at 4 volts, the base is biased at 5 volts, and the collector is biased at 10 volts. Will this circuit perform as desired? Why or why not.

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Please see attached for the solution to the given problems.


1. The NMOS FET, or the MOSFET in general, has its drain current reach a maximum value independent of increasing drain to source voltage because a MOSFET has a fixed sized channel at a certain amount of bias applied to the gate. It is like a tunnel that can only allow a certain amount of current regardless of how much is supplied to the drain to source region.

2. The NPN transistor will act as a current amplifier to be able to provide enough ...

Solution Summary

The solution discusses the NMOS FET and BJT analysis.