Thread Starter


Joined Jan 10, 2006
hello friends,

can you tell me what is major difference we follow in the design of a BJT and a FET amplifier. I know how to find the resistor values in case of a BJT amplifier, how in the case of FET, briefly explain.


Joined Jun 13, 2005
The way to bias a transistor, fet or bipolar, depends on many things. The bias method will effect the gain, frequency, noise, and stability. Assuming we a talking a simple no local feedback circuit, things would start the same. Pick a drain or collector current in the range of the device. Then using the supply voltage choose a load resistor to give a reasonable drain or collector voltage. Usually this would be about the supply voltage divided by two. This is where it gets different. For a bipolar part, use the beta of the device to calculate the base current. A single resistor can then be used to provide the base current by dividing this current into the supply voltage minus the base voltage. For the fet, use the pinch off voltage of the device to calculate the gate voltage. The gate voltage can be provided with a voltage divider. Both of this methods will give you an amplifier with poor DC stability but should give you the basic idea. If you need a more stable circuit look at using some local emitter or source feedback. Good Luck.