Friday, October 31, 2008

Field-Effect Transistor:

The function of Field Effect Transistors is similar to bipolar transistors (especially the type we will discuss here) but there are a few differences. They have 3 terminals as shown below. Two general types of FETs are the 'N' channel and the 'P' channel MOSFETs. Here we will only discuss the N channel. Actually, in this section, we'll only be discussing the most commonly used enhancement mode N channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Its schematic symbol is below. The arrows show how the LEGS of the actual transistor correspond to the schematic symbol.


Current Control:
The control terminal is called the gate. Remember that the base terminal of a bipolar transistor passes a small amount of current. The gate on the FET passes virtually no current when driven with D.C. When driving the gate with high frequency pulsed D.C. or A.C. there may be a small amount of current flow. The transistor's "turn on" (a.k.a. threshold) voltage varies from one FET to another but is approximately 3.3 volts with respect to the source.

When FETs are used in the audio output section of an amplifier, the Vgs (voltage from gate to source) is rarely higher than 3.5 volts. When FETs are used in switching power supplies, the Vgs is usually much higher (10 to 15 volts). When the gate voltage is above approximately 5 volts, it becomes more efficient (which means less voltage drop across the FET and therefore less power dissipation).

MOSFETS are commonly used because they are easier to drive in high current applications (such as the switching power supplies found in car audio amplifiers). If a bipolar transistor is used, a fraction of the collector/emitter current must flow through the base junction. In high current situations where there is significant collector/emitter current, the base current may be significant. FETs can be driven by very little current (compared to the bipolar transistors). The only current that flows from the drive circuit is the current that flows due to the capacitance. As you already know, when DC is applied to a capacitor, there is an initial surge then the current flow stops. When the gate of an FET is driven with a high frequency signal, the drive circuit essentially sees only a small value capacitor. For low to intermediate frequencies, the drive circuit has to deliver little current. At very high frequencies or when many FETs are being driven, the drive circuit must be able to deliver more current.