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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.
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.
High Current Terminals:
Transistor In Circuit:
In the following demo, you can see that there is an FET connected to a lamp. When the voltage is below about 3 volts, the lamp is completely off. There is no current flowing through the lamp or the FET. When you push the button, you can see that the capacitor starts to charge (indicated by the rising yellow line and by the point where the capacitor's charging curve intersects with the white line sweeping from left to right. When the FET starts to turn on, the voltage on the drain starts to fall (indicated by the falling green line and the point where the green curve intersects with the white line). As the gate voltage approaches the threshold voltage (~3.5v), the voltage across the lamp starts to increase. The more it increases, the brighter the lamp becomes. After the voltage on the gate reaches about 4 volts, you can see that the bulb is fully on (it has the full 12 volts across its terminals). There is virtually no voltage across the FET. You should notice that the FET is fully off below 3 volts and fully on after 4 volts. Any gate voltage below 3 volts has virtually no effect on the FET. Above 4 volts, there is little effect.