Introduction to Transistors (Part Two)

Field Effect Transistors

Another transistor design that has become more important than the bipolar transistor is the field-effect transistor (FET). The primary difference between the bipolar transistor and the FET is that the bipolar transistor has two PN junctions and is a current-controlled device, while the FET has only one PN junction and is a voltage-controlled device. Within the FET family, there are two general categories of components. One category is called the junction FET (JFET), which has only one PN junction. The other category is known as the enhancement-type or metal-oxide JET (MOSFET).

Figure 12-225 shows the basic construction of the JFET and the schematic symbol.

In this figure, it can be seen that the drain (D) and source (S) are connected to an N-type material, and the gate (G) is connected to the P-type material. With gate voltage Vgg set to 0 volts and drain voltage Vdd set to some positive voltage, a current flows between the source and the drain, through a narrow band of N-material. If then, Vgg is adjusted to some negative voltage, the PN junction is reverse biased, and a depletion zone (no charge carriers) is established at the PN junction. By reducing the region of noncarriers, it has the effect of reducing the dimensions of the N-channel, resulting in a reduction of source to drain current.

Because JFETs are voltage-controlled devices, they have some advantages over the bipolar transistor. One such advantage is that because the gate is reverse biased, the circuit that it is connected to sees the gate as a very high resistance. This means that the JFET has less of an insertion influence in the circuit. The high resistance also means that less current is used.

Like many other solid-state devices, careless handling and static electricity can damage the JFET. Technicians should take all precautions to prevent such damage.

Metal-Oxide-Semiconductor FET (MOSFET)

Figure 12-226 illustrates the general construction and the schematic symbol of the MOSFET transistor.

The biasing arrangement for the MOSFET is essentially the same as that for the JFET. The term “enhancement” comes from the idea that when there is no bias voltage applied to the gate (G), then there is no channel for current conduction between the source (S) and the drain (D). By applying a greater voltage on the gate (G), the P-channel begins to materialize and grow in size. Once this occurs, the source (S) to drain (D) current Id increases. The schematic symbol reflects this characteristic by using a broken line to indicate that the channel does not exist without a gate bias.

Common Transistor Configurations

A transistor may be connected in one of three different configurations: common-emitter (CE), common-base (CB), and common-collector (CC). The term “common” is used to indicate which element of the transistor is common to both the input and the output. Each configuration has its own characteristics, which makes each configuration suitable for particular applications. A way to determine what configuration you may find in a circuit is to first determine which of the three transistor elements is used for the input signal. Then, determine the element used for the output signal. At that point, the remaining element, (base, emitter, or collector) is the common element to both the input and output, and thus you determine the configuration.

Common-Emitter (CE) Configuration

This is the configuration most commonly used in amplifier circuits because they provide good gains for voltage, current, and power. The input signal is applied to the base-emitter junction, which is forward biased (low resistance), and the output signal is taken off the collector-emitter junction, which is reverse biased (high resistance). Then the emitter is the common element to both input and output circuits. [Figure 12-227]

When the transistor is connected in a CE configuration, the input signal is injected between the base and emitter, which is a low-resistance, low-current circuit. As the input signal goes positive, it causes the base to go positive relative to the emitter. This causes a decrease in the forward bias, which in turn reduces the collector current I_C and increases the collector voltage (E_C being more negative). During the negative portion of the input signal, the voltage on the base is driven more negative relative to the emitter. This increases the forward bias and allows an increase in collector current I_C and a decrease in collector voltage (E_C being less negative and going positive). The collector current, which flows through the reverse-biased junction, also flows through a high-resistance load resulting in a high level of amplification.

Because the input signal to the CE goes positive when the output goes negative, the two signals are 180° out of phase. This is the only configuration that provides a phase reversal. The CE is the most popular of the three configurations because it has the best combination of current and voltage gain. Gain is a term used to indicate the magnitude of amplification. Each transistor configuration has its unique gain characteristics even though the same transistors are used.

Common-Collector (CC) Configuration

This transistor configuration is usually used for impedance matching. It is also used as a current driver due to its high current gain. It is also very useful in switching circuits since it has the ability to pass signals in either direction. [Figure 12-227]

In the CC circuit, the input signal is applied to the base, and the output signal is taken from the emitter, leaving the collector as the common point between the input and the output. The input resistance of the CC circuit is high, while the output resistance is low. The current gain is higher than that in the CE, but it has a lower power gain than either the CE or CB configuration. Just like the CB configuration, the output signal of the CC circuit is in phase with the input signal. The CC is typically referred to as an emitter-follower because the output developed on the emitter follows the input signal applied to the base.

Common-Base (CB) Configuration

The primary use of this configuration is for impedance matching because it has low input impedance and high output resistance. Two factors, however, limit the usefulness of this circuit application. First is the low-input resistance and second is its lack of current, which is always below 1. Since the CB configuration gives voltage amplification, there are some applications for this circuit, such as microphone amplifiers. [Figure 12-227]

In the CB circuit, the input signal is applied to the emitter and the output signal is taken from the collector. In this case, both the input and the output have the base as a common element. When an input signal is applied to the emitter, it causes the emitter-base junction to react in the same manner as that in the CE circuit. When an input adds to the bias, it increases the transistor current; conversely, when the signal opposes the bias, the current in the transistor decreases.

The signal adds to the forward bias, since it is applied to the emitter, causing the collector current to increase. This increase in I_C results in a greater voltage drop across the load resistor RL, thus lowering the collector voltage E_C. The collector voltage, in becoming less negative, swings in a positive direction and is therefore in phase with the incoming positive signal.

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