The Junction Transistor Ppt Presentation

Silicon Transistor

What is Silicon

Silicon is the most popular semiconductor material in the world. It is a gray, crystalline element. It has four electrons, thus possessing the material characteristics of a semiconductor. It is never found as a free element in nature, but its dioxide and other compounds constitute nearly 9/10ths of the earth's crust. Its most common form is sand or sandstone. Initially, it was very hard to use to create transistors due to its high melting point, brittleness, and other difficult characteristics. But, with advanced silicon processing techniques developed in the 1950's (many developed at Bell Labs) silicon eventually became the semiconductor material of choice over germanium, in no small part due to its abundance and durability.

The Junction Transistor

A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector). The device can be characterized as a current amplifier, having many applications for amplification and switching.

Transistor Structure

PNP Junction Transistor Amplifiers

Since a transistor is a threeterminal device and there are four input-output terminals, one of the transistor terminals must be common to the input and output circuits. This leads to the names "common emitter", etc for the three basic types of amplifiers.

NPN Junction Transistor Amplifiers

Since a junction transistor is a three-terminal device and there are four input-output terminals, one of the transistor terminals must be common to the input and output circuits. This leads to the names Common Emitter, Common Collector, Common Base for the three basic types of amplifiers.

NPN CB Mode This configuration is used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.

NPN CC Mode The common collector amplifier, often called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance. It is also termed a "buffer" for this reason.

NPN CE Mode The common emitter configuration lends itself to voltage amplification and is the most common configuration for transistor amplifiers.

Transistor Operation A transistor in a circuit will be in one of three conditions

1. Cut off (no collector current), useful for switch operation. 2. In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications 3. In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications.

Constraints on Transistor Operation

Transistor Switch Example

There is no current to the base, so the transistor is in the cut off condition with no collector current. All the voltage drop is accoss the transistor.

Transistor Switch Example The switch is open

The base resistor is chosen small enough so that the base current drives the transistor into saturation. In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

Transistor as Current Amplifier

The larger collector current IC is proportional to the base current IB according to the relationship IC=βIB, or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.

Transistor CE Amplifier

Transistor CE Amplifier value

without component

CE MODE :INPUT CHARACTERSTIC

Base-Emitter Voltage

The entire normal range of silicon transistor operation involves a change in base-emitter voltage of only about two-tenths of a volt. This is because the base-emitter diode is forward biased. One of the constraints on transistor action is that this voltage remains at about 0.6 volts (often referred to as the diode drop). A small change in VBE can produce a large change in collector current and achieve current amplification.

CE MODE :OUTPUT CHARACTERSTIC

CE MODE :OUTPUT CHARACTERSTIC

Transistor Characteristic Curves (Approximate for 2N2222)

Quiescent point

Base bias

Voltage Divider Bias

Use RC = 1 kΩ, RE = 100 Ω, R2= 2.2 kΩ, VCC = 15 V.