Transistor

In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal.

The transistor is the fundamental building block of modern electronic devices, and is used in radio,telephone,computer and other electronic systems. The transistor is often cited as being one of the greatest achievements in the 20th century, and some consider it one of the most important technological breakthroughs in human history. Some transistors are packaged individually but most are found in intergrated circuit.

Transistor as a switch

Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.

Using the simple transistor circuit it can be seen from the graph, from point a to point b, as the base voltage rises the base and collector current rise exponentially (the a-b segment should be curved), but the collector voltage simultaneously drops because of the collector resistor. Relevant equations:

V_RC = I_C × R_C

V_RC + V_CE = V_CC

If V_CE could fall to 0 (perfect closed switch) then Ic could go no higher than V_CC / R_C, even with higher base voltage and current. The transistor is then said to be saturated. In actuality V_CE drops to roughly V_BE ÷ 2, rising with higher collector currents. Hence, values of input voltage can be chosen such that the output is either completely off,^[8] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier

The above common emitter amplifier is designed so that a small change in voltage in (V_in) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in V_in produce large changes in V_out.

It is important that the operating parameters of the transistor are chosen and the circuit designed such that as far as possible the transistor operates within a linear portion of the graph, such as that shown between A and B, otherwise the output signal will suffer distortion.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive.

Some musical instrument amplifier manufacturers mix transistors and vacuum tubes in the same circuit, as some believe tubes have a distinctive sound.

Advantages

The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are

• Small size and minimal weight, allowing the development of miniaturized electronic devices.
• Highly automated manufacturing processes, resulting in low per-unit cost.
• Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
• No warm-up period for cathode heaters required after power application.
• Lower power dissipation and generally greater energy efficiency.
• Higher reliability and greater physical ruggedness.
• Extremely long life. Some transistorized devices have been in service for more than 30 years.
• Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
• Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.

Limitations

• Silicon transistors do not operate at voltages higher than about 1,000 volts (SiC devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts.
• High power, high frequency operation, such as used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum.
• On average, a higher degree of amplification linearity can be achieved in electron tubes as compared to equivalent solid state devices, a characteristic that may be important in high fidelity audio reproduction.
• Silicon transistors are much more sensitive than electron tubes to an electromagnetic pulse, such as generated by a nuclear explosion.