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Transistors

What is it?

  • Transistors are solid-state devices used for amplifying, controlling and generating electrical signals.

  • They are used widely in electronic equipment such as pocket calculators, radios, remote, electronic circuit and also in communication satellites.

  • These are made of semiconductor such as silicon.

  • As we know that a transistor is a combination of two diodes, we have two junctions here. As one junction is between the emitter and base, that is called as Emitter-Base junction and likewise, the other is a Collector-Base junction.

  • Generally, the transistor has three terminals:

  • Emitter (E)

  • Base (B)

  • Collector(C)

Structure of PNP transistor

Structure of NPN transistor

Symbol

Symbol

  • But in the circuit connections, we need four terminals, two terminals for input and another two terminals for output. 

  • the transistor configurations are three types.

  • The behavior of these three configurations with respect to gain is given below.

Common Base Configuration

  • No current gain.

  • Voltage gain.

Common Collector Configuration

  • Current gain.

  • No voltage gain.

Common Emitter Configuration

  • Current gain.

  • Voltage gain.

  • Transistor works like a switch.

  • It controls the flow of electric current.

(a) cutoff, lamp off

Input voltage NOT sufficient enough to overcome Barrier Potential = OFF

(b) saturated, lamp on

Input voltage sufficient enough to overcome Barrier Potential = ON

Transistors as an Amplifier:

Transistor Configuration:

Transistors as a Switch:

  • The transistor can amplify a weak signal.

  • The low resistance in the input circuit lets any small change in the input signal to result in an appreciable change in the output. 

  • Amplification is a basic function of a transistor.

Types of Transistors

  • This following picture describes the types of transistors.

TRANSISTOR

BJT

FET

JFET

MOSFET

De-MOSFET

E-MOSFET

BJT

FET

  • Bipolar (both e & holes).

  • Handles low current (in mA).

  • Current control current device.

  • Polarity sensitivity.(Emitter & Collector can't be exchanged )

  • Normally OFF switch.

  • Unipolar (either e or holes).

  • Handle high current (in Amp).

  • Voltage control current device.

  • Polarity insensitivity. (Drain & Source can be exchanged )

  • Normally ON switch.

Transistor as a Logic Gate

  • A transistor as NOT gate:

x

y

  • A transistor as OR gate:

A

B

Y

  • A transistor as AND gate:

A

B

Y

Transistor Storage Capacity

  • Dynamic RAM memory uses one transistor and a capacitor to store a bit of information. 

  • The combination of voltage on the row and column lines charges a capacitor.​​

  • Periodically the cell is read and the value rewritten to the capacitor to avoid the charge bleeding off called refreshing.

  • Two transistors can make a flip-flop which will store one bit of digital information.

  • The flip-flop is a positive feedback device that can be set to one or the other state and stay that way until written to again.

  • That is the basic building block of most standard digital circuits.

  • The SRAM, static ram memory cell is a type of flip-flop circuit, usually implemented using FETs.

  • SSD (Solid State Device) used to have billions of transistor to make 1TB storage device.

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