BASIC ELECTRONICS TOPICS COVERED SEMICONDUCTOR DIODE PN JUNCTION

BASIC ELECTRONICS

TOPICS COVERED • • SEMICONDUCTOR DIODE PN JUNCTION DIODE APPLICATION OF DIODE TYPES OF DIODE CLIPPING AND CLAMPING DIODE BIPOLAR TRANSISTOR SINGLE STAGE AMPLIFIER FIELD EFFECT TRANSISTOR

SEMICONDUCTOR DIODE The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite direction (the reverse direction) A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture. These techniques are used to create special-purpose diodes that perform many different functions.

DIODE’S STRUCTURE

PN JUNCTION DIODE A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor creating a potential barrier voltage across the diode junction. A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.

PN JUNCTION DIODE

APPLICATIONS OF DIODE Rectifier circuit Important application of diode is Rectifier circuit, that is rectifying the AC voltage into pulsated DC voltage using Rectification (correction of errors and mistakes) There are two types of rectification circuit§ Half Wave Rectifier

Contd… • Full Wave Rectifier a. Center- Tapped Rectifier Circuit

Continued b. Bridge Rectifier Circuit

Contd… • Demodulation of signals The most common use for diodes is to remove the negative component of an AC signal so it can be worked with easier with electronics. Signal demodulation is commonly used in radios as past of the filtering system to help extract the radio signal from the carrier wave. Other applications of diode • Voltage Reference • Controlling the size of a signal • Mixing signals • Detection signals • Lighting • Lasers diodes

Zener diode A Zener diode is a particular type of diode that, unlike a normal one, allows current to flow not only from its anode to its cathode, but also in the reverse direction, when the Zener voltage is reached. Zener diodes have a highly doped p-n junction. Normal diodes will also break down with a reverse voltage but the voltage and sharpness of the knee are not as well defined as for a Zener diode. Also normal diodes are not designed to operate in the breakdown region, but Zener diodes can reliably operate in this region.

Zener diode Electronic symbol

ZENER DIODE & AVALANCHE BREAKDOWN A conventional solid-state diode allows significant current if it is reverse-biased above its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when carriers in the transition region are accelerated by the electric field to energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.

APPLICATIONS • Waveform clipper: Two Zener diodes facing each other in series will act to clip both halves of an input signal. Waveform clippers can be used not only to reshape a signal, but also to prevent voltage spikes from affecting circuits that are connected to the power supply Examples of waveform clipper

CONTINUED…. Voltage shifter: A Zener diode can be applied to a circuit with a resistor to act as a voltage shifter. This circuit lowers the output voltage by a quantity that is equal to the Zener diode's breakdown voltage Examples of voltage shifter

Contd… A Zener diode can be applied in a voltage regulator circuit to regulate the voltage applied to a load, such as in a linear regulator. Voltage regulator: Examples of voltage regulators

DIODE CLIPPNING CIRCUITS The Diode Clipper, also known as a Diode Limiter, is a wave shaping circuit that takes an input waveform and clips or cuts off its top half, bottom half or both halves together. DIODE CLIPPING CIRCUITS

DIODE CLAMPING CIRCUITS A clamping circuit is used to place either the positive or negative peak of a signal at a desired level. The dc component is simply added or subtracted to/from the input signal. The clamper is also referred to as an IC restorer and ac signal level shifter. A clamp circuit adds the positive or negative dc component to the input signal so as to push it either on the positive side, as illustrated in figure (a) or on the negative side, as illustrated in figure (b)

POSITIVE AND NEGATIVE CLAMPING

TRANSISTORS A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

BIPOLAR TRANSISTOR A bipolar transistor is a semiconductor device commonly used for amplification. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. Physically, a bipolar transistor amplifies current, but it can be connected in circuits designed to amplify voltage or power.

NPN TRANSISTORS In NPN transistor, the flow of current runs from the collector terminal to the emitter terminal. The term ‘NPN’ stands for negative, positive, negative and also known as sinking. The NPN transistor is a BJT, in this transistor, the initial letter ‘N’ specifies a negatively charged coating of the material. Where, ‘P’ specifies a completely charged layer. The two transistors have a positive layer, which are situated in the middle of two negative layers. Generally, NPN transistor is used in various electrical circuits for switching and strengthens the signals that exceed through them.

PNP TRANSISTORS The term ‘PNP’ stands for positive, negative, positive and also known as sourcing. The PNP transistor is a BJT; in this transistor the letter ‘P’ specifies the polarity of the voltage necessary for the emitter terminal. The second letter ‘N’ specifies the polarity of the base terminal. In this kind of transistor, the majority charge carriers are holes. Mainly, this transistor works as the same as the NPN transistor. The required materials used to build the emitter (E), base (B) and collector(C) terminals in this transistor are diverse from those used in the NPN transistor

NPN TRANSISTORS PNP TRANSISTORS

SINGLE STAGE TRANSISTOR AMPLIFIER When in an amplifier circuit only one transistor is used for amplifying a weak signal, the circuit is known as single stage amplifier. Single stage amplifier

When a weak a. c. signal is applied to the base of the transistor, a small base current starts flowing in the input circuit. Due to transistor action, a much larger (β times the base current) a. c. current flows through the load Rc in the output circuit(show in fig. ). Since the value of load resistance Rc is very high, a large voltage will drop across it. Thus, a weak signal applied in the base circuit appears in amplified form in the collector circuit. In this way the transistor acts as an amplifier.

RELATED OTHER TERMS • BIASING CIRCUITS: - The biasing circuit must establish a properating point otherwise a part of the negative half cycle of the signal may be cut off in the output and you will don’t get faithful amplification. • FAITHFUL AMPLIFICATION: - it is is the amplification of a signal, particularly a weak one, by a triode or a transistor such that the signal changes in amplitude but not in shape.

FIELD EFFECT TRANSISTORS It is a three terminal unipolar device in which conduction is manipulated with the help of applied electric field. The name itself gives a brief idea about its working principle, “field effect”, these two words clearly indicates it is a transistor controlled by electric field. Thus, it is also referred as a voltage controlled device in which only majority charge carriers are involved in the conduction mechanism. It comprises of three terminals, i. e. source, gate, and drain.

CIRCUIT DIAGRAM OF FET

CONSTRUCTION & OPERATION OF JFET An n-type channel is formed between two p-type layers which are connected to the gate. Majority carrier electrons flow from the source and exit the drain, forming the drain current. The pn junction is reverse biased during normal operation, and this widens the depletion layers which extend into the n channel only (since the doping of the p regions is much larger than that of the n channel). As the depletion layers widen, the channel narrows, restricting current flow.

N CHANNEL JFET STRUCTURE

APPLICATIONS OF FET • Low noise buffer • Every electronic device produces certain amount of noise but FET causes very little noise. This is especially important near the front-end of the receivers and other electronic equipment because the subsequent stages amplify front-end noise along with the signal. If FET is used at the front-end, we get less amplified noise (disturbance) at the final output. • Buffer. Amplifier A buffer amplifier is a stage of amplification that isolates the preceding stage from the following stage. Because of the high input impedance and low output impedance a FET acts an excellent buffer amplifier.

APPLICATION OF FET § § § Cascode Amplifier Analog Switch Chopper amplifier Multiplexer Current limiter