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• AC stands for alternating current. 

• The AC power source produces potential that varies with time. 

• Creation of AC waves 

1. The electromechanical alternator produces voltage which switches polarity after fixed intervals of time. 
2. When it is represented on a graph, it looks like a sine wave

• Concept of Frequency and Period

1. The horizontal axis of the graph representing AC waves is marked with time and the dimension marked with one cycle of time is measured as period. 
2. Another measurement that can be taken is the back and forth oscillation of the wave called frequency. 
3. Period and Frequency are mathematical reciprocals of each other. 

• Measuring AC voltage and current

1.  AC voltage alters in polarity and current alters in phase. 
2. Hence, the peak of the amplitude of the wave is considered and the peak to peak value is taken. 

• OP – AMP is a high gain voltage amplifier with differential input and single ended output. 
• Working of OP­AMPS: OP­AMP amplifies the difference between the inverting and non inverting input. It works in the open loop mode and closed loop mode. 
• Characteristics of OP­AMP: An ideal op­amp has the following characteristics:

1. Infinite open loop gain 
2. Infinite input impedance 
3. Zero Output impedance 
4. Infinite Bandwidth 

• OP­AMP made of transistors: The diagram shows discrete op amp made of transistors with a gain of 100. This is not widely used.
  
• IC 741: IC 741 is the most common type of op­amp. It consists of differential amplifier, voltage amplifier and output amplifier. 
• IC 741 features : IC 741 has the following features: 

1. Zero common mode gain. 
2. The offset null pins are used to place external resistors. 

• Hybrid: A hybrid op amp consists of three triodes and coupling capacitor at the input. 
• Applications 

1. It is used as low pass or high pass filter. 
2. It is also used in electrocardiogram amplification. 
3. Resistor vs Inductor: Resistor is used to reduce the magnitude of the voltage whereas inductors are used to shift the AC voltage phase.

• Resistor vs Capacitor: A capacitor stores charge whereas resistor uses potential energy to generate power. 
• Capacitor vs inductor: Both capacitor and inductor store energy but capacitor stores it in electric filed whereas inductor stores it in magnetic field. 
• Transistor vs diode: A diode consists of 2 layers whereas transistor consists of 3 layers. A transistor works like diodes placed back to back. 
• Diode vs Zener diode: Diodes can work in both forward and reverse bias but zener diodes are meant to work only in the reverse bias mode. 
• Transformer vs Transistor: A transistor contributes to power gain by taking power from external power supply whereas a transformer is used for impedance matching. 
• Transistor vs op­amp: A transistor needs biasing to turn it on and an op­amp although made up of transistors does not need biasing and can amplify the difference. 
• Simple circuit 

1. A car­ reverse horn. It starts blowing as soon as the car is in the reverse gear. 
2. The circuit of reverse horn consists of resistors, capacitors, transistors and diodes. 
3. A battery of 12 V is supplied to provide power to the circuit. 
4. The power supply passes through the transistor which acts as a switch and provides power to a zener diode. It regulates the current and passes it to the transistors which generate sound waves which are again amplified by another transistor. 

• A bipolar transistor is an active semiconductor device formed by two P­N junctions which amplify electric current 
• Structure of Bipolar transistor

1. A Bipolar transistor consists of three differently doped regions called the Emitter, the Base and the Collector

• NPN 
  

1. It consists of p­doped semiconductors between two n layers 

• PNP 

1. It consists of n­doped semiconductor between two layers of p­doped material.  

• Hetero junction transistor 

1. This type is similar to Bipolar transistor and can handle very high frequencies. 

• Regions of operation: Bipolar transistors have five distinct regions of operation. They are

1. Forward Active 
2. Reverse Active 
3. Saturation 
4. Cut off 
5. Avalanche Breakdown 

• Forward Active: The base emitter junction is forward biased and the base collector junction is reverse biased. 

• Reverse Active: The base emitter junction is reverse biased and the base collector junction is forward biased. 
• Saturation: When both junctions are forward biased, it is in saturation. 
• Cut off: When both junctions are reverse biased, it is in cut off. 
• Avalanche Breakdown region: Due to the electric field created, there is current flowing through good insulators. It is avalanche breakdown. 
• Functions of Bipolar transistor: The functions of a bipolar transistor are to amplify current and to regulate current. 
• Specifications: Voltage between Collector­ Emitter, Voltage between Collector­Base should be above 12V and Voltage between Emitter­Base should be between 5­6V. 
• Applications of Bipolar Transistors: BPT is used an amplifier to amplify input waves in a microphone. It is also used as a switch. It also is a part of Integrated Circuit (IC). 
• Testing of Diodes: Diodes can be tested using multimeter

1. Set the multimeter to diode function. 
2. Anode to positive, Cathode to negative. 
3. Note down the voltage drop. 

• Transistors can be tested using multimeter. 

1. Black probe – Base, Red probe Emitter, Collector. 
2. Red Probe –Base, Black Probe­ Emitter , Collector. 

• If the readings don’t change with the change of probe at base then the transistor is bad. 

• A Zener diode is like any semiconductor diode which allows current to flow in the forward direction but also in the reverse direction during a certain voltage called breakdown voltage 
• Symbol of Zener diode 
  
• Operation of Zener diode: A normal semiconductor diode usually is damaged due to overheating when reverse voltage is applied for long duration. But in a Zener diode, a large amount of current can be passed through it and only a small change would be observed in the voltage level. It is usually used in reverse bias condition.

Types of Zener diodes :-

• Based on the breakdown voltage, Zener diodes can be classified into 2 types: Surface Zener and Sub Surface Zener .

• Surface Zeners

1. Breakdown Voltage: 6.8 – 10 V 
2. NPN transistor where Collector and Emitter are at cathode and Base at Anode 
3. Charges accumulated in the diode cause Zener walk out

• Sub surface Zeners 

1. This diode is similar to surface zener, the only difference being that the avalanche is deeper inside in the structure. 
2. Thus there is no Zener walk out and the zener voltage is maintained.