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A diode is a passive semiconductor device that essentially acts as a one-way switch for current. It allows current to flow easily in one direction, but severely restricts current from flowing in the opposite direction. There are various types of diodes:



A diode can be analyzed as a simple ON/OFF switch like shown below. Whenever the applied voltage (Vs) across the diode exceed the barrier potential voltage Vj of the diode it will start conducting and pass current in one direction. 

During ON state, it passes current in one direction and acts as a short circuit.

During OFF state, it blocks current in both direction and acts as a open circuit.

Forward Bias: Apply voltage across the diode to turn it ON to pass the current.

Reverse Bias: Apply voltage across the diode to turn it OFF to block the current, but still some reverse leakage current will flow. 

Reverse Breakdown: Reverse breakdown voltage is the reverse anode voltage at which the diode conducts a specified amount of reverse current.  Since it's the reverse current across a junction, IR exhibits a knee shaped rise, increasing rapidly once breakdown occurs.

​The diode current is exponentially proportional to its applied voltage (refer equation below), that means small change in the voltage can cause current to increase rapidly. 


A transistor is an active semiconductor device used to control the current (flow of electrons) which can be used as a switch or amplifier or variable resistor.  There are two types of transistors current controlled (BJT) and voltage controlled (MOSFET/JFET/CMOS). 

Bipolar Junction Transistor (BJT)
BJT as Switch

A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers.

Bjt off.JPG
BJT CE Amplifier
CE Amplifier Equations
DC Analysis
AC Analysis
Input and Output Waveform
Op-Amp (Operational Amplifier)

Op-amp is an integrated circuit which is made of transistors that amplifies the difference in voltage between two inputs. Op-amps can be used for various applications, depending on the external components added.

The application configuration of op-amp can be changed based on the type of the feedback applied. 

No feed back op-amp used as comparator

Positive feed back op-amp used as oscillator.

Negative feed back op-amp used as amplifier.

An ideal op amp is usually considered to have the following characteristics:

  • Infinite open-loop gain G = vout / vin

  • Infinite input impedance Rin, and so zero input current

  • Zero input offset voltage

  • Infinite output voltage range

  • Infinite bandwidth with zero phase shift and infinite slew rate

  • Zero output impedance Rout, and so infinite output current range

  • Zero noise

  • Infinite common-mode rejection ratio (CMRR)

  • Infinite power supply rejection ratio.

Amplifier (Negative Feedback )

When the output is fed into negative input terminal of op-amp it is called the negative feedback. This is the configuration used for op-amp amplification. Theoretically op-amp has infinite open loop gain, to make it useful for amplification we need to reduce the gain by providing negative feedback.

The difference between V+ and V- is always zero, thereby whatever voltage at V+ will be same at V- or vice versa. To make this happen op-amp adjust the Vo thru negative feedback.

Vo = Av (V+  V

Inverting Amplifier 
Non Inverting Amplifier 
Oscillator (Positive Feedback )

When the output is fed into positive input terminal of op-amp it is called the positive  feedback. This is the configuration used for op-amp oscillator.

An op-amp with positive feedback tends to stay in whatever output state its already in. It “latches” between one of two states, saturated positive or saturated negative. Technically, this is known as hysteresis.

Comparator (No Feedback )

In an op-amp circuit with no feedback, there is no corrective mechanism, and the output voltage will saturate with the tiniest amount of differential voltage applied between the inputs. The result is a comparator.

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