Common Base configuration of BJT (Voltage amplification)

In this type of configuration we have, the Base terminal of BJT is common with both input and output. Where input is provided to the Emitter and output is collected from the collector. The circuit of the configuration is shown in the figure below. This configuration is used for the voltage gain and current buffer. And we have an input impedance that is low and output impedance is high.

Application

Common base the configuration is not used for the low-frequency input signal and for low frequency operating circuit. Though it is used when there is low input impedance is connected or present. Such as preamplifier, in a where we have low signal and we have to strengthen it out. So there we use a common base configuration.
While mainly it is used for very high frequency and ultra-high frequency. It is because its input capacitance does not affect by the amplification process. Due to which high frequency does not degrade or change.

Circuit:

As you can see in the figure, that base is common with input and output, while Emitter is connected to the input, and the collector is connected to the output. There is a battery connected between base and emitter, to forward-biased base-emitter junction. To forward bias we need to connect the positive side of the battery to the P side of BJT and negative side of the battery to the N side of BJT (In PNP case emitter is P side and Base is N side). There is another battery connected between collector and base to reverse biased the collector-base junction. To do this we have connected the positive side with the base and negative side with the collector.

Working:

Current Gain

Let’s first see the current flow in this configuration. We all know that current flows from the positive terminal of the battery to the negative terminal of the battery. So, in this case, current I_E flows from V_BE towards the emitter, from there some of the current flows towards the base region which is I_B and remaining all the current goes towards the collector. So we can recall the equation,

I_E = I_B + I_C

I_E is the input current and I_C is the output current. So this equation shows that output current I_C can never be greater than input current I_E. So current gain could never be greater than 1. It will be “1” or “less than 1”.

Current Gain equation will be

$$ α= I_C/I_E , where\ "α"\ is \ current\ gain $$

Using this equation we can find out the current gain of a circuit.

Voltage gain

Now let’s see how it provides us the voltage gain (amplification).

As we have the current gain$$  α=I_C/I_E $$ 
And resistance gain              $$ = R_L/R_I_N , $$ So using Ohm’s law(V=IR), We get,
Voltage gain= Current gain x Resistance gain
Voltage gain = $$ I_C/I_E × R_L/R_I_N = {I_C R_L}/{I_E R_I_N} $$

So this equation shows that we can get the desired voltage gain by changing the amount of input and output resistance.

Input characteristics

Now let’s see what the input characteristics of the common base configuration are. Means when we change input voltages V_BE what effects does it has on input current I_E. And keeping the output voltage V_CB constant. We will plot a graph having V_BE on the x-axis as we are changing V_EB that’s why it is on the x-axis. While I_E will be on the y-axis. We will see that there is no current across input when V_EB is 0 to 0.6V. Because the starting voltage of the diode is 0.7V. So when V_EB cross 0.6 V. The I_E will start to grow up and will grow larger with small increase in voltage V_EB. So one case was with V_CB voltage was kept constant at 5V. Now observe another case with V_CB at 7V, and then with 10V. And at last plot a graph of all 3 cases as shown in the figure below.

Output characteristics

Now observe the output characteristics. We will change the output voltage V_CB and observe the change in output current I_C, with input current I_E will remain constant. So V_CB is on the x-axis because we will be changing it and I_C on the y-axis because it will be changed. We will observe that when we keep I_E constant at 0 and will change V_CB, it will have no effect on the I_C. I_C will remain zero at every value of V_CB (0V, 3V, 5V etc). While when we keep I_E constant at 1mA and change V_CB, then we will see I_C will come near to 1mA will remain constant for further increase in V_CB. Now keep I_E constant at 2mA, we will see that after changing V_CB, I_C will come near to I_E, means near to 2mA and will go constant for further increase in V_CB.Will draw plot for all the experiments. And we observe that I_C will never be greater than I_C. As equation 1 tells that.

Bi Polar Junction Transistor (BJT): Introduction, Construction and Working

Introduction:

BJT stands for Bipolar Junction transistor which means two polar junctions. The reason it is called Bipolar is that it uses both electrons and holes for carrying charge or to flow the current. While FET is known as a unipolar transistor.

Construction:

As we have discussed diode before, and how it is formed by the junction of two regions, P doped region, and n doped region. The BJT is formed by the combination of two back to back diodes. The Bipolar junction transistor or BJT has two junctions of the semiconductor. In one of the two types (PNP), a thin n-region is shared by two p-regions. And in another type (NPN), a thin p-region is shared by two n-regions, the figure below explains this.

          Fig (a)

                                                                                 Fig (b)

                                   

This complete structure of BJT has three terminals and has three layers. Each terminal for each layer. And the names of these layers are Emitter, Base, and collector shown in above figure. For NPN the arrow is going away from the Emitter terminal and for PNP the arrow is towards the emitter terminal. The reason is explained later.

 Functions of BJT:

BJT can be used for three purposes,

• Current controlling
• switching
• Amplification

Controlling Current:

BJTs are mostly used for controlling the amount of current flows from emitter to collector or collector to base. These are two cases for two different types of BJT (which are PNP and NPN). In PNP the current goes from Collector to emitter with the combination or addition of base current, see fig (a). While in NPN the current goes from Emitter to collector with separation or removal of base current see fig (b). In all this, the current which goes from emitter to collector or collector to emitter is the controlling current and the current which goes or comes from the base is controlled current. Now all this statement means that, the amount of current which have to go from emitter to collector in NPN transistor is decided by or controlled by the current flows towards the base as shown in the below picture.

           Fig(c)

                                                                                    Fig(d)

         

                                                                                          

Equations of current:

This thing also produce the equation of current,

I_E = I_B + I_C

Where I_E is the Emitter current, I_B is the base current and, I_C is collector current.

I_B = I_E - I_C

I_C = I_E - I_B

Diode or pn junction:Definition, explanation, construction

Definition:

"Diode is an electrical component which allow the current to flow in one direction."

Explanation:

Diode is use to flow the current only in one direction and to block the current, if it flows in other direction. This property of blocking current in one direction and allowing it in only one direction is due to its construction which is explain below. It does not change the current, it just block the current to flow in other direction in a circuit. It is also use to convert AC source into DC source.

By blocking the negative interval and allowing only positive interval and by using some filter, decreases the fluctuation.

Construction:-

The diode is made up by the junction of two semiconductor of same material, lets suppose Silicon (Germanium could also be used). One semiconductor is doped with p type material which is boron and other semiconductor with n type material which is phosphorus. This junction is known as P-N junction. 

                                 

In P there is holes which is majority charge and in N there is electrons which is majority charge.

When we make junction of these (p-type and n-type) then holes which are majority charges in P, moves towards the N, and combine with the electrons, and majority charges in 

N which are electrons moves towards the P junction and combine with the holes.

 During this process there form a junction which is known as PN junction. Which consists both holes and electrons. And no more transfer of electrons and holes occurs(because of this junction wall.).

Working:

In electric Circuit, we take symbol for diode is, 

There are bias (applying any source) condition for semiconductor diode. Forward bias and Reverse bias. Diode works only in forward bias. In reverse bias, it blocks the flow of current in circuit or in other words. It becomes open circuit. which is its main property to allow current in only in one direction.

how to forward bias the diode?

We could do it by connecting positive terminal of voltage source from the anode and negative terminal with the cathode. And for reverse bias its vice versa.

How it allows current to flow in forward bias?

It allows current to flow in forward bias, because we connect positive terminal of voltage source with anode and it attracts electron from the N type. So electrons start to flow towards the anode terminal. And there is junction wall between diode, so to overcome there needs voltage of 0.3 or 0.7 . so could say that it is diode starting voltage.

How it blocks current in reverse bias? 

It blocks current in reverse bias, because the positive terminal of voltage source is connected with the cathode so it attracts electron towards cathode terminal so because of it, negative electrons from P type and positive holes from N type goes into junction and increases it. so junction widens and blocks the transfer.

Capacitor, definition, formula, explanation, Capacitance

Definition:-

"Capacitor is an electrical device which is used to store energy or charges between its plates."

Explanation:-

Capacitor is use to store charge so that we could use it later by discharging it. It stores electrical charge between its parallel plates which are known as conducting plates of capacitor. There is no connection between these conducting parallel plates. They are separated by air or some dielectric. We use dielectric in capacitor to increase its efficiency. By using dielectric we could store more charge between its plates.

Capacitor doesn't allow dc current to flow through it because DC current flows only in one direction. so, it after charging from dc current it allows to flow through it as there is nothing in the circuit.

While it allows AC current to flow through it because ac current changes its direction after half interval, so capacitor charges at first half and discharges at 2nd half.

Capacitance:-

Capacitance is the ability of a capacitor to store charge. Capacitance tells us that how much charge we can store between the plates of capacitor. Capacitance is the unit of capacitor in one way. While capacitance has unit Farad. Farad is define as,

"When there will be 1 volts across capacitor then it will charge one coulomb, its mean capacitor have one farad capacitance."

Formula:-

By seeing definition of farad we could write formula for the capacitance of capacitor,

Q = CV

C = Q/V

AC Generator

Definition:-

"The machine which is used to convert mechanical energy into electrical energy(AC current) is known as AC Generator"

Explanation:-

The device which gives us AC current(Electrical energy) by taking mechanical energy from any source(could be us) is known as Ac generator. Its construction is similar to dc generator and its working is also same almost. 

The above shown diagram is of AC generator. our need is to get the current which changes its direction after every half cycle, so to get this we need to get the current from one slip ring for one half and for other slip ring for the second half. Slip rings are connected at different ends of closed circuit. where load is connected. So that load will get Ac current.

Construction:-

It consists of following parts:-

1.Magnet

2.Slip Rings or Commutator

3.Conducting Coil or Armature

4.Carbon brushes

5. Load  

Working:-

According to the Faraday's Law of electromagnetic induction, whenever a conducting coil is move or rotates across magnetic field emf produces in it and due to this, current starts to flow through it. So in this generator when coil will move by some mechanical force across magnetic field of magnet, then there will produce an EMF and because of this, current will start to flow through it(if coil path is closed or connected to other circuit). Direction of current will be according to Fleming's right hand rule. 

As shown in this diagram, For first half rotation of coil the current will go to the load form one slip ring and for 2nd half rotation of coil the current will go from the other slip rings to the load so it is giving ac current to the load as it is changing its direction after every half cycle.

Generator(DC)

Definition:-

"The device which converts mechanical energy into electrical energy is known as Generator."

Explanation:-

Generator use to produce electrical energy from mechanical energy for our purpose.

Above shown diagram is of DC generator, its construction is same as Electric motor(DC), the difference is that from where we give input in input, from there we get output from generator. the mechanical energy could of any type. for example moving pivot with your hand and by some other source.

Construction:-

It consists of:-

1.Magnet

2.Conducting Coil loop

3.Commutator

4.Carbon brushes

5.Battery

Working:-

According to Faraday's law, whenever a conducting coil will move across magnetic field then there will produce an EMF, and due to this, current will start to flow through coil (if path is closed). the direction of current will be according to Fleming's Right hand rule(Described below). Then this current will go through commutator and through carbon brushes it will go through battery and will charge. That its simple working!

Faraday's Law of electromagnetic induction:-

This law state's that" whenever a conducting coil is move across the magnetic field of magnet, then EMF will produce in coil and current will start to flow through it" Current direction would be according to Fleming right hand rule.

Fleming's Right hand rule:-

According to this law "Straighten your thumb, fore finger and middle finger so that they become perpendicular to each other(right angle to each other) and then your thumb will represent the coil, your fore finger will represent the magnetic field and your middle finger will represent the direction of current flow."

Electric Motor(AC)

Definition:-

"An electric device that converts an AC electric source into mechanical energy is known as AC electric motor"

The device that converts Electrical energy into mechanical energy for our use, we called that device electric motor.

Explanation:-

The above diagram is of ac motor that converts an ac source into mechanical energy. The problem we face in ac motor is that, ac current changes its direction after half cycle. So it is not possible for conducting coil to move in one direction for complete cycle of  ac current. To overcome this problem, we use split rings instead of commutator, so that we could reverse the direction of current. and coil could move in one direction.

Construction:-

It consists of,

1.Magnet

2.Conducting loop

3.Split rings

4.Carbon brushes

5.Ac source

Working:-

AC motor works on the same principle, as DC motor works. When we connect ac current source to the carbon brushes then through split rings current starts to flow through the wire( or conducting loop) so because there is also magnet field through wire and now current flowing through it, So according to Fleming's left hand rule, the coil will start to move. When ac current will change its direction split rings will change their direction and so coil will get same directed current and coil will move in one direction.

Electrical Motor(DC)

Definition:

"Electric motor is a device that converts electrical energy to mechanical energy"
The device which converts electrical energy into other form of energy which is mechanical energy so that we can take benefit from it.

Types of Motor:

There are two basic types of electric motor:-

1.DC motor

2.AC motor

 

DC motor Explanation:

The motor which converts DC power to the mechanical energy or power is known as dc motor.

Above diagram is of dc motor. It consists of a magnet, armature or conducting loops, carbon brushes, commutator, dc source or battery.

Working:-

When we connect the battery with the carbon brushes than through commutator they cause the current to flow through loops, as on the other hand there is also magnetic field from north to south so as we have studied Fleming's left hand rule(will explain below), the loops will move upward from one side and will move downward from other side. When this current carrying conductor(conducting loops) will change position at 90 degree than again according to the Fleming's rule this loop will move as it is going. So that how it works. other end of loop would be connected with the object to whom we want to provide mechanical energy. 

Fleming's Left hand rule:-

This rule states that" hold your left hand and tied your 1st finger, 2nd finger and thumb in such a way that they each other become perpendicular(Right angle) to each other. now if first finger is the direction of magnetic field and 2nd finger represent the current flowing through coil, then thumb will represent the force acting on it."