How is transistor used as a switch or inverter ?


As an electronics student, you may have heard many times that transistor is used as a switch. But did you know how transistor is used a switch ? What properties of transistor allows it to be used as a switch ?


Transistor as you know is basically, a three terminal device consisting of Base, emitter and collector. (If we are talking about BJT)

In digital circuits, we often refer to digital logic “1” and “0” which can be referred as logic “High” or logic “Low”. Almost any electronic device works on these two logic standards. Although there are many logic families like TTL, ECL, CMOS etc (complicated stuff) But, lets talk about the simplest application where a BJT can be used as an INVERTER.

INVERTER is a logic gate which inverts the input. If input is “1” output will be “0” similarly if input is “0” output will be “1”

inverter

But how can a transistor be used as an INVERTER ?

Transistor as inverter

Consider an NPN transistor connected as shown in the above figure. There can be two cases when transistor is used as a switch.

Case 1 : Transistor is in cut off region 

cutoff

  • When Vb (base voltage) is very low, the base current (Ib) will be negligible.
  • Due to this, the collector current (Ic) will also be minimum and transistor will be in cut off region.
  • As the collector current (Ic) is very low, there will be almost no voltage drop across resistance (Rc) thus IcRc = 0 volts

By using Kirchoffs voltage law (KVL) we can say that,

Vcc = Vce + IcRc

But since, IcRc = 0 volts 

Vcc = Vce + 0 = Vce

Thus Vce = Vcc (supply voltage Vcc is usually 5 volts)

For e.g if we give a base voltage (Vb) of 0 volts, we get collector to emitter voltage (Vce) of 5 volts. Thus if we give input as logic “0” we get output as logic “1”

Case 2 : Transistor is in saturation region 

saturation.png

  • When Vb (base voltage) is very high, the base current (Ib) will be maximum.
  • Due to this, the collector current (Ic) will also be maximum and transistor will be in saturation region.
  • That is, if we increase Ib, then Ic will also increase but, beyond a certain point Ic cannot increase and it will attain a maximum value.
  • As the collector current (Ic) is very high, The voltage drop across Rc which is IcRc = very high

By using Kirchoffs voltage law (KVL) we can say that,

Vcc = Vce + IcRc 

But since, IcRc = very high, All the voltage supplied by Vcc will be across IcRc and Vce will be almost 0 volts. But practically in saturation region Vce is very low (0.2 volts) which is almost considered as 0 volts.

Vcc = 0 + IcRc = IcRc

Thus Vce = 0 volts 

For e.g if we give a base voltage (Vb) of 5 volts, we get collector to emitter voltage (Vce) of 0 volts. Thus if we give input as logic “1” we get output as logic “0”


Thus if we summarize the following data we can infer that,

  • When Vb = 0 volts (logic 0) , Transistor will be in cut off region. Vce = 5 volts (logic 1) . Transistor will act as an open switch.

 

  • When Vb = 5 volts (logic 1) , Transistor will be in cut off region. Vce = 0 volts (logic 0) . Transistor will act as a closed switch.

 

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what are resistors ?


  • what is a resistor ?

  • Resistor is like a current limiter which limits the amount of current flowing through a device.

resistor3-500x500          2000px-Resistor_Carbon-film_0.25W_Coloured.svg

Suppose you have to apply a certain current of 5 amperes to a device input and you have 10 amperes source so what would you do ?
solution : Add a resistor in series to limit the current.

led_circuit_single_led

  • This resistance will create a voltage drop (reduces voltage in that area or drops potential) thus reducing the current passing through it.
  • Current is defined as amount of charge flowing in a given time so you basically limit the flow of electrons (or you RESIST the flow of electrons).
  • In the process of limiting the flow of current and reducing voltage, a resistor absorbs electrical energy, which is released in the form of heat.

 

What can you do to reduce the current or limit the current ?

  • What’s going on inside a resistor?

resistcarb

If you break one resistor, and scratch off the outer coating of insulating paint, you might see an insulating ceramic rod running through the middle with copper wire wrapped around the outside. A resistor like this is described as wire-wound. The number of copper turns controls the resistance very precisely: the more copper turns, and the thinner the copper, the higher the resistance. In smaller-value resistors, designed for lower-power circuits, the copper winding is replaced by a spiral pattern of carbon. Resistors like this are much cheaper to make and are called carbon-film. Generally, wire-wound resistors are more precise and more stable at higher operating temperatures.


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Functional block diagram of IC 555


Functional block diagram of IC 555


 

Before starting about this topic, you should go through the basics of IC 555 which were explained in the previous post. The link is given below

https://electroconcepts.wordpress.com/2015/12/21/all-you-need-to-know-about-ic-555/


 

What is a functional block diagram ?

Simply, A “functional block diagram” explains the different components  and their working. These components are fabricated in a special way to form an integrated circuit on a silicon chip. 

The given diagram is (internal block diagram) i.e. the internal structure of IC 555

IC 555 functional block diagram

Fig (1.1) Functional block diagram of IC 555

Here in IC 555, these components are as follows

  1. Comparators
  2. Flipflop
  3. Resistors
  4. Transistor

Now let us study this conceptually about the use of these components

  • Comparators
comparator photo

Fig (1.2) Comparator

  1. A comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. The working of comparator is explained in the above diagram.
  2. In NE 555, the comparators are used as upper comparator (UC) and lower comparator (LC)
  3. These comparators require a reference voltage to give any digital output.
  4. This reference voltage is given by voltage divider circuit which is discussed in next part.
  • Resistors
  1. The three resistors (5k-5k-5k) are used to create a resistive voltage dividing network.
  2. Such that in the above diagram,  voltage at point A is 1/3 Vcc and voltage at point B is 2/3 Vcc which are used for comparison.
  • Flipflop
  1. The flip flop used is SR flipflop which works as follows
  2. When S= 1 then Q = 1 thus the flip flop is set
  3. When R=1 then Q = 0 thus flip flop is reset
  4. The output of UC is connected to Set (S) pin of SR flipflop and the output of LC is connected to Reset (R) pin of SR flipflop.
  • Transistor
  1. The transistor is a NPN BJT (Bipolar Junction Transistor) which is used as a discharge transistor.
  2. It is used for discharging of capacitor through ground and is very essential in multivibrator operations of IC 555.

 

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All you need to know about IC 555


  • All you need to know about IC 555 

 

  • What is IC 555 ?
  1. IC 555 is an monolithic timer IC which can be used for various                applications.
  2. It was introduced in 1971 by an american company called signetics.
  3. In the year 2003, It was estimated that 1 billion units are manufactured every year.
1453706-1

Bunch of timer NE555

 

  • Why the name IC 555 ?
  1. The 555 IC get its name from the three 5k resistors.
  2. These resistors are used as a voltage divider which generates reference voltages for the two comparators (UC and LC) which are fabricated on the IC.
  3. Here’s the actual die photo (published in IEEE spectrum) with the resistors highlighted.
8BdNF

Resistors fabricated on silicon (IC 555)

 

  • What is the basic reason behind its popularity 

 

  • IC 555 is one of the most widely used integrated circuit due to its
  1. Low cost (10-20 Indian rupees)
  2. Large number of applications
    Some of the applications are listed in the next part.

 

  • What are the various applications of IC 555 ?
  1. Timer applications (to introduce a delay)
  2. Multivibrators
  3. Square and triangular wave generators
  4. Ramp signal generator
  5. Voltage controlled oscillator
  6. Pulse width modulator
  7. Missing pulse detector

and many more

 

  • Package
  1. It is a 8 pin DIP (Dual Inline Package) including a ground and Vcc pin for power supply.
  2. The standard 555 package includes 25 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini
    dual-in-line package (DIP-8).

 

  • Why only 5k resistors were used in IC 555 ?

original ic 555

Original IC 555 with 5k resistors highlighted with blue

  1. When 555 IC was first developed in 1971, three 5k resistors were used to develop a voltage divider circuit.
  2. If the reference voltage is to be generated we can even use 10k-10k-10k resitors or some other resistors such as 40k-40k-40k resistors.
  3. The designer had the liberty to select any resistors but why only 5k resistors were selected ?

 

  • Some possible reasons
  1.  Making resistors in silicon is very hard  the available materials are all fairly conductive, so it’s hard to make large-value accurate resistors.
  2. At the time of design of the 555 the minimum feature size was quite large, large enough to be seen with an optical microscope as in that photo.
  3. There’s the additional design constraint that those resistors affect the accuracy of the timer.
  • The choice of resistors for R7, R8, R9 (bipolar version) would be influenced by two things
  1. The desire to minimize power consumption (high value resistors would consume too much chip area)
  2. The desire to minimize temperature variations due to beta changes of darlington pairs.

variant

A 5 volt CMOS version of IC 555 with 40k resistors highlighted

So what do you think about the idea behind choosing the resistors as  5k please let us know in the comments below.


Thanks for reading. Hope you understood it very well. If you have any doubts please comment below. Each and every doubt will be cleared.

Writing this page and making pictorial explanations takes a lot of time and effort. If you like reading this blog, I would request you to subscribe my blog. Also visit my facebook page where I post some stuff related to electronics. Appreciate the support. 🙂

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What are transistors and organic transistors ?


 

Transistors and organic transistors


 

  • What is a transistor ?

In simple words transistor is a three terminal electronic device which is widely used for switching and amplification purposes.

2000px-TO-92,_BC548_(front,_shaded).svg

General purpose BC 548  BJT  (npn transistor)

  • Why was it discovered ?

Transistors have been revolutionary in the field of electronics by allowing engineers to replace bulky vacuum tubes with a tiny and inexpensive component.

 

  • Why is it called transistor ?

Transistor= Transfer + Resistor.
So basically, it is transfer of resistance when transistor operates in its active region the input resistance is high and output resistance is low.
Transistor is a device which transfers its resistance from high to low.

 

Did you know ?

Due to this property transistor amplifies any input signal.
We can control the amplification by changing resistance values of transistors.

20110112

 

  • Analogy 

Consider transistor as a tap which can increase the flow of water applied at its input and you can also regulate the flow at output. You can control the flow of electrons flowing from emitter to collector with the help of base.

 

  • Types of transistors

There are different types of transistors such as JFET, BJT ,heterojunction bipolar transistor, Schottky transistor, avalanche transistor, Darlington transistors.

Transistors-white

Types of transistors

But a type of transistor developed by researchers known as OFET  is rarely heard by anyone.


 

  • Organic field effect transistors 

 

  • Have you ever heard about OFET’s ?

 

  1. It is a part of biodegradable electronics which is a future technology that can prevent generation of organic waste.
  2. OFET’s can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules.
  3. By mechanical transfer of a peeled single-crystalline organic layer onto a substrate.

OFET-2Device_structures

  • Concept of operation

The gate controls the carrier (electrons and holes) movement from the source to the drain.

 

  • How are they prepared?

Thermally oxidized silicon is used to prepare OFET’s .

The silicon dioxide serves as the gate insulator.

The active FET layer is usually deposited onto this substrate using either

(i)   Thermal evaporation

(ii)  Coating from organic solution

(iii)  Electrostatic lamination.

 

The first two techniques result in poly-crystalline active
layers they are much easier to produce, but result in relatively poor transistor performance.

 


Thanks for reading. Hope you understood it very well. If you have any doubts please comment below. Each and every doubt will be cleared.

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A quick reference chart to current parameters


As you know sometimes it becomes quite difficult to remember and re collect the units and dimensions for basic current electricity parameters 

So here is a quick reference to parameter chart and also the relation between them. for any doubts please comment we are here to help you. thank you 🙂

current electricity

 


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ohms law explained


ohms law in a simplified form


The law states that “ As long as the physical state of the conductor remains the same,  the current flowing through the conductor is directly proportional to the voltage (potential difference) applied across it

V ∝ I

V = (constant) I

this constant is “resistance”

thus V = RI

OR  

V=IR


Analogy for ohms law ( to understand )

  • Some people find it difficult to understand ohms law because there exists some unexplained concept.
  • The statement and equation is not sufficient to explain this law.
  • A simplified explanation of ohms law can be given as follows :
ohms law analogy

analogy for ohms law

  • consider two tanks tank A and tank B.
  • tank A is filled with water and tank B is also filled with water.
  • tank A is kept at a particular height as compared to tank B.
  • now both the tanks are connected with a pipe so as to allow the flow of water from tank A to tank B.
  • the water will flow from tank A to tank B due to more pressure in tank A.

 Now two simple observations from this experiment  :

1) if you increase the pressure in tank A, the “water flow” will increase similarly if you increase the voltage across the conductor the “current flow” will increase.

2) if you decrease the diameter of pipe , the “water flow” will decrease similarly the resistance opposes the “current flow” in conductor .

small diameter of pipe will act as a resistance.

The current flowing in a conductor is “analogous” to water flowing from tank A to    tank B.

now you can refer to ohms law that if you increase the voltage the flow of current (opposite to the flow of electrons) will increase as there is some difference in potential between two terminals. if you remember this analogy ohms law is easy to understand.

THANK YOU FOR READING 🙂


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All you need to know about IC 7805


  • What is an Integrated Circuit (IC)?

An integrated circuit (IC) is a semiconductor wafer on which thousands or millions of tiny resistors, capacitors, and transistors are fabricated. An IC can function as an amplifier,oscillator, timer, counter, computer memory, or microprocessor. A particular IC is categorized as either linear (analog) or digital, depending on its intended application.

  • What is 7805 IC?

7805 IC is a part of 78xx series of ICs which are used as voltage regulators.The Regulator ICs need minimum 1.5 higher input voltage than their voltage rating. For example 7805 IC needs minimum 6.5 volts and maximum 35 volts to give 5 volt output.

IC 7805


circuit designs of IC 7805 to monitor the output voltage:

ic7805schem

  • An input voltage ranging from 7.5v  to 35v is connected to pin 1 of IC.
  •  pin 2 of IC is connected to ground.
  • A 100uf capacitor is connected between pin 1 and ground to steady the slow changes in the voltage applied at the input of the circuit.
  • Another 10uf capacitor is connected between pin 3 and ground to steady the slow alterations in the output voltage.
  • The output terminal is connected to pin 3 and a 100nf capacitor is connected between output terminal and ground to bypass very small period spikes to the earth with no influence on the other components..
  • Thus at the output terminal we get 5 v output.

APPLICATIONS:

  • As a portable charger using this circuit with certain modifications.
  • To provide regulated voltage supply to micro controllers.
  • To step  down Ac voltages to Dc voltages using bridge rectifier circuit.

PRECAUTIONS:

  • Do not provide input voltage beyond specified range as with increase in voltage excess electricity is liberated in form of heat from IC 7805 damaging the IC.
  • Use heat sink for input voltages beyond 20-25v.

Thanks for reading. Hope you understood it very well. If you have any doubts please comment below. Each and every doubt will be cleared.

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