COMPARISON OF SCR BJT MOSFET

We all know about SCR, BJT, MOSFET here we going to familiarize the different characters of each one.

• SCR

It is also called thyristor.
 It have 4 terminals.
Minority carrier device.
High voltage high current device.
Low switching speed.
Low resistive input impedance.
Only single pulse required for turn on.
Can be connected in series easily with voltage equalizing circuits.
It have thermal run away.
Can be paralleled with forced current sharing circuits.
Most robust devices.
Less temperature sensitive.
No second breakdown.

Phototransistor

It is the device which works on the principle of absorption of of light is phototransistor. Which is more sensitive than a pn photodiode. The phototransistor is an npn transistor usually connected in  CE configuaration with the base kept open. The radiation is allowed to fall near the junction.
If light radiation is now to fall near th collector junction additional minority carriers are photo generated. These carriers contribute to the reverse current in exactly the same manner, as do the thermally generated minority carriers.

WORKING OF GENERAL TRANSISTORS NPN & PNP

We all know about the transistors they have most important role in electronic field. Here am going to explain about how the transistor work , both PNP and NPN. Mostly the transistors used as switches in electronic circuits.

 NPN means two N layers are sandwitched between one P layer. N region are rich in  with free electrons . And P region have holes and less number of free electrons. P region is the base in NPN transistors. When NPN  transistor in free state they don't have connection with collector and emitter because of the P region between it.

MODULATION AND CLASSIFICATION

Modulation is the very important technique used in communication system. The purpose of modulation system is to deliver a message signal from an information source in recognizable form to a user destination. For this the transmitter modifies the message signal into a suitable for transmission over the channel. This modification is achieved by means of a precise called "modulation". The modulation involves varying some parameter of a carrier wave in accordance with the message signal. The carrier is the high frequency wave than the message signal. The receiver recreate the original signal from the modulated signal. The recreation is done by the process called "demodulation". It is the reverse process of modulation.


 Classification of Modulation

 
.In continuous modulation , a sinusoidal wave is the carrier.
 .AM- When the amplitude of the carrier wave is change in accordance with the message signal , it is called amplitude modulation.
 .FM- The instantaneous frequency of the the carrier is change in accordance with the amplitude of the message signal.
 .AM- The angle of the carrier is change in accordance with the carrier.
 .PM- The phase of the carrier is change in accordance with the amplitude of the message signal.
 .Angle Modulation- The angle of the carrier is change in accordance with the carrier. In pulse modulation , the carrier is the periodic sequence of rectangular pulses.
 .PAM-In which carrier signal is sampled at regular intervals and each the signal is made proportional to the amplitude of the amplitude of the message signal.
 .PWM- It is also called pulse duration modulation or pulse length modulation. In this the width of each pulse is made proportional to the amplitude of the signal at the instant.
 .PPM- The amplitude and width of the pulse is kept constant and the position of the each pulse is varied in accordacne with modulating signal.

COLOUR CODING OF CAPACITORS

In electronic field the most important one thing is knowing componets and the values of that conponent. In this we going to familiarising with calculation of capacitance of capacitor .
In small value capacitors like mica capacitors , value of capacitors will be written on it in a coded form. The method of reading the value is explained below.

ASTABLE MULTIVIBRATOR USING NE555

In this am going to explaining about astable multivibrator using ne555. Am already explained the astable multivibrator using BC107 and OPAMP in my previous post.




The working of astable using ne555 is starting by the capacitor C.
Initially capacitor C starts charging through Ra and Rb towards Vcc with a time constant (Ra+Rb)C. Durring this time R=0, S=1, Q*=0 and ouput is high. When capacitor voltage equals (2/3)Vcc the upper comparator triggers the control flip flop so that Q*(Q bar)=1 . This makes transistor Q1 ON and the capacitor C starts discharging towards ground through Rb and transistor Q1 with a time constant RbC.


Durring thr discharge of the timing capacitor C, a it reaches Vcc/3 the lower comparator is triggerd and at this stage S=1, R=0, which turns Q*=0 . This makes transistor Q1 OFF and again capacitor C starts to charge. Thus the capacitor periodically charges and discharges between (2/3)Vcc . Output amplitude of the square wave swings between 0.3V to applied Vcc.



The charging periode of the capacitor Tc=0.69(Ra+Rb)C . The discharging periode of Td=0.69RbC . If Tc is equal to Td , the duty cycle of the output waveform will be 1/2 . In order to make Tc and Td equal to each other , a diode is connected in parallel with Rb such that capacitor charges through Ra and diode, and discharges through Rb . Another method is using a resistor of same value of Ra and Rb between the pin number 7 and the junction formed by Ra and Rb. The difference we can seen in the output waveform.



Components for wiring this circuit


IC NE555
Ra - 6.8K
Rb - 6.8K
C - 0.1μF
C1 - 0.01μF
Vcc - 10V

FUNCTIONAL BLOCK DIAGRAM OF NE555

In this we are going to study about functional block diagram of ne555. It is also called 555 timer. This IC used for generating accurate time delay or oscillation. The maximum operating frequency is excess of 500kHz. The 555 timer can be used with supply voltage in the range from +5V to +18V and can drive load up to 200mA.




Functional diagram of 555




The 5K intrenal resistance act as a voltage devider network, providing (2/3) Vcc at inverting terminal of upper comparator (UC) and (1/3)Vcc at the noninverting terminal of lower comparator (LC). In the stable state , the Q*(Q bar) output of the control flip flop is high. This makes the output low because of the buffer which basically is an inverter.


Pin 1 - It is ground

Pin 2 (Trigger) - This pin is used to feed the trigger input when the chip is set up as a monostable multivibrator. When a trigger of amplitude greater than (1/3)Vcc is applied to this terminal , circuit switches to quasi stable state.

Pin 3 - This is output

Pin 4 (Reset) - This terminal is used to reset the output of the circuit irrespective of th input. A logic low input will be reset the output. For normal operation this pin is connected to Vcc.

Pin 5 (Control) - Viltage applied to this terminal will control the instant at which the comparator switches and hence the pulse width of the output. When this pin is not used it is bypassed to ground using a 0.01μF capacitor.

Pin 6 (Threshold) - If the voltage applied to the threshold terminal is greater than (2/3)Vcc, upper comparator switches to +Vsat and flip flop output get reset.

FAMILIARIZATION OF MULTIMETER

Multimeter is a very popular electronic measuring device. It is an essential equipment to troubleshoot an electronic circuit.
Multimeter is used to measure various electrical quantities such as voltage , current and resitance. It work in different modes namely ammeter, voltmeter and ohmmeter. The mode can be set using a selector switch provided in the multimerer.

Both AC and DC voltages and current can be measured using multivibrator. They are available in two types analog and digital. Analog multimeter have a needle moving over a caliberated scale while the digital multimeter give the digital display of the measured quantities.

FAMILIARISATION OF OP-AMP

Operational amplifier, in short , op-amp is a versatile device used to amplify AC and DC signal. Though it was originally designed for computing mathematical operations such as addition, multiplication, differentiation, integration etc., it is widely used for veriety of applications like oscillators ,fillters regulators, clipping circuits waveform generators etc.
The symbol of op-amp represent a circuit with two input , an output terminal and two bias supply points.
The 741 it is frequency compensated and shot circuit protected IC. 741C is its commertial version of the operating temperature range from 0 to +70degree C.
741 need +ve and -ve dc source for bias supply connections V+ and V- . This is provided by either a dual power supply or two power supplies. When dual power supplies used its positive terminal.is connected to V+ pin of the ic and -v terminal is connected to V- pin of the IC. The ground terminal of the dual power supply is connected to the ground point of the circuit. When two power supplies are used +ve terminal of the power one  supply and -v terminal of other supply are connected to V+ and V- pins of IC rspectivly. Remaining trminals of the power supplies are connected ti the groun point.of the circuit. When several op-amp are used in a circuit, one power supply is enough to drive all op-amps because of the low power consumption of op-amps.
The output saturation voltages in general are about 2V below the magnitude of the dc power supply levels.

SCHMITT TRIGGER CIRCUIT USING BC107

The schmitt trigger is an emitter coupled bistable multivibrator with its cross-coupling is removed . It is comparator that is used to convert a periodical random analog wave to square wave having the same frequency of the analog wave. Due to this schmitt trigger is called a squaring circuit.
In this circuit diagram output of this circuit goes high level whan the amplitude of the input goes have above a predetermined level called upper treshold point(utp) output of this circuit goes low level when the amplitude of the input signal goes below a predetermined level called lower treshold point (ltp). The schmitt trigger compares the input analog wave form  with respect to the preset values of UTP and LTP . So it is known as two level comparator.
Without any input signal , transistor Q1 stays in state and Q2 in saturation state. Current Ie2 flows trougth the common emmiter resistor Re causing a potential drop equal to Ie2Re. This point called UTP. When the input amplitude increase and reaches UTP Q1 turns ON. Subsequently Q2 turns OFF and the output raises to Vcc. Now current Ie2 become zero and Ie2 starts flowing through Re. The minimum voltage required to hold the transistor Q1 ON is equal to Vbe cut in + Ie1Re1. When the amplitude of the input sinwave become lessthan this, Q1 turns off, in turns Q2 turns on and the output voltage drops. The value of the input voltage at which transistor Q2 turns on is called LTP. 
components for wiring this circuit
Q1 = Q2= BC107
Rb = 47K
Rc1 = 4.7K
Rc2 = 1.5K
R1 = 10K
R2 = 33K
Re = 3.3K
C1 = 4.7pF
Vin = 20Vpp
Vcc = 8V
Vo= output

SELECTIVE FILTER CIRCUIT

Filters are networks designed to pass only certain desired frequency band. It can be broadly classified as passive or active filtes according to the device used to implement them. High pass and low pass filters are there.
It is a combination of low pass and high pass filtes. For the selection of particular frequency. Low pass filter passes low frequency readily and attenuates high frequencies. High pass filtes passes high frequency and attenuate low frequencies.
     
This circuit is designed by the equation f=1/2 pi RC .
f= input signal frequency.
Low pass filter passes a frequency less than f1 and high pass filtes passes frequency greater than f2.  R1, C1 is for low pass and R2, C2 for high pass. By selection  frequency in between two frequency application, high frequency taken as f1 and low frequency taken as f2. And finding the different values C1 for f1 and C2 for f2. And design the circuit like this.   Input is 5Vpp.
R1=6.8K
R2=6.8K

Jones chopper

Jones chopper is an example of class D commutation in which a charged capacitor is switched by an auxilary SCR to commutate the main SCR. In this circuit SCR1 is the main switch and SCR2 is the auxilary switch which is of lower capacity than SCR1 and is used to commutate SCR1 by a reverse voltage developed across the capacitor C. "The special feature of the circuit is the tapped autotransformer T through a portion of which the load current flows".
Working   
        When T1 is ON, capacitor C discharges resonantly through T1, L1, D1 . This discharge current doesnot flow through L2 and back to the battery because of transformer action of T. The load current is picked up by T1 and the freewheel diode D1 is reverse biased. As the capacitor voltage swings negative, the reverse bias on diode D2 decreases.  This continues upto a time  pi(L1C)^1/2.
            When T2 is on the negative voltage on capacitar C is applied across T1 and it becomes OFF.The load current which is normally constant starts to flow in T2 and capacitor C. The capacitor C charged positively at first upto a voltage equal to supply voltage Vdc.The freewheel diode become forward bias and begins to pickup load current. And capacitor current starts to reduce. After this the energy 1/2LI^2 is the inductance L2 is forced in to the capacitor C.Charging is positively to 1/2CV^2  the capacitor current continues to decrease as a result current through T2 decreases gradually become OFF. The cycle repeat when T1 is again turned ON.
Advantage
The main advantage of JONES chopper over other  the circuit is that
* It allows the use of higher voltage and lower microfarad commutating capacitor. This is because the trapped energy of inductor L2 can be forced in to the commutating capacitor rather than simply charging the capacitor by supply voltage.
* In this circuit there is no starting problem and anyone of the SCR can be turned on initialy there is great flexibility in condrol also.

Wein bridge oscillator using JFET

The wein bridge osillator employs as balanced wein bridge as the feedback network. Two stage common source amplifier provide 360 degree phase shift to the signal.
  The attenuation of the bridge is calculated to be 1/3 at resonant frequency. There for the amplifirer stage should provide a gain of 3 to.make loop gain unity. Gain preferred to slightly greater than 3 to compensate for the losses occuring in the circuit. Since gain of two stage amplifier is the product of individual stages, over all gain may become very high.
components for wiring the circuit
J1=J2= BFW10
Rd1 - 2.2K
Rd2 - 2.2K
Rs1 - 1K
Rs2 - 1.2K
Rg - 1M
R - 1.5k
Cc1 - 10F
Cc2 - 10F
C - 0.01F
Vdd - +12V

Oscillators

An oscilletor is an electronic device which convert dc power from the supply into ac power in the load. An oscillator generates a signal with out any external input. These are classified in different ways.
According to the principle involved: the oscillations can occur either through +ve feed back mechanism or negative resistance effect.
According to the type of wave form produce: oscillator can generate either sinwave as output or any waveform other than sinusoidal.
According to the feed back circuit: The feed back circuit may contain resistors or capacitors or may contain inductors or capacitors for frequency varietion.
According to the frequency of output signal: the oscillation can be audio frequency AF oscillator (upto 20Kz) or radio frequency oscillators (upto 300MHz) or ultra high frequency UHF oscillators (upto 3GHz) or microwave oscillators (above 3GHz).

Frequency response of an Amplifier

It is a high pass RC circuit. Frequency response indicates the variation of gain of an amplifier for different signal frequencies. Usually , an amplifier can't respond uniformly for all freqencies. Depending on the type of the amplifier the variation differs. In general, the performance of an amplifier remains uniform for mid frequencies. This is because coupling capacitors , bipass capacitors and junction capacitors have negligible effect at mid frequencies. At, low frequencies, the gain of the amplifier is limited by coupling capacitors and emitter by pass capacitors whereas at high frequencies, it is limitted by junction capacitance ot the transistor.
  In general, the low frequency response of an amplifier is governed by the high pass circuit.
Specifications
Vi - input voltage
Vo - output voltage
C - capacitor
R - resistor