Transistor Terminals

 Transistor  terminals:

1.Emitter

2.Base

3.Collector

Emitter:

It is left hand section (or region) of the transistor and its main function is to supply majority charge carriers(electrons in case of NPN transistors and h

oles in case of P-N-P transistors) to the base.The emitter is always forward biased w.r.t base so that it is able to supply majority charge carriers to the base.The emitter is heavily doped so that it may be able to inject a large number of charge carriers.It is of moderate size in order to maintain heavy doping without diluting it or mesh formation in it.

Base:It is the middle section of the transistor and is very lightly doped to reduce the recombination within the base so as to increase collector current and is very thin(of the order of micrometer)in comparison to either emitter or collector so that it may pass most of the injected charge carriers to the collector.

Base form two junctions i.e,emitter junction and collector junction,each having its own barrier voltage.The forward-biased emitter-base junction offers low resistance to the emitter current whereas the collector-base junction,which is reverse biased,offers high resistance to the collector current.As the resistance of emitter-base junction is very small as compared to that of collector-base junction,therefore,the forward bias applied to the emitter-base  junction is usually very small whereas the reverse bias on the collector-base junction is much large.

collector:

It is right hand section of the transistor and its main function is to collect majority charge carriers.Collector is always reverse biased so as to remove the charge carriers away from  its junction with the base.It is moderately doped to avoid the chances of mesh formation even after taking the carriers from the emitter.Large in size to withstand the temperature generated at the collector.

FM Receiver

FM receiver invariably make use of superheterodyne configuration,and the basic block diagram is the same as that for an AM receiver.However,the antenna,RF amplifier and local oscillator usually operate in the VHF of 88-108 MHz and have an intermediate frequency (IF) of 10.7 MHz  with a bandpass of 2 X 75 kHz i.e.,150kHz.

In both AM and FM receivers,the purpose of stage following the IF amplifier is to extract the information from the carrier signal.Since the information was originally impressed on the carrier by modulating its frequency,the FM detector must sense frequency-not amplitude-variations.Thus,basic difference between FM and AM receiver lie in the detector (or demodulator) circuit.Also,since FM signals may occupy a wider band for given modulating signal,the RF and IF bandwidths are typically greater.Since FM is usually confined to higher frequencies,this wider bandwidth need is  more tolerable .

The audio portion of an FM receiver is identical to that for an AM receiver,except that the audio portion of an FM broadcast receiver is usually capable of amplifying without distortion a wider frequency range than an AM broadcast receiver-typically 15 kHz versus 5 kHz.

The functions of components other than limiter and de-emphasis network are the same as explained in case of superheterodyne receiver.In FM receiver limiter is provided to remove all amplitude variations,caused by noise from IF signal which might have crept into the FM signal.The de-emphasis network is provided to reduce the amplitude of high frequencies in the audio signal which was earlier increased at the transmitting station. 

Essentials of an Oscillator circuit

Oscillator circuit must have the following three elements:

1.Oscillatory circuit or frequency determine element.

2.Amplifier.

3.Positive feedback network.

The Oscillatory circuit or a frequency-determining element can  be an inductance-capacitance network(L-C tank),resistance-capacitance (R-C network,or a quartz crystal,depending on the frequency and waveshape desired.

The electronic amplifier receives dc power from the battery or dc power supply and converts it into ac power for supply to the tank circuit.The oscillations occurring in the tank circuit are applied to the input of the electronic amplifier.Because of the amplifying properties of the amplifier,we get increased output of these oscillations.This amplified output of oscillations is because of dc power supplied from the external source( a battery or power supply).The output of the amplifier can be supplied to the tank circuit to meet the losses.

The feedback network supplies a part of the output power to the frequency-determining element in correct phase to aid the oscillations.In other words,feedback circuit must be a positive  feedback network.The block diagram of an oscillators is shown in fig.

Types of Transistor Oscillators

A transistor can be operated as an oscillator for producing continuous undamped oscillations of any desired frequency if tank and feedback circuits are properly connected to it.All oscillators under different names have similar function i.e.they generate continuous undamped output.However,they differ in methods of supplying energy to the tank or oscillatory circuit to meet the losses and the frequency ranges over which they are used.

The frequency spectrum over which oscillators are employed to produce sinusoidal signals is extremely wide (from less than 1 Hz to many GHz).However,no signal oscillator design is practical for generating signals over this entire range.Instead,a variety of designs are employed,each of which generates sinusoidal outputs most advantageously over various portions of the frequency spectrum.

Oscillators,which use inductance-capacitance(L-C)circuits as their tank or oscillatory circuits,are very popular for generating high frequency (e.g.10 kHZ to 100 MHz) outputs.The most widely used LC oscillators.Although they slightly differ from one another in their electronic circuitry but they have virtually identical frequency ranges and frequency and frequency-stability characteristics,However,such oscillators are not suitable for generating low-frequency sinusoidal outputs.This is due to the fact that the components required in construction of low-frequency L-C resonant circuits are too bulky and heavy. So resistor-capacitor(R-C) oscillators are generally employed for generating low-frequency sinusoidal signals.Two most common R-C oscillators are the Wien bridge and phase and phase shift types.

Type of oscillator                                                                          Frequency Ranges 

Wien bridge oscillator                                                                1 Hz-1 MHz

phase shift oscillator                                                                   1Hz-10 MHz

Hartly oscillator                                                                           10 kHz-100 MHz

Colpitt's oscillator                                                                       10 kHz-100 MHz

Negative resistance oscillator                                                        > 100 MHz

Crystal oscillator                                                                          Fixed frequency

Frequency Stability Of Oscillator

An Oscillator having initially been set at a particular frequency does not maintain its initial frequency,but instead drift and wander about in frequency,sometime uniformly in one direction,sometimes quite erratically.The frequency stability of an oscillator is a measure of its ability to maintain as nearly a fixed frequency as possible over as long a time interval as possible.These deviations in frequency are caused due to variations in the values of circuit features(circuit components,transistor parameters,supply voltage,stray capacitance,output load etc.)that determine the oscillator frequency.

As the circuit operates,it warms up.Consequently the values of resistors,inductors and capacitors,which are frequency determining factor in such circuits,are changed with temperature.Variations in temperature also cause variations in transistor parameters.However,the variation or drift in oscillator frequency is slow because of slow variations in R,L,C and transistor parameters with time.

The other major factor responsible for deviation in frequency is variations in power supply(operating voltage applied to the active device).However,this problem can be overcome by using regulated power supply.

Any variation in load coupled to the tank circuit may cause a change in effective resistance of the circuit by transformer action which,in turn,causes the drift in frequency.

Other factors responsible for drift in oscillators frequency are:

Inter-element capacitance,operating  point of active device and mechanical vibrations.The effects of variations in inter element capacitance can be neutralized introducing a swamping capacitor across the offending elements (the introduced capacitor becomes the part of the tank circuit).The operating point Q of the active device is so chosen as to restrict the operation of the device on the linear portion of its characteristic because operation on nonlinear portion causes the variations in transistor parameters which,in turn,affect the oscillator frequency stability.The instability in oscillator frequency caused due to mechanical vibrations can be avoided by isolating the oscillator circuit from the source of mechanical vibrations.

TRANSISTOR CIRCUIT CONFIGURATIONS

A transistor is a tree-terminal device (having three terminals namely emitter,base and collector)but we require four terminals-two for the input and two for the output for connecting it in a circuit.Hence one of the terminals of the transistor is made common to the input and output circuits.Thus there are three types of configurations for operating of a transistor.These configurations are common base(CB),common emitter(CE) and common collector(CC) configurations.The term 'common' is used to denote electrode that is common to input and output circuits.Since the common terminal is usually grounded,these configurations are also referred to as grounded base,grounded emitter and grounded collector configurations,as shown in fig.1(a),(b) and (c) respectively.Each configuration has its own merits and demerits.It is to be remembered that regardless of the circuit configurations,the emitter is always forward biased and the collector is always reverse biased.

Optoelectronics

Optoelectronics is the technology that combines optics and electronics and the devices based on this technology are known as optoelectronic devices.These devices include emitters,sensors and optocouplers or optoisolators.These devices may be categorised as follows:

1.Devices that detect optical signals through electronic processes,such as photodetectors.

2.devices that convert optical radiation into electrical energy,such as photovoltaic device or solar cell.

3.Devices that convert electrical energy into optical radiation,such as light-emitting diodes(LEDs) and the LASER(Light Amplification by Stimulated Emission of Radiation) diode.

The devices that convert electrical energy into optical radiations are known as emitters.The devices that convert optical radiations into electrical energy or detect optical signals through electronic process fall under the category of sensors.The optocouplers combine light generating and light sensitive units in a signal package.The present-day family tree of optoelectronic devices is shown in fig.

solar cells

The characteristics of photodetectors and solar cells are dependent on optical energy,which is absorbed in a semiconductor and creates excess EHPs generating photocurrents. photodetectors are the semiconductor devices that can be used to detect the presence of photons and convert optical signals into electrical signals.

example of  optoelectronic