BJT AC Modelling

In amplifier output sinusoidal signal is greater than the input sinusoidal signal, or the output ac power is greater than the input ac power.
The question then arises as to how the ac power output can be greater than the input ac power.
Conservation of energy dictates that over time the total power output, Po , of a system cannot be greater than its power input, Pi , and that the efficiency defined by h = Po>Pi cannot be greater than 1.
But here output ac Power is due to both input ac power & dc biasing both.
The control mechanism is such that the application of a relatively small signal to the control mechanism can result in a substantial oscillation in the output circuit.
The superposition theorem is applicable for the analysis and design of the dc and ac components of a BJT network, permitting the separation of the analysis of the dc and ac responses of the system.
one can make a complete dc analysis of a system before considering the ac response and once the dc analysis is complete, the ac response can be determined using a completely ac analysis.

BJT TRANSISTOR MODELING

A model is a combination of circuit elements, properly chosen, that best approximates the actual behavior of a semiconductor device under specific operating conditions.
Once the ac equivalent circuit is determined, the schematic symbol for the device can be replaced by this equivalent circuit and the basic methods of circuit analysis applied to determine the desired quantities of the network.
Hybrid equivalent network was employed the most frequently.
Specification sheets included the parameters in their listing, and analysis was simply a matter of inserting the equivalent circuit with the listed values.
The drawback to using this equivalent circuit, however, is that it is defined for a set of operating conditions that might not match the actual operating conditions.
the use of the re model became the more desirable approach because an important parameter of the equivalent circuit was determined by the actual operating conditions.
To understand the effect that the ac equivalent circuit will have on the analysis to follow, consider the circuit of Fig

Because we are interested only in the ac response of the circuit, all the dc supplies can be replaced by a zero-potential equivalent (short circuit) because they determine only the dc (quiescent level) of the output voltage and not the magnitude of the swing of the ac output. This is
clearly demonstrated by Fig.

The coupling capacitors C₁ and C₂ and bypass capacitor C₃ were chosen to have a very small reactance at the frequency of application.
Therefore, they, too, may for all practical purposes be replaced by a low-resistance path or a short circuit.
This will result in the shorting out of the dc biasing resistor R_E .
By defining Z_i, Z_o, I_i, and I_o, establishing a common ground and rearranging the elements the equivalent circuit is shown below.

therefore, the ac equivalent of a transistor network is obtained by:

Setting all dc sources to zero and replacing them by a short-circuit equivalent
Replacing all capacitors by a short-circuit equivalent
Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 and 2
Redrawing the network in a more convenient and logical form

THE re TRANSISTOR MODEL

Common Emitter Configuration

The equivalent circuit for the common-emitter configuration will be constructed using the device characteristics and a number of approximations.
Starting with the input side, we find the applied voltage Vi is equal to the voltage V_be with the input current being the base current I_b.

Because the current through the forward-biased junction of the transistor is I_E , the characteristics for the input side appear simply as that of a forward-biased diode.

For the equivalent circuit, therefore, the input side is simply a single diode with a current Ie.

If we redraw the collector characteristics to have a constant β(another approximation) and the entire characteristics at the output section can be replaced by a controlled current source whose magnitude is β times of the base current as shown in Fig.

The equivalent model can be difficult to work with due to the direct connection between input and output networks.
It can be improved by first replacing the diode by its equivalent resistance as determined by the level of I_E.
The diode resistance is determined by r_D = 26 mV/I_D.
Using the subscript e because the determining current is the emitter current will result in re=26mV/I_E.

The result is that the impedance seen looking into the base of the network is a resistor equal to β times the value of r_e , as shown in Fig.
The collector output current is still linked to the input current by β as shown in the same figure.
Improved BJT equivalent circuit is given below.

but aside from the collector output current being defined by the level of beta and I_B , we do not have a good representation for the output impedance of the device.
In reality the characteristics do not have the ideal appearance of parallel straight line.
Rather, they have a slope that defines the output impedance of the device.
The steeper the slope, the less the output impedance and the less ideal the transistor.
In any event, an output impedance can now be defined that will appear as a resistor in parallel with the output as shown in the equivalent circuit.

Common Base Configuration

For the common emitter configuration the use of a diode to represent the connection from base to emitter is same for the common base configuration.
The pnp transistor employed will present the same possibility at the input circuit.
The result is the use of a diode in the equivalent circuit as shown in Fig.
For the output circuit, we find that the collector current is related to the emitter current by alpha.
In this case, however, the controlled source defining the collector current is opposite in direction to that of the controlled source of the common emitter configuration.
The direction of the collector current in the output circuit is now opposite that of the defined output current.
For the ac response, the diode can be replaced by its equivalent ac resistance determined by r_e = 26mV/I_E.
An additional output resistance can be determined from the characteristics in much the same manner as applied to the common emitter configuration.
In general, common base configurations have very low input impedance because it is essentially simply re .
Typical values extend from a few ohms to perhaps 50 ohm.
The output impedance r_o will typically extend into the megaohm range.
Because the output current is opposite to the defined Io direction, you will find in the analysis to follow that there is no phase shift between the input and output voltages.
For the common emitter configuration there is a 180 degree phase shift.

Common Collector Configuration

For the common collector configuration, the model defined for the common emitter configuration is normally applied rather than defining a model for the common collector configuration.

COMMON EMITTER FIXED BIAS CONFIGURATION

The input signal V_i is applied to the base of the transistor, whereas the output V_o is off the collector.
In addition, the input current I_i is not the base current, but the source current.
The output current I_o is the collector current.

The small signal ac analysis begins by removing the dc effects of V_CC and replacing the dc blocking capacitors C₁ and C₂ by short circuit equivalents, resulting in the network of Fig.

Substituting the re model for the common-emitter configuration results in the network of Figure shown below.

The negative sign in the resulting equation for Av reveals that a 180 degree phase shift occurs between the input and output signals.

Electrical technology is used for generating, storing, regulating, transferring, and using electrical energy for solving different purpose in real life. Example: power plant generators, Air Conditioners, electric light bulb, Electric Fan, Electric Heater etc.

Conventional & Non-conventional ways of Electricity Generation:

Most electricity in the world is conventionally generated. Example: using coal, oil, natural gas, nuclear energy, or hydropower

Electricity is normally generated in Power plants.
Capacity of a Powerplant is the amount of electricity it can produce when it is running at full blast. It is other wise known as name plate capacity.
Name plate capacity or the maximum amount of power is typically measured in megawatts (MW) or kilowatts (KW).
The energy produced is defined in the units of KWh (Kilowatt Hour) or MWh(megawatts hour). It can be defined as:
Total KWh energy consumed= KWh rating X Number of hours it worked.
Capacity factor (net) of a Power plant: The ratio of the net electricity generated, for the time considered, to the energy that could have been generated at continuous full-power operation during the same period.
Example: Let a Powerplant has nameplate capacity of 3,942 MW & its annual generation was 31,200,000 MWh

Hydropower plants

Hydropower plants use water to generate electricity. When flowing water is captured and turned into electricity, it is called hydroelectric power or hydropower.
There hydroelectric facilities are powered by the kinetic energy of flowing water as it moves downstream.
Turbines and generators convert this kinetic energy into electricity, which is then fed into the electrical grid to be used in homes, businesses,
and by industry.
Tehri Hydropower Complex in Uttarakhand is the largest hydroelectric power plant in India with a capacity of 2400MW (Mega Watt)

Thermal Power station

A thermal power station is a power station in which heat energy is converted to electricity.
Water is heated into steam, which is used to drive the turbine of electrical generator.
After it passes through the turbine the steam is condensed in a steam condenser and recycled to where it was heated.
This is known as a Rankine cycle.
Coal has been used widely in thermal power station to heat the water & steam generation for decades.
Use of Gases have also increased now a days as an alternative of Coal because of its low price.
Vindhyachal Thermal Power Station in Madhya Pradesh is the biggest thermal power plant in India, with an installed capacity of 4,760MW.

Nuclear powerplants

Nuclear powerplants are also a type of Thermal Power plant.
Nuclear Power plant satisfies both economic and environmental protection goals.
So, the energy produced from Nuclear Power plant is a clean & green
energy.
Heat source for a Nuclear Power plant is the fission reaction of radioactive elements in a nuclear reactor.
This heat of nuclear reactor heats the reactor coolant which may be water or gas, or even liquid metal, depending on the type of reactor.
The reactor coolant then goes to a steam generator and heats water to produce steam.
The pressurized steam is then fed to a multi-stage steam turbine for electricity generation.
Uranium-238 (U-238) and Uranium-235 (U-235) are normally used as fuel for fission reaction
Kudankulam Nuclear Power Plant in Tamil Nadu is the highest capacity nuclear plant in India with an installed capacity of 2000 MW.

Nuclear powerplants

There are various Renewable Energy Sources also present which contributes for electricity generation.
The conversion of solar radiation directly into electrical power is done in Solar Power Plants.
Thermal energy is transformed into electrical energy using photovoltaic panels.
Large number of panels are installed in an optimal configuration and harvest light energy from the sun and convert it into electrical energy
which feeds into the grid.
Harvested thermal energy is converted into direct current (DC) electricity using solar panels.
To convert this direct current (DC) electricity to alternating current (AC) electricity, an essential component inverter is used.
Inverter is a component which converts direct current (DC) electricity to alternating current (AC) electricity.
Bhadla Solar Park of 2,250MW is India’s biggest solar power plant in the state of Rajasthan.
Wind mill converts the kinetic energy of moving air into mechanical energy that can be either used directly to run the machine or to run the
Induction generator to produce electricity.
Electricity produced from Tides of sea is known as Tidal Energy.