In this context, “subtleties” refer to the subtle details, complexities, and peculiarities of the design, analysis, and operation of the common emitter configuration. These complexities include areas such as charge handling, impedance control, response optimization, and understanding capacitance and inductance. Furthermore, they clarify coupling and uncoupling techniques and the importance of bypass arrangements. Understanding these facets is crucial for electronics engineers and hobbyists to build efficient and stable amplifiers. Additionally, it helps those who study electronics understand the underlying principles of transistor-based circuits.
Sender connection principle
= (l C + l b ) + l CBO or
I C (1-α) = αI b + I CBO
I C = (α/(1-α))I b + I CBO /(1-α)
- A common emitter configuration is a transistor amplifier circuit that uses an NPN transistor.
- Base current (IB) is the input current supplied by the base-emitter connection.
- Collector current (IC) is the output current that flows through the collector-emitter connection.
- It provides current amplification, where a small change in IB results in a larger change in IC.
- The output signal at the collector-emitter connection is phase inverted compared to the input signal at the base-emitter connection.
- For stable, linear transistor operation, the correct bias voltage is essential.
- Applications include audio amplifiers, high frequency circuits, and signal processing circuits where weak signal amplification is required.
Common Emitter Configuration
The common emitter circuit is a basic structure of a transistor amplifier widely used in electronic circuits. In this configuration, the emitter terminal is common to the input and output circuits, while the base and collector terminals serve as the input and output terminals, respectively. When a small input voltage is applied to the base terminal, it controls the current flowing between the collector and emitter, thereby amplifying the input signal.
Current Gain Factor
The relationship between the collector current and the base current is called the DC direct current transfer relationship or DC gain. It is denoted by β. Direct current . It was supposed to be DC's beta.
Common emitter DC gain β Direct current = L C /L b
Where I C and I b are the collector and base currents of a given operating point in the linear range, as the value of the base current is in microamperes and low, the value of β Direct current is in the range of 10 to 500 depending on the type of transistor.
Common emitter AC gain β Forward current is the ratio of the small change in collector current ΔI b to constant collector-emitter voltage V CE .
Electric potential and potential difference
Electric potential, also called voltage, is a fundamental concept in electromagnetism. A scalar quantity describes a specific point in an electric field. In simpler terms, it indicates the amount of electrical potential energy that a positive test charge would possess if placed at that specific point in the electric field.
The electrical potential (V) at a point is measured in volts (V). It is calculated as the work done to move a positive test charge (q) to the point from an infinitely large distance relative to the electric field, divided by the charge itself:
V = qW
Where:
- V is the electrical potential in volts (V).
- W is the work done in joules (J).
- q is the test charge in coulombs (C).
Electric potential is fundamental to understanding electric fields and the interaction of charged particles.
Potential difference
The potential difference or voltage difference or voltage drop represents an electric field. It is a vector quantity that indicates the size and direction of the electrical potential difference between two points.
The potential difference (ΔV or V_ab) between two points “a” and “b” is calculated as the change in electrical potential energy experienced by a positive test charge (q) when it moves from point “a” to point "B":
ΔV=Vb−Va
- V_b is the electrical potential at point “b” in volts (V).
- V_a is the electrical potential at point “a” in volts (V).
Potential difference is a fundamental concept in electrical circuits because it determines the flow of electrical charge (current) from a higher potential to a lower potential. It is the driving force behind the movement of electrons in a circuit. It is crucial in several electrical applications, including powering electronic devices and generating electrical currents in power systems.
Relationship between α Direct Current and β Direct Current
Division by I C
I E /I C = 1+(I b /I C )
α = β / (β+1) ——> ii
α(β+1) =β
αβ+α =β
α = β – αβ = β (1 – α)
β = α / (α+1) ——-> iiii
α = β / (β+1)
1-α = 1 – (β / (β+1)) ——>iv
From equation (iii) it can be seen that β approaches infinity as α approaches 1.
This means that the current gain of the transistors in an emitter circuit is very high. For this reason, transistors are used in common emitter circuits.
We know that I C = (αI b / (1-α)) + I CBO / (1-α) ->v
I C = βI b + (β+1)I CBO ——->vi
EU CEO = (β+1)I CBO ——>vii
I C = βI b + I CEO ——->viii
EU CEO = (α / (1-α)) I CBO ——->ix
EU CEO = I CEO / (1-α)
The leakage current in the emitter circuit is greater than that in the base circuit. EU CE is the collector current that flows when the base-emitter circuit is left open and the collector-base junction is reverse biased. It flows in the same direction as the normal collector current through the transistor and is temperature dependent.
Emitter circuit properties
The figure above shows the experimental setup to determine the static characteristic of an NPN transistor in an emitter circuit. Two variably regulated power supplies, VBB and VCC, are connected to the base and collector terminals of a transistor.
Input characteristic (V SER versus me b )V CE = constant
From the input characteristics we can derive the following important points:
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There is a limit, a turn-off voltage Vγ, below which the base current I b is very small. The turn-on voltage value is 0.5 V for Si transistors and 0.1 V for GE transistors.
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After turning on the voltage, the base current is I b increases rapidly with a small increase in the base-emitter voltage V SER . However, it should be noted that the value of the base current does not increase as quickly as that of the input characteristic of a common base configuration. This means that the input dynamic resistance is small in the common emitter configuration, but slightly larger than that in the CB configuration.
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For a fixed value of V SER EU b increases with reduced V CE . A large value of V CE leads to a larger reverse voltage at the collector base PN junction. This increases the depletion zone and reduces the effective base width, thus reducing the base current.
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Dynamic or AC input resistance can be determined from the input characteristic. It is the ratio of a small change in base-emitter voltage to the resulting change in base current at a constant emitter voltage.
Input resistance R Ó = (ΔV SER /ΔI b ) a television CE = constant
Output characteristics (V CE versus i C ) at i b =constant
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The output characteristics of the emitter circuit curves are obtained by recording V CE against I C for different values of I b. The collector current varies with V CE for values between 0V and 1V. The collector current varies with V CE for values between 0V and 1V. After this collector the current is I C becomes almost constant and reaches saturation values. Transistors are operated in the range above the knee voltage. This area is called the active area. The experiment is repeated for different values of I. b .
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Output characteristics can be divided into three areas. They are
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Saturation range
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Exclusion area
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Active region
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In the figure above, the active area is the area to the right of the ordinate V CE = a few tenths of voltage and above I b = 0.
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Ideally, if V CE exceeds 0.7V, the base-collector junction becomes reverse biased and the transistor enters the active or linear region of its operation. Since the base-collector junction is reverse biased, I C stabilizes and remains almost constant for a given value of I b as V CE continues to increase. EU C increases very slightly when V CE increases due to the widening of the base collector depletion zone. This phenomenon is called the proximity effect.
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If the base current I b is zero, there is a small collector current. This is called leakage current. However, in practice, when the base current is zero, the collector current is zero. In this condition, the transistor is said to be off. The small collector current is called reverse collector current.
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If you CE reaches a sufficiently high voltage, the reverse-biased collector junction collapses and therefore the collector current increases rapidly. If you CE voltage exceeds 40 V, the collector diode breaks down and the normal function of the transistor is lost. The transistor is not intended to operate in the breakdown region. This effect is commonly known as the punch-through effect.
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The dynamic output resistance can be determined from the output characteristic curve.
Principle of electronics
Dynamic output resistance R out = (ΔV CE /ΔI C ) at I b = constant
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The reciprocal of the slope of the output characteristic in this range gives the output resistance. The Rout value is between 10KΩ and 50KΩ.
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The output characteristic can be used to determine the current gain of the small signal emitter or AC beta (β). Alternating current ) from a transistor. This can be done by selecting the two points M and N on the characteristic curve and writing down the corresponding values of ΔI. E and ΔI b .
