unit BJT.pdf

# Bipolar Junction Transistor (BJT) ## Introduction - The transistor was invented by Dr. William Shockley and Dr. John Bardeen at Bell Laboratories in America in 1951. - For the first time in 1952, the transistor was used in a telephone switching circuit. Since then, it has revolutionized the fiel...

# Bipolar Junction Transistor (BJT) ## Introduction - The transistor was invented by Dr. William Shockley and Dr. John Bardeen at Bell Laboratories in America in 1951. - For the first time in 1952, the transistor was used in a telephone switching circuit. Since then, it has revolutionized the field of electronics. - The transistor has replaced bulky vacuum tubes in most of the electronic circuits. - The transistor is a basic building block of all modern electronic systems. - It is a three terminal device. - The output voltage, current, and power are controlled by the input current in a transistor. Therefore, it is also called a current controlled device. ## Transistor configuration - **Emitter:** It is an outer region situated in one side of the transistor. - The function of the emitter is to inject charge carriers (electrons in the case of NPN transistors and holes in the case of PNP transistors) into the base. - Since the emitter has to supply a large number of charge carriers, so it is heavily doped. The more the doping, the more charge carriers will be available. - **Base:** It is the middle region of the transistor. The base is a very thin and lightly doped region. The function of this base region is to pass all the charge carriers (electrons and holes) onto the collector. - **Collector:** It is the other outer region situated in the other side of the transistor. - The doping of the collector is between the heavy doping of the emitter and light doping of the base. - The function of the collector is to collect charge carriers (electrons and holes). - The collector region is physically larger than the emitter region. ## Transistor Symbols There are two types of transistors: NPN and PNP - **NPN:** Emitter (N), Base (P), Collector (N) - **PNP:** Emitter (P), Base (N), Collector (P) ## Types of transistors - Out of these two types of transistors, the NPN transistor is mostly used. The reason for this is that in NPN transistors, the current conduction is mainly by electrons, whereas in PNP transistors, the current conduction is mainly by holes. Since electrons are more mobile than holes, the conduction is higher in NPN transistors. ## Transistor as a two port network - A general two port network is shown. But in a BJT, it is a three terminal device where one terminal is made common to the input and output. - The direction of the voltage and currents entering is positive, and outward is inverse. ## Transistor circuit configurations There are three transistor circuit configurations namely: - Common base configuration - Common emitter configuration - Common collector configuration ## Unbiased transistor - When no battery is connected between the different terminals of a transistor. ## Biasing of a transistor - When DC voltage is applied across the different terminals, then this process is called biasing. - Since there are two junctions in a transistor (emitter-base junction (EBJ) and collector-base junction (CBJ)), there are four possible ways of biasing for those two junctions. - These possible ways are also called modes of operation of a transistor. | Case | Emitter-base Junction | Collector-bare Junction | Region of Operation | |---|---|---|---| | 1 | Forward-biased | Reverse-biased | Active | | 2 | Forward-biased | Forward-biased | Saturation | | 3 | Reverse-biased | Reverse-biased | Cut-off | | 4 | Reverse-biased | Forward-biased | Inverted | ## Common Base Amplifier - **DC load line:** With the help of the DC load line, we can easily analyze the performance of an amplifier circuit. - **Quiescent Condition:** The condition of having no input AC signal is also known as the Quiescent Condition. - **Applying KVL** to the collector portion of the circuit: $V_{CC} = I_CRC + V_{CE}$ ## Selection of operating point - To study the effect of biasing condition on the performance of a transistor, it is necessary to draw a DC load line on the output characteristics of a transistor. - Show three operating points, these are: - Q1: If point Q1 is near the saturation region, therefore, the positive cycle of the signal will be clipped off. Hence, distortion will occur. Thus, Q1 is not the suitable operating point. - Q2: If point Q2 is near the cut-off region, therefore the negative cycle will be clipped off. Hence, again distortion will occur. Thus, point Q2 is also not the suitable operating point. - Q3: If Q3 is chosen the operating point, then the output characteristics of the transistor will lie in the active region. Thus, the signal will not be clipped off, and there will be no distortion at all. This is the suitable operating point. ## Leakage Current in a Common-Base Transistor - The circuit of a common base NPN transistor. The polarity of the battery $V_{EE}$ is such that it makes the emitter-base junction forward bias. The battery $V_{CC}$ sets the collector-base junction in reverse bias. - **Switch S in closed position:** In this case, electrons are injected into the base region due to the forward bias. The most of these electrons are attracted by the collector due to reverse bias. - **Switch S is opened:** Let us assume that the switch S is opened. In this case, the emitter-base junction is open circuit. Therefore, there is no emitter current which means that there is no collector current. Since the collector-base junction is reverse biased, minority currents diffuse across the collector base junction, which produces a small current. This small current is called leakage current. - **The leakage current is denoted by $I_{CBO}$ or $I_{CO}$** - $I_{CBO}$ means The leakage current between the collector and the base with the emitter open. - It is also known as reverse saturation current - $I_C = \alpha I_E + I_{CBO}$ ## Leakage Current in a Common- Emitter Configuration - In a Common-emitter Configuration with NPN transistor, if the switch S is closed, then usual current $I_E$, $I_C$ and $I_B$ flow. It means that emitter current is the sum of the base current and collector current. - When the switch is opened, the emitter-base junction is an open circuit and the leakage current flows between the collector and emitter. This leakage current is due to thermally generated minority carriers $I_{CBO}$, but also due to the movement of holes that flow across the emitter-base junction. Due to the flow of holes, the emitter-base junction is slightly forward biased because the current is supplied to the transistor. The small base current is denoted by $I_{CEO}$. The small base current $I_{CEO}$ is equal to the addition of collector currents $I_{CBO}$. ## ON condition - $I_E = I_C$ (ideal) - $I_E = I_B+I_C$ - $I_C = \beta I_B$ ## OFF condition - $I_C = I_{CBO} + \beta I_{CBO}$ - $I_C = \beta I_B + I_{CBO} + \beta I_{CBO}$ - $I_C = \beta I_B+ I_{CBO}(1+\beta)$ ## Stability Factor (S) - The stability factor (S) is a measure of the bias stability of a transistor circuit. - It may be observed that the higher the value of the stability factor, the poorer is the stability, whereas the lower the value of the stability factor, the better is the stability. - The stability factor is expressed as the rate of change of collector current with respect to the reverse saturation current while keeping base current $I_B$ and the common emitter current gain $\beta$ constant. - $S = \frac{dI_C}{dI_{CBO}}$ - We know that: - $I_C = \beta I_B+ I_{CBO}(1+\beta)$ - Differentiate both sides with respect to $I_{CBO}$ - $1 = B \frac {dI_B}{dI_{CBO}} + (1+B) \frac{dI_{CBO}}{dI_{CBO}}$ - Since $I_B$ is constant, $\frac {dI_B}{dI_{CBO}}$ = 0 - $1 = (1+B) \frac{dI_{CBO}}{dI_{CBO}}$ - $S = \frac{dI_C}{dI_{CBO}} = (1+B)$ - $S = \frac{1+ \beta \frac{dI_B}{dI_C}}{dI_C/ dI_{CBO}} = \frac{1+ \beta}{dI_C/ dI_{CBO}}$, for common-base - $S = \frac{1 + \beta \frac{dI_B}{dI_C}}{dI_C/ dI_{CBO}} = 1 + \beta$, for common-emitter ## Method of Transistor biasing There are following four methods of biasing which are most widely used: - Fixed bias - Collector-to-base bias - Self bias or voltage divider bias - Emitter bias - **Fixed Bias:** In this circuit, two DC batteries $V_{BB}$ and $V_{CC}$ are used. The battery $V_{BB}$ is a low value battery and $V_{CC}$ is a high value battery. - **Collector-to-Base Bias:** This circuit has the advantage of providing a stable operating point. - **Self Bias or Voltage Divider Bias:** The circuit is stable and has a good operating point. - **Emitter Bias:** The circuit provides a stable operating point and has the advantage of a good voltage gain and current gain. ## Hybrid parameters (h-parameters) - Hybrid parameters are also h-parameters. - Hybrid parameters are very popular and widely used because they give accurate result and can be measured very easily. - $V_2 = h_{11}I_1 + h_{12}V_2$ - $I_2 = h_{21}I_1 + h_{22}V_2$ ## Determination of h-parameters - **Put $V_2 = 0$** - $V_1 = h_{11}I_1$ - $h_{11} = \frac{V_1}{I_1}$ (Input impedance) - **Put $I_1 = 0$** - $V_1 = h_{12}V_2$ - $h_{12} = \frac{V_1}{V_2}$ (Reverse Vol. gain) - **Put $I_2 = 0 $** - $I_1 = h_{22}V_2$ - $h_{22} = \frac{I_1}{V_2}$ (Output Conductance/Admittance) ## h-parameter representation of three configurations | h-parameter | CB | CC | CE | |---|---|---|---| | $h_i$ | $h_{ib}$ | $h_{ic}$ | $h_{ie}$ | | $h_r$ | $h_{rb}$ | $h_{rc}$ | $h_{re}$ | | $h_f$ | $h_{fb}$ | $h_{fc}$ | $h_{fe}$ | | $h_o$ | $h_{ob}$ | $h_{oc}$ | $h_{oe}$ | | $h_{12}$ | $h_{re}$ | $h_{ro}$ | $h_{re}$ | | $h_{21}$ | $h_{fb}$ | $h_{fc}$ | $h_{fe}$ | | $h_{22}$ | $h_{ob}$ | $h_{oc}$ | $h_{oe}$ | ## Hybrid equivalent circuit - $V_1 = h_{11}I_1 + h_{12}V_2$ - $I_2 = h_{21}I_1 + h_{22}V_2$ - The circuit is shown in the image. ## Current gain - $A_i = \frac{I_2}{I_1} = -\frac {I_2}{I_1}$ (from diagram) - $V_2 = I_2R_L$ ## Voltage gain - $A_v = \frac{V_2}{V_1}$ - $A_v = -\frac { I_2R_L}{V_1}$ (Since $V_2 = -I_2R_L$) - $A_v = -A_i\frac{R_L}{R_i}$ ## Output resistance - The output resistance may be calculated by reducing the source voltage to zero and load resistance $R_L$ to infinity and driving the output terminals by a voltage generated $V_2$. This arrangement is the O/P resistance. - The O/P resistance $R_o$ is the ratio of voltage $V_2$ and the current drawn from the voltage source $I_2$. - $R_o = \frac{V_2}{I_2}$ - $I_2 = h_{f1}I_1 + h_{o}V_2$ - Apply KVL in IP side of the circuit - $R_sI_1 + h_{11}I_1 + h_{r1}V_2 = 0$ - $I_1(R_s + h_{11}) = -h_{r1}V_2$ ## h-parameter for Common-Emitter Amplifier - **Current gain:** The general expression is: - $A_i = \frac{-h_{fe}}{1 + h_{oe}R_L}$ - **For common-emitter amplifier:** - $A_i = \frac{-h_{fe}}{1 + h_{oe}R_L}$ - **Voltage gain:** the general expression is: - $A_v = \frac{A_iR_L}{R_i}$ - **Output resistance:** - $R_o = R_s + h_{ie}$ - $R_o = R_s.h_{oc} + \frac{1}{\Delta h}$ (where $\Delta h = h_{ie}h_{oe} - h_{re}h_{fe}$) - **Input resistance:** The general expression is: - $R_i = h_{ie}+A_i.h_{re}R_L$ - **For common-emitter amplifier:** - $R_i = h_{ie}+A_i.h_{re}R_L$ ## h-parameter for Common-Collector Amplifier - **Current gain:** The general expression is: - $A_i = \frac{-h_{fc}}{1 + h_{oc}R_L}$ - **For common-collector amplifier:** - $A_i = \frac{-h_{fc}}{1 + h_{oc}R_L}$ - **Input resistance:** the general expression is: - $R_i = h_{ic} + A_i.h_{rc}R_L$ - **For common-collector amplifier:** - $R_i = h_{ic} + A_i.h_{rc}R_L$ - **Voltage gain:** - $A_v = \frac{A_iR_L}{R_i}$ - **Output resistance:** - $R_o = Rs + h_{ic}$ - $R_o = Rs.h_{oc} + \frac{1}{\Delta h}$ (where $\Delta h = h_{ic}h_{oc} - h_{rc}h_{fc}$)