Lecture 3B Transistors - PDF

DAR ES SALAAM INSTITUTE OF TECHNOLOGY DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ENGINEERING FUNDAMENTALS OF ANALOG ELECTRONIC – ETU 07122 LECTURE-3 B TRANSISTOR CLASS: BENG24COE-1 & 3 LECTURER: Ms...

DAR ES SALAAM INSTITUTE OF TECHNOLOGY DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION ENGINEERING FUNDAMENTALS OF ANALOG ELECTRONIC – ETU 07122 LECTURE-3 B TRANSISTOR CLASS: BENG24COE-1 & 3 LECTURER: Ms. Justiner Joseph Transistor When a third doped element is added to a crystal diode in such a way that two pn junctions are formed, the resulting device is known as a transistor. The transistor—an entirely new type of electronic device—is capable of achieving amplification of weak signals in a fashion comparable and often superior to that realised by vacuum tubes.  A transistor consists of two pn junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types. Accordingly ; there are two types of transistors, namely; (i) n-p-n transistor (ii) p-n-p transistor An n-p-n transistor is composed of two n-type semiconductors separated by a thin section of p type as shown in Fig (i). However, a p-n-p transistor is formed by two p- sections separated by a thin section of n-type. As shown in Fig (ii). In each type of transistor, the following points may be noted : (i) These are two pn junctions. Therefore, a transistor may be regarded as a combination of two diodes connected back to back. (ii) There are three terminals, one taken from each type of semiconductor. (iii) The middle section is a very thin layer. This is the most important factor in the function of a transistor.  Trans - istor A transistor transfers a signal from a low resistance to high resistance. The prefix ‘trans’ means the signal transfer property of the device while ‘istor’ classifies it as a solid element in the same general family with resistors.  Naming the Transistor Terminals A transistor (pnp or npn) has three sections of doped semiconductors. The section on one side is the emitter and the section on the opposite side is the collector. The middle section is called the base and forms two junctions between the emitter and collector.  Naming the Transistor Terminals (i) Emitter. The section on one side that supplies charge carriers (electrons or holes) is called the emitter. The emitter is always forward biased w.r.t. base so that it can supply a large number of majority carriers (ii) Collector. The section on the other side that collects the charges is called the collector. The collector is always reverse biased. Its function is to remove charges from its junction with the base. (iii) Base. The middle section which forms two pn-junctions between the emitter and collector is called the base. The base-emitter junction is forward biased, allowing low resistance for the emitter circuit. The base-collector junction is reverse biased and provides high resistance in the collector circuit. Facts about transistors The transistor has three regions, namely ; emitter, base and collector. The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. The base is lightly doped and very thin ; it passes most of the emitter injected charge carriers to the collector. The collector is moderately doped. The emitter diode is always forward biased whereas collector diode is always reverse biased. The resistance of emitter diode (forward biased) is very small as compared to collector diode (reverse biased). (i) Working of npn transistor. The forward bias causes the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current IE. As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore, only a few electrons (less than 5%) combine with holes to constitute base current IB. The remainder (more than 95%) cross over into the collector region to constitute collector current IC. Types of Transistors There are two basic transistor types : (i) The bipolar junction transistor (BJT) and (ii) Field effect transistor (FET). Note that when we use the term transistor, it means bipolar junction transistor (BJT). The symbols used for npn The symbols used for pnp  Transistor Circuit as an Amplifier A transistor raises the strength of a weak signal and thus acts as an amplifier. In order to achieve faithful amplification, the input circuit should always remain forward biased. To do so, a d.c. voltage VEE is applied in the input circuit in addition to the signal as shown in figure below. This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the input circuit forward biased regardless of the polarity of the signal. Transistor Connections (Configurations) A transistor can be connected in a circuit in the following three ways : (i) common base connection (ii) common emitter connection (iii) common collector connection  Common Base Connection Fig (i) shows A common base npn transistor circuit whereas, Fig.(ii) shows the common base pnp transistor circuit. 1. Current amplification factor (α). It is the ratio of output current to input current. In a common base connection, the input current is the emitter current IE and output current is the collector current IC. The ratio of change in collector current to the change in emitter current at constant collector base voltage VCB is known as current amplification factor i.e. Expression for collector current. The whole of emitter current does not reach the collector. It is because a small percentage of it, as a result of electron-hole combinations occurring in base area, gives rise to base current. (i) That part of emitter current which reaches the collector terminal i.e. αIE. (ii) The leakage current I eakage. This current is due to the movement of l minority carriers across base-collector junction on account of it being reverse biased. This is generally much smaller than αIE. It is clear that if IE = 0 (i.e., emitter circuit is open), a small leakage current still flows in the collector circuit. This ILeakage is abbreviated as ICBO meaning collector-base current with emitter open. The ICBO is indicated in figure shown If only d.c. values are considered, then α = IC/IE. ○○○ ○○○○ ○○○○ ○○○○ ○○○○ ○○○○ ○○ Example In a common base connection, current amplification factor is 0.9. If the emitter current is 1mA, determine the value of base current. Example. For the common base circuit shown in Figure below, determine IC and VCB. Assume the transistor to be of silicon. Applying Kirchhoff’s voltage law to the emitter-side loop, we have, Applying Kirchhoff’s voltage law to the collector-side loop, we have,  Characteristics of Common Base Connection 1. Input characteristic. It is the curve between emitter current IE and emitter- base voltage VEB at constant collector-base voltage VCB. (i) The emitter current IE increases rapidly with small increase in emitter-base voltage VEB. (ii) The emitter current is almost independent of collector-base voltage VCB. 2. Output characteristic. It is the curve between collector current IC and collector-base voltage VCB at constant emitter current IE.  Common Emitter Connection 1. Base current amplification factor β. In common emitter connection, input current is IB and output current is IC. The ratio of change in collector current (IC) to the change in base current (IB) is known as base current amplification factor i.e. In almost any transistor, less than 5% of emitter current flows as the base current. Therefore, the value of β is generally greater than 20. Usually, its value ranges from 20 to 500.  Relation between β and α A simple relation exists between. This can be derived as follows : Write Expression for collector current.  Measurement of Leakage Current A very small leakage current flows in all transistor circuits. However, in most cases, it is quite small and can be neglected. (i) Circuit for ICEO test. Fig shows the circuit for measuring ICEO. Since base is open (IB = 0), the transistor is in cut off. Ideally, IC = 0 but actually there is a small current from collector to emitter due to minority carriers. It is called ICEO (collector-to-emitter current with base open). (ii) Circuit for ICBO test Figure shows the circuit for measuring ICBO. Since the emitter is open (IE = 0), there is a small current from collector to base. This is called ICBO (collector-to-base current with emitter open). This current is due to the movement of minority carriers across base collector junction. The value of ICBO is also small. If in measurement, ICBO is excessive, then there is a possibility that collector-base is shorted. Example Find the value of β if (i) α = 0.9 (ii) α = 0.98 (iii) α = 0.99. Solution Example 2 Find the α rating of the transistor shown in Figure below. Hence determine the value of IC using both α and β rating of the transistor. Characteristics of Common Emitter Connection The important characteristics of this circuit arrangement are the input characteristics and output characteristics. Input characteristic. It is the curve between base current IB and base- emitter voltage VBE at constant collector-emitter voltage VCE. Output characteristic. It is the curve between collector current IC and collector-emitter voltage VCE at constant base curent IB. Common Collector Connection Transistor as an Amplifier in CE Arrangement Examples