Bipolar Transistor Basics:Transistors are three terminal active devices made from different semiconductor materials that can act as either an insulator or a conductor by the application of a small signal voltage. The transistor's ability to change between these two states enables it to have two basic functions: "switching" (digital electronics) or "amplification" (analogue electronics). The bipolar transistors have the ability to operate within three different regions: Active Region - the transistor operates as an amplifier and Ic = .Ib Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation) Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0There are two basic types of bipolar transistor construction, PNP and NPN. The Bipolar Transistor basic construction consists of two PN-junctions producing three connecting terminals with each terminal being given a name to identify it from the other two. These three terminals are known and labelled as the Emitter ( E ), the Base ( B) and the Collector ( C ) respectively. Bipolar Transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a current-controlled switch. The principle of operation of the two transistor types PNP and NPN, is exactly the same the only difference being in their biasing and the polarity of the power supply for each type.Bipolar Transistor Configurations:There are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output. Each method of connection responding differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit arrangement. Common Base Configuration - has Voltage Gain but no Current Gain. Common Emitter Configuration - has both Current and Voltage Gain. Common Collector Configuration - has Current Gain but no Voltage Gain.The Common Base (CB) Configuration:As its name suggests, in the Common Base or grounded base configuration, the Base connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or less, in other words the common base configuration "attenuates" the input signal.This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-phase. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics. Its output characteristics represent that of a forward biased diode while the input characteristics represent that of an illuminated photo-diode. Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly load resistance RL to input resistance Rin giving it a value of Resistance Gain. Then the voltage gain ( Av ) for a common base configuration is therefore given as:Where: Ic/Ie is the current gain, alpha ( ) and RL/Rin is the resistance gain. The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency response.The Common Emitter (CE) ConfigurationIn the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of bipolar transistor connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased PN-junction, while the output impedance is HIGH as it is taken from a reverse-biased PN-junction. In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance ( RL ) is connected in series with the collector, the current gain of the common emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek symbol of Beta, ( ). As the emitter current for a common emitter configuration is defined as Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of . Note: that the value of Alpha will always be less than unity.Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the physical construction of the transistor itself, any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ). Then, small changes in current following in the base will thus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and 200 for most general purpose transistors. By combining the expressions for both a, and Beta, the mathematical relationship between these parameters and therefore the current gain of the transistor can be given as:Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal. Then to summarise, this type of bipolar transistor configuration has a greater input impedance, current and power gain than that of the common base configuration but its voltage gain is much lower. The common emitter configuration is an inverting amplifier circuit. This means that the resulting output signal is 180o "out-of-phase" with the input voltage signal.The Common Collector (CC) ConfigurationIn the Common Collector or grounded collector configuration, the collector is now common through the supply. The input signal is connected directly to the base, while the output is taken from the emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms while having relatively low output impedance.

The common emitter configuration has a current gain approximately equal to the value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin andVout are "in-phase". It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.The NPN TransistorIn the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two basic forms. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-Positive) type, with the most commonly used transistor type being the NPN Transistor. We also learnt that the junctions of the bipolar transistor can be biased in one of three different ways Common Base, Common Emitter and Common Collector.The construction and terminal voltages for an NPN transistor are shown above. The voltage between the Base and Emitter ( VBE ), is positive at the Base and negative at the Emitter because for an NPN transistor, the Base terminal is always positive with respect to the Emitter. Also the Collector supply voltage is positive with respect to the Emitter ( VCE ). So for an NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter. Then the voltage sources are connected to an NPN transistor as shown. The Collector is connected to the supply voltage VCC via the load resistor, RL which also acts to limit the maximum current flowing through the device. The Base supply voltage VB is connected to the Base resistor RB, which again is used to limit the maximum Base current. We know that the transistor is a "current"operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched "fully-ON". However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input.The transistor current in an NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( ). The value of can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Icprovides the output. Note that Beta has no units as it is a ratio. Also, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( ), and is a function of the transistor itself (electrons diffusing across the junction). As the emitter current Ie is the product of a very small base current plus a very large collector current, the value of alpha , is very close to unity, and for a typical low-power signal transistor this value ranges from about 0.950 to 0.999The values of Beta vary from about 20 for high current power transistors to well over 1000 for high frequency low power type bipolar transistors. The value of Beta for most standard NPN transistors can be found in the manufactures datasheets but generally range between 50 - 200. The equation above for Beta can also be re-arranged to make Ic as the subject, and with a zero base current ( Ib = 0 ) the resultant collector current Ic will also be zero, ( x 0 ). Also when the base current is high the corresponding collector current will also be high resulting in the base current controlling the collector current. One of the most important properties of the Bipolar Junction Transistor is that a small base current can control a much larger collector current. Consider the following example.The Transistor as a SwitchWhen used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its "active" region, that is the linear part of the output characteristics curves are used. However, both the NPN & PNP type bipolar transistors can be made to operate as "ON/OFF" type solid state switches by biasing the transistors base differently to that of a signal amplifier. Solid state switches are one of the main applications for the use of transistors, and transistor switches can be used for controlling high power devices such as motors, solenoids or lamps, but they can also used in digital electronics and logic gate circuits. If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the " I-V " characteristics curves we have seen previously. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its "fully-OFF" (cut-off) and "fully-ON" (saturation) regions as shown below.Area at the bottom of the curves represents the "Cut-off" region while the blue area to the left represents the "Saturation" region of the transistor. Both these transistor regions are defined as:1. Cut-off RegionHere the operating conditions of the transistor are zero input base current ( IB ), zero output collector current IC and maximum collector voltage VCE which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched "Fully- OFF".Cut-off CharacteristicsThen we can define the "cut-off region" or "OFF mode" when using a bipolar transistor as a switch as being, both junctions reverse biased, VB < 0.7v and IC = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.2. Saturation RegionHere the transistor will be biased so that the maximum amount of base current is applied, resulting in maximum collector current resulting in the minimum collector emitter voltage drop which results in the depletion layer being as small as possible and maximum current flowing through the transistor. Therefore the transistor is switched "Fully-ON".

Then we can define the "saturation region" or "ON mode" when using a bipolar transistor as a switch as being, both junctions forward biased, VB > 0.7v and IC = Maximum. For a PNP transistor, the Emitter potential must be positive with respect to the Base. Then the transistor operates as a "single-pole single-throw" (SPST) solid state switch. With a zero signal applied to the Base of the transistor it turns "OFF" acting like an open switch and zero collector current flows. With a positive signal applied to the Base of the transistor it turns "ON" acting like a closed switch and maximum circuit current flows through the device.An example of an NPN Transistor as a switch being used to operate a relay is given below. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inductive load when the transistor switches "OFF" and so protect the transistor from damage. If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown. Transistor as a Switch Summary Transistor switches can be used to switch and control lamps, relays or even motors. When using the bipolar transistor as a switch they must be either fully-OFF or fully-ON. Transistors that are fully "ON" are said to be in their Saturation region. Transistors that are fully "OFF" are said to be in their Cut-off region. When using the transistor as a switch, a small Base current controls a much larger Collector load current. When using transistors to switch inductive loads such as relays and solenoids, a "Flywheel Diode" is used. When large currents or voltages need to be controlled, Darlington Transistors can be used.What is the need for biasing?In order to produce distortion free output in amplifier circuits, the supply voltages and resistances establish a set of dc voltage VCEQ and ICQ to operate the transistor in the active region. These voltages and currents are called quiescent values which determine the operating point or Q-point for the transistor. The process of giving proper supply voltages and resistances for obtaining the desired Q-Point is called Biasing. The circuits used for getting the desired and proper operating point are known as biasing circuits. To establish the operating point in the active region biasing is required for transistors to be used as an amplifier. For analog circuit operation, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) when input is applied. For digital operation, the Q point is placed so the transistor does the contrary - switches from "on" to "off" state. Often, Q point is established near the center of active region of transistor characteristic to allow similar signal swings in positive and negative directions. Q-point should be stable. In particular, it should be insensitive to variations in transistor parameters (for example, should not shift if transistor is replaced by another of the same type), variations in temperature, variations in power supply voltage and so forth. The circuit must be practical: easily implemented and cost-effective.TYPES OF BIAS1) Base-Current Bias (Fixed Bias):The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS. It consisted basically of a resistor (RB) connected between the collector supply voltage and the base. Unfortunately, this simple arrangement is quite thermally unstable. If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). This reaction to temperature is undesirable because it affects amplifier gain (the number of times of amplification) and could result in distortion.Merits:It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB). A very small number of components are required.Demerits:The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable. Changes in Vbe will change IB and thus cause RB to change. This in turn will alter the gain of the stage.When the transistor is replaced with another one, considerable change in the value of can be expected. Due to this change the operating point will shift. For small-signal transistors (e.g., not power transistors) with relatively high values of (i.e. between 100 and 200), this configuration will be prone to thermal runaway. In particular, the stability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately +1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors.Self-Bias or Collector-to-base bias:A better method of biasing is obtained by inserting the bias resistor directly between the base and collector. By tying the collector to the base in this manner, feedback voltage can be fed from the collector to the base to develop forward bias. This arrangement is called SELF-BIAS. Now, if an increase of temperature causes an increase in collector current, the collector voltage (VC) will fall because of the increase of voltage produced across the load resistor (RL). This drop in VC will be fed back to the base and will result in a decrease in the base current. The decrease in base current will oppose the original increase in collector current and tend to stabilize it. The exact opposite effect is produced when the collector current decreases.Self-bias has two small drawbacks:(1) It is only partially effective and, therefore, is only used where moderate changes in ambient temperature are expected; (2) it reduces amplification since the signal on the collector also affects the base voltage. This is because the collector and base signals for this particular amplifier configuration are 180 degrees out of phase (opposite in polarity) and the part of the collector signal that is fed back to the base cancels some of the input signal. This process of returning a part of the output back to its input is known as DEGENERATION or NEGATIVE FEEDBACK. Sometimes degeneration is desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose.TRANSISTOR AS AN AMPLIFIERAmplifiers are used extensively in electronic circuits to make an electronic signal bigger without affecting it in any other way. Generally we think of Amplifiers as audio amplifiers in the radios, CD players and stereo's we use around the home. In this amplifier tutorial section we looked at the amplifier which is based on a single bipolar transistor as shown below, but there are several different kinds of transistor amplifier circuits that we could use.Small Signal Amplifiers Small Signal Amplifiers are also known as Voltage Amplifiers. Voltage Amplifiers have 3 main properties, Input Resistance, Output Resistance and Gain. The Gain of a small signal amplifier is the amount by which the amplifier "Amplifies" the input signal. Gain is a ratio of input divided by output, therefore it has no units but is given the symbol (A) with the most common types being, Voltage Gain (Av), Current Gain (Ai) and Power Gain (Ap) The power Gain of the amplifier can also be expressed in Decibels or simply dB. In order to amplify all of the input signal distortion free in a Class A type amplifier, DC BaseBiasing is required. DC Bias sets the Q-point of the amplifier half way along the load line. This DC Base biasing means that the amplifier consumes power even if there is no input signal present. The transistor amplifier is non-linear and an incorrect bias setting will produce large amounts of distortion to the output waveform. Too large an input signal will produce large amounts of distortion due to clipping, which is also a form of amplitude distortion. Incorrect positioning of the Q-point on the load line will produce either Saturation Clipping orCut-off Clipping. The Common Emitter Amplifier configuration is the most common form of all the general purpose voltage amplifier circuit using a Bipolar Junction Transistor. The Common Source Amplifier configuration is the most common form of all the general purpose voltage amplifier circuit using a Junction Field Effect Transistor.Large Signal AmplifiersLarge Signal Amplifiers are also known as Power Amplifiers. Power Amplifiers can be sub-divided into different Classes, for example Class A Amplifiers where the output device conducts for all of the input cycle, Class B Amplifiers, where the output device conducts for only 50% of the input cycle and Class AB Amplifiers, where the output device conducts for more than 50% but less than 100% of the input cycle.An ideal Power Amplifier would deliver 100% of the available DC power to the load. Class A amplifiers are the most common form of power amplifier but only have an efficiency rating of less than 40%. Class B amplifiers are more efficient than Class A amplifiers at around 70% but produce high amounts of distortion. Class B amplifiers consume very little power when there is no input signal present. By using the "Push-pull" output stage configuration, distortion can be greatly reduced. However, simple push-pull Class B Power amplifiers can produce high levels of Crossover Distortion due to their cut-off point biasing. Pre-biasing resistors or diodes will help eliminate this crossover distortion. Class B Power Amplifiers can be made using Transformers or Complementary Transistors in its output stage.How does a small change in base current result in a large change in collector current in a BJT??Think of the base-emitter junction as a regular diode junction. In order for charge to flow, the junction must be forward biased with a voltage drop of about 0.7V (for silicon) across it. Now what does this actually mean? Well for a start it means current can flow from emitter to base. Furthermore it means that electrons in the emitter will flow over to the base-emitter junction and holes from the base will flow to the base-emitter junction. Because of the nature of holes and electrons, they combine at the junction. But here is where the magic is, the base has been dopedin such a way that only few holes are available (about 1 hole for every 100 or so electrons). The more electons you put into the base, the less holes will be available for the electrons from the emitter (becuase most of the holes-electrons combinations are taken up by electrons entering the base), hence more emitter electrons will enter the collector region and there will be a stronger flow of charge. So what happens to 99% of the electrons? Well as you probably already know, the base layer is very thin. There also exist a depletion region between the base and collector regions. It is this depletion region that the electrons must cross to enter the collector region. Electrons then come under the influence of a strong electric field (which is between the base and collector region) that "sucks" the electrons. The electrons then leave the transistor through the collector terminal as current. In other words it all started with a forward biased diode junction (base-emitter junction) which provided the necessary holes and electrons, the electrons then combined with holes. The remaining electrons were then collected by the collector and emitted as current. The collector collects the electrons emitted by the emitter. Its actually small changes in the voltage applied across the baseemitter terminals which causes the current that flows from emitter to collector to change significantly. Its this applied voltage that causes the depletion region between the base and emitter to get thinner and charges start to cross it. A change in voltage causes a change in current, this is roughly like a resistor. But the voltage that controls the current flow in on an entire different region (base-emitter), yet its effects are seen on the collector side of the transistor. So its effects are transferred. Hence the name transistor.THERMAL RUNAWAYAs a transistor heats, its junction temperature increases. This increases the collector current, which forces the junction temperature to increase further, producing more collector current, etc., until the transistor is destroyed. Or Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in Collector-to-Emitter leakage current. This is frequently seen in a pushpull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce 1/2Vcc Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.As the temperature of a bipolar transistor rises, it's voltage drop tends to go down, in other words, it becomes even more conductive. It thus allows more current to pass and tends to want to blow itself up. The maximum average power in which a transistor can dissipate depends upon the construction of transistor and lie in the range of few milliwatts and 200W. The maximum power is limited by the temperature that the collector Base junction can withstand. The maximum power dissipation is usually specified for the transistor enclosure is 25 degree celsius. The junction temperature may increase either because of rise in ambient temperature or because of self heating. The problem of self heating arises due to dissipation of power at the collector junction. The leakage current Icbo is extremely temperature dependent and increases with the rise in temperature of collector-base junction. With the increase in collector current Ic, collector power dissipation increases which raises the junction temperature that leads to further increase in collector current Ic. The process is cumulative and may lead to the eventual destruction of transistor. This phenomenon is known as THERMAL RUNAWAY of transistor. In practice the Thermal Runaway can be prevented by a well designed circuit called as STABILIZATION Circuitry.