UNIT-III Bipolar Junction Transistor

Transcription

1 DC UNT-3.xplain the construction and working of JT. UNT- ipolar Junction Transistor A bipolar (junction) transistor (JT) is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. ipolar transistors are so named because their operation involves both electrons and holes. Charge flow in a JT is due to bidirectional diffusion of charge carriers across a junction between two regions of different charge concentrations. An NPN transistor can be considered as two diodes with a shared anode. n typical operation, the base-emitter junction is forward biased and the base collector junction is reverse biased. n an NPN transistor, for example, when a positive voltage is applied to the base emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base. To minimize the percentage of carriers that recombine before reaching the collector base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime. n particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector base junction. The thin shared base and asymmetric collector emitter doping is what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series. Transistor 'alpha' and 'beta' The proportion of electrons able to cross the base and reach the collector is a measure of the JT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by β F or h fe ; it is approximately the ratio of the DC collector current to the DC base current in forward-active region. t is typically greater than 00 for small-signal transistors but can be smaller in transistors designed for high-power applications. Another important parameter is the common-base current gain, α F. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and Alpha and beta are more precisely related by the following identities (NPN transistor): GRT-C G.Surekha Page

2 DC UNT-3 Fig Simplified cross section of a planar NPN bipolar junction transistor A JT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP, and n type, p type and n type in a NPN transistor. ach semiconductor region is connected to a terminal, appropriately labeled: emitter (), base () and collector (C). The base is physically located between the emitter and the collector and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section view of a JT indicates that the collector base junction has a much larger area than the emitter base junction. Small changes in the voltage applied across the base emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. JTs can be thought of as voltage-controlled current source, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base. NPN The symbol of an NPN ipolar Junction Transistor. GRT-C G.Surekha Page 2

3 DC UNT-3 NPN is one of the two types of bipolar transistors, in which the letters "N" (negative) and "P" (positive) refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. n other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter. The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode. PNP The other type of JT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor. The symbol of a PNP ipolar Junction Transistor. PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base in common-emitter mode is amplified in the collector output. n other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter. The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode. Regions of operation Applied voltages Mode < < C Forward active < > C Saturation > < C Cut-off ipolar transistors have five distinct regions of operation, defined by JT junction biases. The modes of operation can be described in terms of the applied voltages (this description applies to NPN tranistors; polarities are reversed for PNP transistors): GRT-C G.Surekha Page 3

4 DC UNT-3 Forward active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by β F ). Saturation: base higher than emitter, but collector is not higher than base. Cut-Off: base lower than emitter, but collector is higher than base. t means the transistor is not letting conventional current to go through collector to emitter. n terms of junction biasing: ('reverse biased base collector junction' means Vbc < 0 for NPN, opposite for PNP) Forward-active (or simply, active): The base emitter junction is forward biased and the base collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, β F, in forward-active mode. f this is the case, the collector emitter current is approximately proportional to the base current, but many times larger, for small base current variations. Saturation: With both junctions forward-biased, a JT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. Cutoff: n cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch. ipolar Transistor Construction GRT-C G.Surekha Page 4

5 DC UNT-3 The construction and circuit symbols for both the NPN and PNP bipolar transistor are shown above with the arrow in the circuit symbol always showing the direction of conventional current flow between the base terminal and its emitter terminal, with the direction of the arrow pointing from the positive P-type region to the negative N-type region, exactly the same as for the standard diode symbol. 2. xplain C configuration with the help of input and output characteristics. There are basically three possible ways to connect a ipolar Transistor within an electronic circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.. Common ase Configuration - has Voltage Gain but no Current Gain. 2. Common mitter Configuration - has both Current and Voltage Gain. 3. Common Collector Configuration - has Current Gain but no Voltage Gain. The Common ase Configuration. As its name suggests, in the Common ase or Grounded ase configuration, the AS 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 less than "", or in other words it "Attenuates" the signal. The Common ase Amplifier Circuit This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are n-phase. This type of arrangement is not very common due to its unusually high voltage gain characteristics. ts Output characteristics represent that of a forward biased diode while the nput characteristics represent that of an illuminated photo-diode. Also this type of configuration has a high ratio of Output to nput resistance or more importantly "Load" resistance (R L ) to "nput" resistance (R in ) giving it a value of "Resistance Gain". Then the Voltage Gain for a common base can therefore be given as: GRT-C G.Surekha Page 5

6 DC UNT-3 Common ase Voltage Gain The Common ase circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response. nput/ Output Characteristics Common-ase: o nput characteristics: The junction is essentially the same as a forward biased diode, therefore the current-voltage characteristics is essentially the same as that of a diode: Output characteristics: Also the collector-base voltage V C > 0 helps enhance the current to some extent. As the C junction is reverse biased, the current C depends totally on. When =0, C = C0 is the current caused by the minority carriers crossing the pn-junction. This is similar to the diode current-voltage characteristics seen before, except both axes are reversed (rotated 80 degrees), as both voltage C and current C are defined in the opposite directions. When is increased C =α + CO is increased correspondingly. Higher V C can slightly increase α and there by C. As C < C configuration does not have current-amplification effect. GRT-C G.Surekha Page 6

7 DC UNT-3 3. xplain C configuration with the help of input and output characteristics. The Common mitter Configuration. n the Common mitter or Grounded mitter 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 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 junction, while the output impedance is HGH as it is taken from a reverse-biased junction. The Common mitter Amplifier Circuit n 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 e = c + b. Also, as the load resistance (R L ) is connected in series with the collector, the Current gain of the Common mitter Transistor Amplifier is quite large as it is the ratio of c/b and is given the symbol of eta, (β). Since the relationship between these three currents is determined by the transistor itself, any small change in the base current will result in a large change in the collector current. Then, small changes in base current will thus control the current in the mitter/collector circuit. y combining the expressions for both Alpha, α and eta, β the mathematical relationship between these parameters and therefore the current gain of the amplifier can be given as: GRT-C G.Surekha Page 7

8 DC UNT-3 Where: "c" is the current flowing into the collector terminal, "b" is the current flowing into the base terminal and "e" 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 ase configuration but its Voltage gain is much lower. The common emitter is an inverting amplifier circuit resulting in the output signal being 80 o out of phase with the input voltage signal. Common-mitter: nput characteristics: Same as in the case of common-base configuration, the junction of the commonemitter configuration can also be considered as a forward biased diode, the currentvoltage characteristics is similar to that of a diode: The collector-emitter voltage V C has little effect on. Output characteristics: The C junction is reverse biased, the current depends on the current. When =0, C = CO the current caused by the minority carriers crossing the pn-junctions. When is increased C is correspondingly increased by β. GRT-C G.Surekha Page 8

9 DC UNT-3 Various parameters of a transistor change as functions of temperature. For example, β increases along with temperature. 4. xplain CC configuration with the help of input and output characteristics. The Common Collector Configuration. n the Common Collector or Grounded Collector configuration, the collector is now common and the input signal is connected to the ase, while the output is taken from the mitter load as shown. This type of configuration is commonly known as a Voltage Follower or mitter Follower circuit. The mitter 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, and it has relatively low output impedance. The Common Collector Amplifier Circuit The Common mitter configuration has a current gain equal to the β value of the transistor itself. n 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 base currents combined, the load resistance in this type of amplifier 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: GRT-C G.Surekha Page 9

10 DC UNT-3 This type of bipolar transistor configuration is a non-inverting amplifier circuit in that the signal voltages of Vin and Vout are "n-phase". t has a voltage gain that is always less than "" (unity). The load resistance of the common collector amplifier configuration receives both the base and collector currents giving a large current gain (as with the Common mitter configuration) therefore, providing good current amplification with very little voltage gain. 5.Give the Comparisions between C,C,CC configurations. with the characteristics of the different transistor configurations given in the following table: Characteristic Common Common Common ase mitter Collector nput impedance Low Medium High Output impedance Very High High Low Phase Angle 0 o 80 o 0 o Voltage Gain High Medium Low Current Gain Low Medium High Power Gain Low Very High Medium 6. What is the Relation between, &? RLATON TWN, & a) We know = C +, ut C =. = & + - = or = ( - ) Dividing both sides by, c C C ( - ) GRT-C G.Surekha Page 0

11 DC UNT-3 Or or (b) C OR C C Or OR C C Or. or = (c), Substituting = - C. C, by diving Numerator & Denominator on R.H.S, by. / / / C Putting the value of = / +. ( or ) ( or ) 7. xplain early effect or base width modulation in C configuration. ARLY FFCT OR AS WDTH MODULATON. As V CC made to increase the reverse bias, the space charge width between collector and base tends to increase. This results in decrease of effective width of the base. This dependence of base width on collector voltage is known as arly ffect. This decrease of effective base width has three consequences. GRT-C G.Surekha Page

12 DC UNT-3 (i) There is less chance of recombination in base region and c increases causing to increase with increase in V C. (ii) The charge gradient is increased within the base and current of minority carries injected across emitter junction increases. (iii) For extremely large V C, the effective base width becomes zero causing voltage breaks down in the transistor. This phenomenon is called the Punch through effect. Problems. n common base connection = ma, C = 0.95 ma calculate value of. = C = 0.95 = 0.05mA. 2. n a C configuration current amplification factor is 0.90 and emitter current is ma. Determine base current. = 0.9, = ma = C ; C =. = 0.9 x = 0.9mA = C = 0.9 = 0.mA. 3. A JT has = 0 A, =.99 and CO = A what is collector current. C = + (+ ) CO = C = 99 x 0 + ( + 99) = = 090 A =.09 ma. GRT-C G.Surekha Page 2

13 DC UNT-3 4. A transistor operating in C configuration has C = 2.98mA, = 3.0mA and co = 0.0mA. What current will flow in collector circuit of that transistor when connected in C configuration and base current is 30 A. Given : n C C = 2.98mA, = 3.0mA = CO = 0.0mA. n C = 30 A C =? C = C = + ( + ) co. = 99 x 30 x (00)0.0 x 0-3 = 2.97 x x 0-3 = 3.97mA 5. Given an NPN transistor for which = 0.98, co = 2 A O =.6 A. A C configuration is used and V CC = 2V and R C = 4.0K. What is the min. base current required in order that transistor enter in to saturation region. Given = 0.98, CO = C = 2 A, O = CO =.6 A. V CC = 2 V, V C = V CC = 2V, R L = 4.0K =? (n saturation) Solution:- Where Transistor is in saturation V C = 0.2 (Assumed) V RL = =.8 Volts. c V R RL L ma We know GRT-C G.Surekha Page 3

14 DC UNT-3 c = + ( +) CO x 0-3 = 49 + (49+) 2 x A = A 49 = ( ) A = 2850 A = A 6. Calculate the values of, dc and dc for a transistor with c = 3 A, =200mA, CO = 6 A. Also determine the new level of c which will result in reducing = 00mA. Given a. c = 3 A = 200mA CO = 6 A =? dc =? dc =? b. find c, when = 00 ma a. When = 200mA c cannot be 3 A. as c = x. Assume c = 3 Amperes C Then dc = = 3/200 x 0 3 = 65 = C + or = Amperes.= 3.2 Amperes We can also use the formulae CO, Which will also result c 3.2 Amperes. dc C b. C = dc. = 65 x 200 x 0-3 = 6.5 Amperes GRT-C G.Surekha Page 4

15 DC UNT-3 7. A transistor operating in C configuration has c = 2.98 ma, - =3.00mA and co = 0.0mA. What current will flow in the collector circuit of the transistor when connected in C configurat- -ion with base current of 30 A. Given c = 2.98 ma f = 30 A, c =? = 3 ma co = 0.0 ma c = + ( +) CO and, C = c = 99 x 30 x (99+) x 0.0 x 0-3 = 3970 A = 3.97 ma. 8. The reverse saturation current in a transistor is 8 A. f the transistor common base current gain is 0.979, calculate the collector and emitter current for 40 A base current. Given CO = CO = 8 A C & for = 40 A = ; d CO A c = = = 2245 A. 9. Given an NPN transistor for which = 0.98, CO = 2 A and O =.6 A. A common emitter connection is used and V CC = 2V and R L = 4.0K. what is the minimum base current required in order that transistor enter into saturation region. Given = 0.98 CO = 2 A for csat=? O =.6 A V CC = 2V R L = 4.0 K. When the transistor is in saturation c = csat and V C of ideal transistor = 0 volts. csat V R cc L mA c and mA Or 6 A. 49 GRT-C G.Surekha Page 5

16 DC UNT-3 0.The current gain of a transistor in C circuit is 49. Calculate C gain and find base current where the emitter current is 3mA. Given = 49 To find =? for = 3mA. = ( +) or A GRT-C G.Surekha Page 6