Electronic Circuits Bipolar Junction Transistors Manar Mohaisen Office: F208 Email: manar.subhi@kut.ac.kr Department of EECE
Review of Precedent Class Explain the Operation of the Zener Diode Explain Applications of the Zener Diode Explain the Operation and Applications of the Varactor Diode Explain the Operation and Applications of the Optical Diodes Other Types of Diodes
Class Objectives Bipolar Junction Transistor (BJT) BJT Characteristics and Parameters BJT as an Amplifier and as a Switch Phototransistor and its Applications Transistor Categories and Packaging Discussions
Bipolar Junction Transistor C (collector) C B (base) n p n Base-Collector junction Base-Emitter junction B p n p npn pnp E (emitter) E BJT Consists of three regions called emitter, base, and collector. The base region is thin and lightly doped. The emitter region is heavily doped wide region. The controller region is moderately doped wide region. Consists of two pn junctions Namely, base-emitter junction and base-controller junctions.
BJT Operation Base Current Electrons flow from the emitter to the base generating hole current. Electrons from the emitter recombine with the holes in the base. Since the base region is thin The number of holes is small. The number of electrons combining with the holes is also small. These valence electrons become free in the metallic base. This is called the base current. Bipolar Junction Transistor contd.
BJT Operation contd. Collector Current Base region is thin. most of the electrons entering the base don t combine with holes. The uncombined free electrons are attracted by the collector supply voltage. The uncombined free electrons move through the collector into the external circuit. Bipolar Junction Transistor contd.
Bipolar Junction Transistor contd. Transistor Currents + I C + I C I C I C + I B n p + I B I B p n I B n p I E I E I E + I E + (1) BJT currents (2) DC Beta I I = I + I β = C E B C DC I B (3) DC Alpha I α = C DC I E
BJT Characteristics and Parameters Transistor DC Model [Unsaturated BJT] Input circuit: forward-biased diode with a current of I B. Output circuit: a dependent current source of β DC I B.
BJT Circuit Analysis V BE = 0.7 V Forward-biased diode. Therefore, I B is given by: I C is given by: V CE is given by: V = V I R CE CC C C V CB is given by: BJT Characteristics and Parameters contd. V = V V R B BB BE I B I C V = = β BB V R DC B I B BE V = V V CB CE BE
Example 4-2 β DC = 150 Currents: Voltages: BJT Characteristics and Parameters contd. I I V V 5V 0.7V 430 μ A = BB BE = = B RB 10kΩ β = I = 150 430 A = 64.5 ma C DC B I = I + I = E B C μ 64.9mA V = V I R = 10V (64.5mA)(100 Ω ) = 3.55V CE CC C C V V V = = 3.55V 0.7V = 2.85V CB CE BE
Collector Characteristic Curves Saturation region Both junctions are forwardbiased V CC = 0. I C is independent of I B. BJT Characteristics and Parameters contd. As V CC increases, I C and V CE increase. Ideally, when V CE exceeds 0.7, the base-collector junction becomes reverse-biased. The BJT goes into the linear region (active region). In linear region, I C is dependent on I B. I C B Saturation region Active region Breakdown region C I C = β DC I B A 0 0.7 V V CE(max) V CE
BJT Characteristics and Parameters contd. Collector Characteristic Curves contd. I C At higher voltages of V CE The base-collector junction goes in breakdown. B Active region Breakdown region C The collector current increases rapidly. A transistor should never be operated in the breakdown region. Saturation region A 0 0.7 V V CE(max) V CE
BJT Characteristics and Parameters contd. Cutoff This region of operation happens when I B = 0. There will be a very small collector leakage current I CEO. This I CEO is neglected and therefore V CE = V CC. Both junctions are reverse-biased.
Saturation BJT Characteristics and Parameters contd. As V BB increases, the base-emitter junction is forward-biased. The base current I B increases. The base current increases and the collector current increases as a result (I C = β DC I B ). As a result, V CE decreases till the saturation value V CE (sat) and the base -controller junction becomes forward-biased (V CE = V CC I C R C ). I C does not increase anymore even when I B increases. Saturation occurs below the knee of the collector curves A few tenths of a volt.
BJT Characteristics and Parameters contd. DC Load Line In between cutoff and saturation along the load line is the active region. Details will be covered in Chapter 5. I C I C(sat) Saturation I B = 0 Cutoff 0 V CE(sat) V CC V CE
BJT Characteristics and Parameters contd. Example 4-4 Determine whether or not the transistor is in saturation or not. V CE(sat) = 0.2 V. Solution If the transistor is in saturation, then I C(sat) V V CC C(sat) = = 10V 0.2V = 9.8mA R 1.0 kω B If the transistor is not in saturation, then V V I = BB BE = 2.3 V = B 0.23 ma RB 10kΩ I = β I = (50)(0.23mA) = 11.5 ma C DC B and Because I C can t be greater than I C(sat), the transistor is saturated.
BJT Characteristics and Parameters contd. More About β DC β DC is not really constant but rather varies with I C and the temperature. Usually we specify the minimum β DC in the transistor datasheet.
Maximum Transistor Rating BJT Characteristics and Parameters contd. Limitations (maximum ratings) are specified by the manufacturer for: Collector-to-base voltage Collector-to-emitter voltage Collector current Power dissipation Note that Collector-to-emitter voltage and collector current can t be maximum at the same time. If V CE is maximum, I C is given by: If I C is maximum, V CE is given by: I V C CE = = P V D(max) P CE D(max) I C
Example 4-5 Collector-to-emitter voltage = 6 V. Maximum power rating = 250 mw. Find the maximum collector current. BJT Characteristics and Parameters contd. Solution: I C PD(max) = = 250 mw = 41.7 ma V 6V CE
Example 4-6 P D(max) = 800 mw, V CE(max) = 15 V, and I C(max) = 100 ma. Determine the maximum value of V CC before exceeding the rating. Solution: BJT Characteristics and Parameters contd. I B V V 195 μa = BB BE = B RB IC = βdc I = (100)(195μA) = 19.5mA VR = I C CRC = (19.5mA)(1.0k Ω ) = 19.5 V I C is much less than the maximum collector maximum current. I C is dependent on I B, and will not change as long as I B is fixed. Therefore, V = V + V 15V 19.4 V 34.5 V CC(max) CE(max) R = + = Has the maximum power rating been exceeded? No. P = V I = = D C CE(max) C (15V)(19.5mA) 293 mw
BJT Characteristics and Parameters contd. Derating P D(max) P D(max) is usually specified at 25 o C. For higher temperatures, P D(max) is less. Derating is calculated using a derating factor. Example A derating factor of 2 mw/ o C This means that the maximum power dissipation is reduced 2 mw for each degree Celsius increase in temperature.
The BJT as an Amplifier Voltage Gain Defined as the ratio between output voltage and input voltage. Let us define r e as the resistance between base and emitter. Then, Vb Voltage gain Example: r e = 50 Ω R C = 1.0 k Ω V I R = I ' ere c e C A v V I R R = V I r r = c b e C ' e e C ' e A v R r C 1000 ' = = 20 50 e
The BJT as a Switch Switch Condition in Cutoff V = V CE(cutoff) CC Conditions in Saturation V V CC CE(sat) I = C(sat) R C I B(min) = I β C(sat) DC +V CC +V CC +V CC +V CC R C I C = 0 R C R C I C(sat) R C I C(sat) 0 V R B I B = 0 C E +V BB R B I B + C E Cutoff open switch Saturation closed switch
The BJT as a Switch contd. Application of a Transistor Switch In cutoff diode is off. In saturation diode is on. Example The diode requires 30 ma to be on. Use double the minimum base current to ensure saturation. V CC = 9V, V CE(sat) = 0.3 V, R C = 220 Ω, R B = 3.3 kω. Determine the value of the square wave. V V V CC LED CE(sat) I = = 9V 1.6V 0.3V = C(sat) 32.3mA R 220Ω I B(min) DC C IC(sat) = = 32.3mA = 646 μ A β 50 V = 2 I R + V = (1.29mA)(3.3k Ω ) + 0.7 V = 4.96V in B(min) B BE
The Phototransistor Phototransistor The base current is produced when light strikes the sensing semiconductor base region. In absence of light, there will be a small leaking current I CEO. When the light strikes the collector-base pn junction A base current, I λ, is produced that is directly prop. with the light intensity. I = β I C DC λ +VCC Except for the base current generation, The phototransistor behaves as a conventional BJT. The base-collector region is wider than that in the case of the BJT. To produce greater current. Emitter n p n Base Collector Light R C V OUT
The Phototransistor contd. Three-lead phototransistor The base lead is electrically available where the phototransistor can be used as a BJT with or without light-sensitivity. Two-lead phototransistor The base lead is not electrically available. This is the usual configuration.
The Phototransistor contd. Collector Characteristics Curves Each curve corresponds to a certain value of light intensity. I C (ma) 10 50 mw/cm 2 +V CC 8 6 40 mw/cm 2 30 mw/cm 2 R C V OUT 4 20 mw/cm 2 2 10 mw/cm 2 Dark current 0 5 10 15 20 25 30 V CE (V) Phototransistors are not sensitive to all lights They are sensitive to wavelengths in the red and infrared bandwidths.
The Phototransistor contd. Applications Light activating. Light deactivating.
The Phototransistor contd. Optocouplers A device that has light-based connection. Types of Optocouplers LED-to-photodiode LED-to-phototransistor Current transfer ration Ration between change in input current (LED s) and that of the out current.
The Phototransistor contd. Applications of the Optocouplers Isolate sections of the circuit that are incompatible in terms of current or voltage. Ex.: Isolate low-current control or circuits from noisy power supplies or high current motors, etc. Ex.: Isolating patients from monitoring instruments. Ex.: Traffic lights where control circuit is isolated from the power circuit.
Transistor Categories and Packaging General-purpose / Small-signal Transistors Used for low- or medium-power amplifiers or switches. The packages are either metal or plastic. Certain types of packages contain multiple transistors. 2 Base 3 Collector 12 3 1 Emitter 1 2 3 1 Base 3 Collector 2 Emitter 2 Base 3 Collector 1 Emitter TO-92 SOT-23 32 1 TO-18
Transistor Categories and Packaging contd. Power Transistors Used to handle large currents ( > 1 A) and/or large voltages. A power amplifier is used to drive the speakers. Usually it has a metal case for the collector Usually connected to a heat sink for heat dissipation.
RF Transistors Operate at extremely high-frequencies. Usually used in communication systems. Transistor Categories and Packaging contd.
Discussion & Notes K K A K A A A A K K K K A K A K K A K A K A