Transistor Biasing and Stabilization, Study notes of Engineering

Download Transistor Biasing and Stabilization and more Study notes Engineering in PDF only on Docsity! Transistor Biasing and Stabilization By Mr. Aniket Kumar Assistant Professor, Department of Electronics & Communication Engineering Shobhit Institute of Engineering & Technology (Deemed to-be University) NH-58, Modipuram, Meerut – 250 110, India Chapter-4 Transistor Biasing and Stabilization Biasing We known that transistor can operate in any of three regions of operation namely cutoff, active region and saturation. To operate the transistor in these regions the two junction of a transistor should be forward or reversed biased as shown in table Region of operation Base Emitter Junction Collector base junction Application Cut off Reversed bias Reversed bias As a switch Active Forward bias Reversed bias Amplifier Saturation Forward bias Forward bias As a switch In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. This process is called as biasing of transistor. Voltage divider bias (VDB) The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can recognize it by the voltage divider in the base circuit. Accurate VDB Analysis The Key idea is for the base current to be much smaller than the current through the voltage divider. When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating conditions VDB load line & Q point The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact location determined by the biasing. Large variations in current gain have almost no effect on the Q point because this type of bias sets up a constant value of emitter current. Two –Supply emitter bias This design uses two power supplies: one positive and the other negative . The idea to set up a constant value of „emitter current‟. Other types of bias This section introduced negative feedback, a phenomenon that exits when an increase in an output quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage- divider bias. PNP Transistors These pnp devices have all current & voltages reversed from their npn counterparts. They may be used with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down configuration. Reverse Feedback ratio If some percentage of an amplifier‟s output signal is connected to the input, so that the amplifier amplifies part of its own output signal, we have what is known as feedback. Feedback comes in two varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback reinforces the direction of an amplifier‟s output voltage change, while negative feedback does just the opposite. Emitter current Collector current IE = VE/ RE IC ≈ IE Collector voltage Collector – emitter voltage VC = VCC - ICRC VCE= VC - VE TSEB (Two supply emitter bias) Derivations Base voltage Emitter current VB≈ 0 VC= VCC- ICRC Collector Voltage (TSEB) Collector-emitter Voltage (TSEB) VC=VCC -ICRC VCE= VC+ 0.7V Long & Short Questions Q.1. What is meant by transistor –biasing ? Define Stability factor. Or What do you understand by transistor by transistor biasing ? Why it is necessary ? Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor. Necessary of transistor biasing  To active an transistor, biasing is essential. For proper working it is essential to apply to apply voltages of correct polarity across its two junctions.  If it is not biased correctly it would work inefficiently and produce distortion in the output signal  Q-point is not middle Output signal is distorted & the signal is clipped  Further for various applications , BJT is biased as shown in table  I n o In order to have these applications , we need to connect external DC power supplies with correct polarities & magnitude. This process is called as biasing of transistor. Stability Factor The stability of Q point of transistor amplifier depends on the following three parameters : 1. Leakage current ICO 2. βdc 3. Base to emitter voltage The effect of these parameters can be expressed mathematically by defining the stability factors 1. Stability factor S = Δ𝐼𝐶 Δ𝐼𝐶𝑂 Constant VBE & βdc This represents the change in collector current due to change in reverse saturation current ICO .The other two parameters that means VBE & βdc are assumed to be constant. 2. Stability factor S‟ = Δ𝐼𝐶 Δ𝑉𝐵𝐸 Constant ICO & βdc Region of operation Base Emitter Junction Collector base junction Application Cut off Reversed bias Reversed bias As a switch Active Forward bias Reversed bias Amplifier Saturation Forward bias Forward bias As a switch S‟ represents the change in IC due to change in VBE at constant ICO & βdc 3. Stability factor S” = Δ𝐼𝐶 βdc Constant ICO & 𝑉𝐵𝐸 Total change in collector current Δ𝐼𝐶= S. Δ𝐼𝐶𝑂 + S‟. Δ𝑉𝐵𝐸 + S”. βdc  Ideally the values of all the stability factors should be zero and practically they should be as small as possible.  Practically the value of S is significantly higher than the other two stability factor. Hence while comparing the biasing circuits, the values of S is more significant. Q.2. What are the various methods used for transistor biasing? Explain one method & State its advantage & disadvantages. Biasing is a technique to aid VBB in the input circuit which is separate from the VCC used in the output circuit. The following are the most commonly used method for transistor biasing are as below : 1. Fixed bias circuit (Single base resistor biasing) or base bias 2. Collector to base bias circuit 3. Voltage divider bias circuit (VDB) or self bias 4. Emitter bias or modified fixed bias circuit Fixed bias circuit (Single base resistor biasing) or base bias The simplest of all biasing is fixed bias ckt. as shown in fig.  Before biasing we were using two separate power supplies i.e. VCC & VBB to bias a transistor.  But in this circuit only one power supply has been used to supply power to both collector as well as base.  RB is the single base biasing resistor , hence this circuit is also called as single base resistor biasing. Related Short Answer Questions (i) What do you mean by biasing of a transistor ? Explain with examples This is because , current through R1 & R2 is approx. same and is equal to I. Collector circuit The collector circuit as shown in fig., the voltage across emitter resistance RE can be as follows : VE= IERE = VB- VBE ∴ IE= (VB- VBE)/ RE Applying the KVL to the collector circuit we get -VCC + ICRC + VCE+ IERE= 0 ∴ VCC= ICRC + VCE+ IERE VCE= VCE - IERE -ICRC Bias stabilization  If IC increases due to change in temp. or β  Then IE increases  Hence drop across RE increases (VE= IERE)  But VB is constant. Hence VBE decreases.  Hence IB decreases.  Hence IC also deceases. Thus the compensation for increase in IC is achived. Q.4. Draw the circuit diagram of Collector to base bias of a transistor . Explain its working. Collector to base bias shown in fig. is also known as collector-feedback bias. Historically , this was another attempt at stabilizing the Q point. Again, the basic idea is to feed back a voltage to the base in an attempt to neutralized any change in collector current. Like emitter-feedback bias circuit , collector feedback bias circuit uses –ve feedback in an attempt to reduce the original change in collector current. Analysis Applying KVL in the base circuit we have -VCC + RC(IC+ IB) +IBRB+ VBE = 0 ∵ IB= IC/β ∴ IB= 𝑉𝐶𝐶 − 𝑉𝐵𝐸 𝑅𝐶+ 𝑅𝐵 /β Similarly applying KVL on collector side We have VC= VCE = VCC - ICRC Q.5. Draw the circuit diagram of two supply emitter bias of a transistor . Explain its working. Sometimes electronic equipment has a power supply that produces both +ve and –ve supply voltages. The –ve supply forward biases the emitter diode. The +ve supply reverse bias the collector diode. This circuit is derived from emitter bias, for this reason , we refer to it as two-supply emitter bias (TSEB). Analysis VB ≈ 0 V Applying KVL from emitter to base loop in anticlockwise we have VEE - IERE - VBE =0 ∴ - IE = (- VEE + VBE ) / RE = (-VEE + 0.7 ) / RE ∴ IE = (VEE - 0.7 ) / RE V(RE)= VEE - 0.7 V Applying KVL on collector side we have - VCC+ ICRC + VC= 0 ∴ VC = VCC- ICRC VCE= VC - VE Q.7. Compare Fixed bias, Collector to base bias & Voltage divider bias circuits. Sr. No. Parameter Fixed bias Collector to base bias Voltage divider bias 1. Emitter Resistance Not used Not used Used 2. -ve Feedback Not used Included Included 3. Stability S= (1+ β) S = (1+ β)/ [1+ S=(1+ β ) * [ 1+ 𝑅𝐵 𝑅𝐶 ] / factor β( 𝑅𝐶 𝑅𝐶+𝑅𝐵 )] [1+ β +𝑅𝐵 𝑅𝐶 ] 4. Q-Point stability Poor Moderate Good 5. Configuration Numerical Q.1. Determine IC, VE, VB& VC for the voltage divider configuration .If β=20, R1= 62KΩ, R2= 9.1 KΩ , RC= 3.9 KΩ ,RE= 0.68 KΩ &VCC= 16V Exp: As the biasing is voltage divider We have VB= VCC. R2/ (R1+R2) ∴ VB= 16 x 9.1𝑘Ω 62𝑘Ω+9.1𝑘Ω = 2V ∵ VE= VB- VBE ∴ VE= 2V -0.7V = 1.3V ∵IE= VE/ RE ∴IE= 1.3V/ 0.68KΩ = 1.23mA ∵ IC= α .IE= β /(β+1) IE ∴ IC= 1.23mA x 20/21 = 1.17mA Q.2. For the fixed bias circuit determine IB, IC, VCE, VB, VC&VBC for the following parameters RB = 240 KΩ , RC= 2.2 KΩ , VCC= 12 V & β=50 Exp: As it is fixed bias We have IB = (VCC – VBE)/ RB ∴ IB= (12-0.7)/ 240 KΩ = 47.08μA ∵IC= β IB ∴ IC= 50 x 47.08μA = 2.35mA ∵VCE = VCC- ICRC ∴VCE = 12 – 2.35mA x 2.2 KΩ = 6.83 V ∵ Emitter terminal is grounded ∴ VB= VBE= 0.7V VC = VCE= 6.83 V ∵VBC= VB- VC