* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter4 DC Biasing BJT (part c)
Pulse-width modulation wikipedia , lookup
Ground loop (electricity) wikipedia , lookup
Stepper motor wikipedia , lookup
Power inverter wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Three-phase electric power wikipedia , lookup
Electrical ballast wikipedia , lookup
History of electric power transmission wikipedia , lookup
Electrical substation wikipedia , lookup
Distribution management system wikipedia , lookup
Power electronics wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
History of the transistor wikipedia , lookup
Semiconductor device wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Current source wikipedia , lookup
Schmitt trigger wikipedia , lookup
Surge protector wikipedia , lookup
Buck converter wikipedia , lookup
Rectiverter wikipedia , lookup
Power MOSFET wikipedia , lookup
Opto-isolator wikipedia , lookup
Alternating current wikipedia , lookup
Voltage regulator wikipedia , lookup
Stray voltage wikipedia , lookup
Voltage optimisation wikipedia , lookup
DMT 121 – ELECTRONIC 1 Chapter 4(c) DC Biasing – Bipolar Junction Transistors (BJTs) Collector Feedback Bias An improved level of stability can also be obtained by introducing a feedback path from collector to base. If IC tries to increase, it drops more voltage across RC, thereby causing VC to decrease. When VC decrease, there is a decrease voltage across RB, which decrease IB. The decrease in IB produce less IC which in turn, drops less voltage across RC and thus offsets the decrease in VC. These feedbacks keep the Q-point stable. VC IC VRC VC VRB IB IC VRC offset the decrease in VC Collector Feedback Bias Base – Emitter Loop VCC – IC'RC – IBRB – VBE – IERE = 0 Actual case IC' = IC + IB Approximation can be employed : IC' IC = IB and IE IC VCC – VBE - IB (RC + RE) – IBRB = 0 Solving for IB, yields VCC VBE IB RB ( RC RE ) Collector Feedback Bias Collector – Emitter Loop VCC – IC'RC – VCE – IERE = 0 Approximation can be employed : IC' IC and IE IC VCC – VCE - IC (RC + RE) = 0 VCE = VCC – IC (RC + RE) Collector Feedback Bias - Summary Circuit recognition The base resistor is connected between the base and the collector terminals of the transistor. Q-Point Stability IC is dependent to βDC and VBE, IB is dependent to VBE -ve feedback effect the Q-point for stability. Advantage: A simple circuit with relatively stable Q-point. Disadvantage: Relatively poor ac characteristics. Applications: Used primarily to bias linear amplifiers. Collector Feedback Bias - Summary Q-point relationships: VCC VBE IB RB ( 1) RC IC IB VCE VCC IC ( RC RE ) EXAMPLE Determine the values of ICQ and VCEQ for the amplifier shown in figure below.VCC = 10 V, RB = 180 kΩ, RC = 1.5kΩ and β =100 VC TROUBLESHOOTING Shown is a typical voltage divider circuit with correct voltage readings. Knowing these voltages are required before logical troubleshooting can be applied. We will discuss some of the faults and symptoms. TROUBLESHOOTING R1 Open With no bias the transistor is in cutoff. Base voltage goes down to 0V. Collector voltage goes up to 10 V (VCC). Emitter voltage goes down to 0V. TROUBLESHOOTING Resistor RE Open: Transistor is in cutoff. Base reading voltage will stay approximately the same. Collector voltage goes up to 10V(VCC). Emitter voltage will be approximately the base voltage + 0.7V. TROUBLESHOOTING Base Open Internally: Transistor is in cutoff. Base voltage stays approximately the same. Collector voltage goes up to 10V(VCC). Emitter voltage goes down to 0V. TROUBLESHOOTING Open BE Junction: Transistor is in cutoff. Base voltage stays approximately the same. Collector voltage goes up to 10V(VCC) Emitter voltage goes down to 0V. TROUBLESHOOTING Open BC Junction: Base voltage goes down to 1.11V because of more base current flow through emitter. Collector voltage goes up to 10V (VCC). Emitter voltage will drop to 0.41V because of small current flow from forward biased baseemitter junction. TROUBLESHOOTING RC Open: Base voltage goes down to 1.11V because of more current flow through the emitter. Collector voltage will drop to 0.41V because of current flow from forward biased collector-base junction. Emitter voltage will drop to 0.41V because of small current flow from forward biased base-emitter junction. TROUBLESHOOTING R2 Open: Transistor pushed close to or into saturation. Base voltage goes up slightly to 3.83V because of increased bias. Emitter voltage goes up to 3.13V because of increased current. Collector voltage goes down because of increased conduction of transistor. SUMMARY The purpose of biasing is to establish a stable operating point (Q-point). The Q-point is the best point for operation of a transistor for a given collector current. The dc load line helps to establish the Q-point for a given collector current. The linear region of a transistor is the region of operation within saturation and cutoff. SUMMARY Voltage-divider bias is most widely used because it is stable and uses only one voltage supply Base bias is very unstable because it is dependant. Emitter bias is stable but require two voltage supplies. Collector-back is relatively stable when compared to base bias, but not as stable as voltage-divider bias.