Electronics 1 : Chapter # 05 : DC Biasing BJT
- 2. Biasing Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal.
- 3. Operating Point The DC input establishes an operating or quiescent point called the Q-point.
- 4. The Three States of Operation ā¢ Active or Linear Region Operation BaseāEmitter junction is forward biased BaseāCollector junction is reverse biased ā¢ Cutoff Region Operation BaseāEmitter junction is reverse biased ā¢ Saturation Region Operation BaseāEmitter junction is forward biased BaseāCollector junction is forward biased
- 5. DC Biasing Circuits ā¢ Fixed-bias circuit ā¢ Emitter-stabilized bias circuit ā¢ Voltage divider bias circuit ā¢ DC bias with voltage feedback ā¢ Emitter Follower configuration ā¢ Common base configuration
- 6. Fixed Bias
- 7. The Base-Emitter Loop From Kirchhoffās voltage law: Solving for base current: +VCC ā IBRB ā VBE = 0 B BECC B R VV I ļ ļ½
- 8. Collector-Emitter Loop Collector current: From Kirchhoffās voltage law: BII C ļ¢ļ½ CCCCCE RIVV ļļ½
- 9. Transistor Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value. CR CCV CsatI ļ½ V0CEV ļ
- 10. Load Line Analysis ICsat IC = VCC / RC VCE = 0 V VCEcutoff VCE = VCC IC = 0 mA ā¢ where the value of RB sets the value of IB ā¢ that sets the values of VCE and IC The Q-point is the operating point: The end points of the load line are:
- 11. Circuit Values Affect the Q-Point more ā¦
- 12. Circuit Values Affect the Q-Point more ā¦
- 13. Circuit Values Affect the Q-Point
- 14. Emitter-Stabilized Bias Circuit Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit.
- 15. Base-Emitter Loop From Kirchhoffās voltage law: 0R1)I(-RI-V EBBBCC ļ½ļ«ļ¢ 0RI-V-RI- EEBEBBCC ļ½ļ«V EB BECC B 1)R(R V-V I ļ«ļ¢ļ« ļ½ Since IE = (ļ¢ + 1)IB: Solving for IB:
- 16. Collector-Emitter Loop From Kirchhoffās voltage law: 0 CC V C R C I CE V E R E I ļ½ļļ«ļ« Since IE ļ IC: )R(RIāVV ECCCCCE ļ«ļ½ Also: EBEBRCCB CCCCECEC EEE VVRIāVV RI-VVVV RIV ļ«ļ½ļ½ ļ½ļ«ļ½ ļ½
- 17. Improved Biased Stability Stability refers to a circuit condition in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor Beta (ļ¢) values. Adding RE to the emitter improves the stability of a transistor.
- 18. Saturation Level VCEcutoff: ICsat: The endpoints can be determined from the load line. mA0I VV C CCCE ļ½ ļ½ ERCR CCV CI CE V0V ļ« ļ½ ļ½
- 19. Voltage Divider Bias This is a very stable bias circuit. The currents and voltages are nearly independent of any variations in ļ¢.
- 20. Exact Analysis
- 22. Approximate Analysis Where IB << I1 and I1 ļ I2 : Where ļ¢RE > 10R2: From Kirchhoffās voltage law: 21 CC2 B RR VR V ļ« ļ½ E E E R V I ļ½ BEBE VVV ļļ½ EECCCCCE RIRIVV ļļļ½ )R(RIVV II ECCCCCE CE ļ«ļļ½ ļ
- 23. Voltage Divider Bias Analysis Transistor Saturation Level EC CC CmaxCsat RR V II ļ« ļ½ļ½ Load Line Analysis Cutoff: Saturation: mA0I VV C CCCE ļ½ ļ½ V0VCE ERCR CCV CI ļ½ ļ« ļ½
- 24. DC Bias with Voltage Feedback/ Collector Feedback Configuration Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. In this bias circuit the Q-point is only slightly dependent on the transistor beta, ļ¢.
- 25. Base-Emitter Loop )R(RR VV I ECB BECC B ļ«ļ¢ļ« ļ ļ½ From Kirchhoffās voltage law: 0RIāVāRIāRIāV EEBEBBCCCC ļ½ļ¢ Where IB << IC: C I B I C I C I' ļļ«ļ½ Knowing IC = ļ¢IB and IE ļ IC, the loop equation becomes: 0RIVRIRIāV EBBEBBCBCC ļ½ļ¢ļļļļ¢ Solving for IB:
- 26. Collector-Emitter Loop Applying Kirchoffās voltage law: IE + VCE + IāCRC ā VCC = 0 Since Iļ¢C ļ IC and IC = ļ¢IB: IC(RC + RE) + VCE ā VCC =0 Solving for VCE: VCE = VCC ā IC(RC + RE)
- 27. Saturation Analysis Transistor Saturation Level EC CC CmaxCsat RR V II ļ« ļ½ļ½ Load Line Analysis Cutoff: Saturation: mA0I VV C CCCE ļ½ ļ½ V0VCE E R C R CC V C I ļ½ ļ« ļ½
- 28. Emitter follower Configuration Voltage is taken off from collector Saturation:
- 29. Emitter follower Configuration (cont.)
- 31. Common Base Configuration (cont.)
- 34. Transistor Switching Networks ā¢Transistors with only the DC source applied can be used as electronic switches. ā¢Transistors as inverters
- 35. Switching Circuit Calculations C CC Csat R V I ļ½ dc Csat B I I ļ¢ ļ¾ Vi = 5V Saturation current: To ensure saturation:
- 36. Vi= 0
- 37. Emitter-collector resistance at saturation and cutoff: Csat CEsat sat I V R ļ½ CEO CC cutoff I V R ļ½
- 38. Switching Time Transistor switching times: dron ttt ļ«ļ½ fsoff ttt ļ«ļ½ Rise time 10% to 90%rt dt ft st Delay time Storage time Fall time 90% to 10%
- 39. PNP Transistors The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction.
- 40. PNP Transistors (cont.) Base-emitter loop Collector-emitter loop
- 41. Troubleshooting Hints ā¢ Approximate voltages ā VBE ļ .7 V for silicon transistors ā VCE ļ 25% to 75% of VCC ā¢ Test for opens and shorts with an ohmmeter. ā¢ Test the solder joints. ā¢ Test the transistor with a transistor tester or a curve tracer. ā¢ Note that the load or the next stage affects the transistor operation.