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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MALVINO & BATES SEVENTH EDITION Electronic PRINCIPLES.

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Presentation on theme: "Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MALVINO & BATES SEVENTH EDITION Electronic PRINCIPLES."— Presentation transcript:

1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MALVINO & BATES SEVENTH EDITION Electronic PRINCIPLES

2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Transistor Biasing Chapter 8 8

3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Topics Covered in Chapter 8 Voltage-divider bias Accurate VDB analysis VDB load line and Q point Two-supply emitter bias Other types of bias Troubleshooting PNP transistors

4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Voltage divider bias Base circuit contains a voltage divider Most widely used Known as VDB

5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE R1R1 R2R2 Voltage divider bias circuit { R 1 and R 2 form a voltage divider

6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC R1R1 R2R2 +V BB Divider analysis: V BB = R2R2 R 1 + R 2 V CC ASSUMPTION: The base current is normally much smaller than the divider current.

7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I E = V BB - V BE RERE V C = V CC - I C R C V CE = V C - V E I C  I E +V CC RCRC RERE V BB To complete the analysis: Now the circuit can be viewed this way:

8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The six-step process 1. Calculate the base voltage using the voltage divider equation. 2. Subtract 0.7 V to get the emitter voltage. 3. Divide by emitter resistance to get the emitter current. 4. Determine the drop across the collector resistor.

9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The six-step process (Continued) 5. Calculate the collector voltage by subtracting the voltage across the collector resistor from V CC. 6. Calculate the collector-emitter voltage by subtracting the emitter voltage from the collector voltage.

10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. VDB analysis The base current must be much smaller than current through the divider With the base voltage constant, the circuit produces a stable Q point under varying operational conditions

11 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE R1R1 R2R2 Is the divider a stiff source? { Find the Thevenin resistance. R TH = R 1 R 2

12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE V BB R TH A Thevenin model of the bias circuit:

13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE The 100:1 rule applied to the bias circuit: R IN V BB R TH R TH < 0.01 R IN When this rule is met, the divider is stiff.

14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Firm voltage divider Used because divider resistors (e.g. R 1 and R 2 ) in a stiff design would be too small The collector current will be about 10% lower than the stiff value

15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE R1R1 R2R2 Sometimes a firm divider is chosen. R 1 R 2 < 0.1  dc R E A closer approximation: I E = V BB - V BE R E +  dc R 1 R 2

16 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. VDB load line and Q point VDB is derived from emitter bias The Q point is immune to changes in current gain The Q point is moved by varying the emitter resistor

17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two-supply stiff emitter bias: 10 V 3.6 k  2.7 k  1 k  2 V Assume 0 V I E = V EE - 0.7 V RERE I E = 2 V - 0.7 V 1 k  = 1.3 mA

18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 V 3.6 k  2.7 k  1 k  2 V V C = 10 V - (1.3 mA)(3.6 k  ) = 5.32 V V CE = 5.32 V - (-0.7 V) = 6.02 V Find the voltages:

19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RBRB Base bias: The least predictable Q point moves with replacement Q point moves with temperature Not practical

20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE RBRB Emitter-feedback bias: Better than base bias Q point still moves Not popular

21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RBRB Collector-feedback bias: Better than emitter-feedback bias Q point still moves Some applications because of circuit simplicity

22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RBRB RERE Collector- and emitter -feedback bias: Better than emitter-feedback bias Not as good as voltage-divider bias Limited application

23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two-supply emitter bias: Very stable Requires 2 supplies Note: Also called TSEB

24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC RCRC RERE R1R1 R2R2 Voltage divider bias: Very stable Requires 1 supply The most popular Note: Also called VDB

25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Troubleshooting Voltage measurements Use 10 MΩ input voltmeter Troubles include: Opens Shorts Faulty transistors

26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PNP transistor The base is n-type material The collector and emitter are p-type material The emitter arrow points in Can be used with a negative power supply

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Electron flowConventional flow ICIC IBIB IEIE ICIC IBIB IEIE PNP transistor symbol and current flow

28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. -V CC RCRC RERE R1R1 R2R2 PNP Biasing with a negative supply

29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V EE RERE RCRC R2R2 R1R1 PNP Biasing with a positive supply

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