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BJT Emitter Stabilized Bias

BJT Emitter Stabilized Bias. ELEC 121. Improved Bias Stability. The addition of R E to the Emitter circuit improves the stability of a transistor output

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BJT Emitter Stabilized Bias

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  1. BJT Emitter Stabilized Bias ELEC 121

  2. Improved Bias Stability • The addition of RE to the Emitter circuit improves the stability of a transistor output • Stability refers to a bias circuit in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor forward current gain () • The temperature (TA or ambient temperature) surrounding the transistor circuit is not always constant • Therefore, the transistor  is not a constant value ELEC 121

  3. BJT Emitter Bias • Draw Equivalent Input circuit • DrawEquivalent Output circuit • Write necessary KVL and KCL Equations • Determine the Quiescent Operating Point • Graphical Solution using Loadlines • Perform a Computational Analysis ELEC 121

  4. Emitter-Stabilized Bias Circuit Adding an emitter resistor to the circuit between the emitter lead and ground stabilizes the bias circuit over Fixed Bias ELEC 121

  5. Base-Emitter Loop ELEC 121

  6. Equivalent Network ELEC 121

  7. Reflected Input impedance of RE ELEC 121

  8. Base-Emitter Loop Applying Kirchoffs voltage law: - VCC + IB RB + VBE +IE RE = 0 Since: IE = ( + 1) IB We can write: - VCC + IB RB + VBE + ( + 1) IBRE = 0 Grouping terms and solving for IB: Or we could solve for IE with: ELEC 121

  9. Collector-Emitter Loop ELEC 121

  10. Collector-Emitter Loop Applying Kirchoff’s voltage law: - VCC + IC RC + VCE + IE RE = 0 Assuming that IE IC and solving for VCE: VCE=VCC – IC (RC + RE) If we can not use IE IC the IC = IE and:VCE=VCC – IC (RC + RE) Solve for VE: VE = IE RE Solve for VC: VC = VCC - IC RC or VC = VCE + IE RE Solve for VB: VB = VCC - IB RB or VB = VBE + IE RE ELEC 121

  11. Transistor Saturation At saturation, VCE is at a minimum We will find the value VCEsat = 0.2V For load line analysis, we use VCE = 0 To solve for ICSAT, use the output KVL equation: ELEC 121

  12. Load Line Analysis The load line end points can be calculated: At cutoff: At saturation: ELEC 121

  13. Emitter Stabilized Bias Circuit Example ELEC 121

  14. Design of an Emitter Bias CE Amplifier Where .1VCC VE  .2VCC And .4VCC VC  .6VCC ELEC 121

  15. Emitter Bias with Dual Supply ELEC 121

  16. Emitter Bias with Dual Supply Input Output ELEC 121

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