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BJT Emitter Stabilized BiasPowerPoint Presentation

BJT Emitter Stabilized Bias

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### BJT Emitter Stabilized Bias

ELEC 121

- 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

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

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

ELEC 121

Equivalent Network

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Reflected Input impedance of RE

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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

ELEC 121

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

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:

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Emitter Bias with Dual Supply

ELEC 121

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