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Emitter degeneration is a common technique used in transistor amplifier circuits to improve stability and linearity. It involves adding a resistor in the emitter leg of a bipolar junction transistor (BJT) to influence the circuit’s behavior. This article explains the theory behind emitter degeneration, how to perform relevant calculations, and the benefits it provides in circuit design.
Theory of Emitter Degeneration
Emitter degeneration introduces negative feedback into the transistor circuit. By placing a resistor in the emitter, the voltage across it varies with the emitter current. This variation reduces the gain sensitivity to transistor parameter changes, such as beta (β). As a result, the circuit becomes more stable and less affected by transistor variations or temperature changes.
Calculations for Emitter Degeneration
The key calculation involves determining the emitter resistor value (RE). It is typically chosen to set the desired gain and stability. The approximate voltage gain (AV) with emitter degeneration is given by:
AV ≈ RC / (RE + (1 + β)RE)
where RC is the collector resistor. To achieve a specific gain, RE can be calculated based on the desired AV and known resistor values. Additionally, the emitter resistor affects the bias point and must be selected to ensure proper transistor operation.
Circuit Benefits of Emitter Degeneration
Adding an emitter resistor offers several advantages:
- Improved stability: Reduces the effect of transistor parameter variations.
- Enhanced linearity: Minimizes distortion in the output signal.
- Controlled gain: Allows precise setting of the amplifier’s gain.
- Temperature compensation: Mitigates the impact of temperature changes on circuit performance.