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There are multiple hFE options for BD139s, with the higher hFE being slightly more expensive. What advantages or disadvantages, if any, would the higher hFE parts have in emitter follower circuit?

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Advantages of higher hFE:

  • Higher input impedance to the circuit
  • Lower output impedance

This way the emitter follower load effects to previous and next stages are reduced.

Disadvantages:

  • Depending on the polarization of the transistor, high hFE can affect negatively the stability due to variations on temperature.

References: Emitter Follower, Ruye Wang, Electronic Circuits - I. pp. 1-21, A.P.Godse, U.A.Bakshi

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For a given bias current, the relationship between input impedance and hFE is one of proportionality.

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given the beta results from carriers injected by the emitter, attempting to impact carriers arriving from the base pin and yet missing thus continue on into the collector, with THINNER BASE regions producing HIGHER BETA, we have some interesting changes in behavior with high beta.

Higher beta means thinner base regions.

Thinner base regions mean FASTER TRANSISTORS.

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