Is BJT Impedance Proportional to Ic?

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SUMMARY

The discussion centers on the relationship between BJT impedance and collector current (Ic). It establishes that the transconductance (gm) is defined as gm = Ic/Vt, where Vt is the thermal voltage. The impedance of the current source in a BJT is indeed 1/gm, confirming that as Ic increases, gm also increases, thereby affecting the impedance. The input resistance of a diode-connected BJT is represented as (rbe || 1/gm).

PREREQUISITES
  • Understanding of BJT operation and equivalent circuit models
  • Knowledge of transconductance (gm) and its significance in BJTs
  • Familiarity with thermal voltage (Vt) and its role in semiconductor physics
  • Basic concepts of input resistance in electronic circuits
NEXT STEPS
  • Study the derivation and implications of transconductance (gm) in BJTs
  • Explore the concept of input resistance in diode-connected BJTs
  • Learn about thermal voltage (Vt) and its effects on semiconductor behavior
  • Investigate the impact of varying collector current (Ic) on BJT performance
USEFUL FOR

Electrical engineers, electronics students, and anyone involved in the design and analysis of BJT circuits will benefit from this discussion.

likephysics
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In the equivalent diagram of BJT, Vbe and rbe represent the base voltage and resistance.
gmVbe is a current source, which represents the current gain.

Since gm = Ic/Vt

Vt/Ic = 1/gm
Does this mean the impedance of the current source is 1/gm ?
I know this sounds a bit weird. It's because of a homework problem posted here -
https://www.physicsforums.com/showthread.php?t=569541
(the input resistance of the diode connected BJT is (rbe||1/gm).
 
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likephysics said:
In the equivalent diagram of BJT, Vbe and rbe represent the base voltage and resistance.
gmVbe is a current source, which represents the current gain.

Since gm = Ic/Vt

Vt/Ic = 1/gm
Does this mean the impedance of the current source is 1/gm ?
I know this sounds a bit weird. It's because of a homework problem posted here -
https://www.physicsforums.com/showthread.php?t=569541
(the input resistance of the diode connected BJT is (rbe||1/gm).

You're confusing up things. gm means trans-conductance. it is derived as gm = dIc/dVbe = Ic/Vt. This Vt is thermal equivalent of voltage and is constant for a particular temperature.
It only means gm is proportional to Ic. And it does mean the current source has 1/gm impedance.
 

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