MOSFET Small-Signal Amplification

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SUMMARY

The discussion focuses on the small-signal amplification of a MOSFET, specifically the relationship between transconductance (gm) and output conductance (gDS) in the context of circuit analysis. The equation gm * ΔUGS + gDS * ΔUDS = 0 is derived under the assumption that the drain current (I0) remains constant. Participants clarify that this derivation is consistent with Kirchhoff's Current Law (KCL) applied at the drain, leading to the conclusion that changes in gate voltage (Vg) and drain-source voltage (Vds) must balance each other out to maintain a constant current.

PREREQUISITES
  • Understanding of MOSFET operation and characteristics
  • Familiarity with small-signal models in electronics
  • Knowledge of Kirchhoff's Current Law (KCL)
  • Basic grasp of transconductance (gm) and output conductance (gDS)
NEXT STEPS
  • Study the derivation of the small-signal model for MOSFETs
  • Learn about the impact of transconductance on amplifier performance
  • Explore practical applications of KCL in circuit analysis
  • Investigate the relationship between Vgs, Vds, and drain current in MOSFET circuits
USEFUL FOR

Electrical engineers, students studying circuit design, and professionals involved in analog electronics who seek to deepen their understanding of MOSFET amplification techniques.

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Hi everyone,

I am trying to learn about circuits from some slides I obtained and I am stuck on a slide.

Here is a screenshot of it (it is in German): http://i.imgur.com/HCtWF.png

It says: Example: Illustration of maximum achievable amplification of a MOSFET with the use of the parameters gm (transconductance) and gDS (output conductance).

On the left is the circuit configuration. On the right is the effect of an input voltage change with x-axis drain voltage and y-axis drain current. It also has the derivation for the amplification.

What confuses me is how they obtained:

gm * ΔUGS + gDS * ΔUDS = 0

as a result of I0 being constant. Perhaps I am not understanding the graph completely. Can someone point me in the correct direction?

For reference:
gm = dID / dUG with UD const.
gDS = dID / dUD with UG const.

Many thanks.
 
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The change in I0 is represented by the sum of ( gm * change in Vg + gd * change in Vd ). You know this already if you understand the definition of gm and gd. Now they assumed the current to be constant. Thus they had to equate this to zero.
 

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