Cs_mosfet Amplifier Troubleshooting

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Discussion Overview

The discussion revolves around troubleshooting a common-source MOSFET amplifier, specifically focusing on discrepancies between simulated and actual hardware gain measurements. Participants explore potential reasons for the observed differences in gain values.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant reports a significant difference in gain between simulation (-11.3) and hardware (-40 to -45) for a common-source MOSFET amplifier using a BS170 MOSFET.
  • Another participant questions the validity of the assumed transconductance (gm) value of 1 mA/V for the real MOSFET, suggesting the need to measure the threshold voltage (Vgs(th)) and the K-factor.
  • Further inquiries are made regarding the verification of the correct transistor and resistor usage, as well as the DC operating point of the circuit.

Areas of Agreement / Disagreement

Participants express uncertainty about the accuracy of component parameters and the setup, indicating that multiple factors could contribute to the discrepancies in gain measurements. No consensus is reached on the specific cause of the issue.

Contextual Notes

Limitations include potential inaccuracies in component specifications, measurement techniques, and the need for verification of circuit conditions.

pforpashya
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Hi all,

I am studying cs_mosfet amplifier and i have simulate one simple amplifier with VDD=+12v Vs=1v ID=0.5mA gm=1mA/v Vd=6v. I am getting a signal that is amplified by gain -11.3 by calculation and also on simulation but when doing on hardware i am getting gain around -40 to -45
I am using Bs 170 mosfet.

I have tried various combination of capacitors but still gain comes around -40...can anybody suggest me where is the mistake?
 
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can anybody tell me how to attach file here?
 
How can you be sure that your real MOSFET Bs170 will indeed have gm = 1m?
Do know how to measure Vgs(th) and how you can find K - factor ?

PS Go advanced ---> Attach Files
 
Last edited:
Can you verify you are using the correct transistor?

Can you verify you are using the correct resistor?

Have you verified the DC operating point?
 

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