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Impedance calculation (CS stage)

Hello.

I am trying to calculate the output impedance for the following circuit in SPICE notation (CS stage with drain-gate feedback):

  • input signal vin 1 0

  • output signal (output port, where impedance should be calculated) vout 3 0

  • resistors rs 1 2 rf 2 3

  • nmos (drain, gate, source, bulk) mosfet1 3 2 0 0

Unfortunately, I am getting odd results in SPICE, probably because the circuit I entered was missing the DC bias.

But, apart from SPICE, I would be mainly interested in the analytical solution as a function of rs and rf...

Can anyone help ?

Thanks !

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Approximate MOSFET as a controlled current (drain-to-source of value gm*Vgs) with drain-to-gate capacitance, Cgd, and gate-to-source capacitance, Cgs.

For paper analysis you don't include a bias source, you've assumed a gm value.

For simulation you use a proper MOSFET model and include a bias source so as to cause enough current to flow to get a good gm. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

     Thinking outside the box... producing elegant solutions. 

"It is not in doing what you like, but in liking what you do that 
is the secret of happiness."  -James Barrie
Reply to
Jim Thompson

It occurs to me that there is a fundamental problem: Only a resistor, RF, from drain-to-gate forces the MOSFET to have (at DC), VDS=VGS, in other words, the _diode_ connection... so impedance measurements will be at the "knee"... not a very useful value. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 

     Thinking outside the box... producing elegant solutions. 

"It is not in doing what you like, but in liking what you do that 
is the secret of happiness."  -James Barrie
Reply to
Jim Thompson

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