I want to ask a question about common source circuit with inductive load.
For example, there is a circuit as in the picture. I removed coupling capacitor and arrange it as below to make it simple, though I know it is only in theory not practical.
VGS: Bias voltage to make M operate in saturation region.
vgs: Small signal voltage that need to be amplified.
Let's assume that VGS= constant so that the transistor is in saturation region.
And the small signal:
\(v_{gs} = V_{0} sin( \omega t)\)
The transconductance of the transistor, gm, is:
\(g_{m} = \mu _{n} c_{ox} \frac{W}{L}( V_{GS} - V_{TH} ) = constant \)
Drain current, iD:
\(i_{D}= g_{m} v_{gs}= g_{m} V_{0}sin( \omega t)\)
The voltage across the inductor:
\(v_{L} = L \frac{d i_{L} }{dt} = L \frac{d}{dt} (g_{m} V_{0}sin( \omega t)) = \omega Lg_{m} V_{0}cos( \omega t)\)
The voltage at D:
\( v_{D}= -v_{L} + V_{DD} = - \omega Lg_{m} V_{0}cos( \omega t) + V_{DD}\)
If gm, ω, VGS are fixed, then the magnitude of vL is directly proportional with L. We can make it lager arbitrarily by choosing the value of L.
And therefore, the voltage swing at D can be very large beyond the range from 0 to 2VDD, right?
I have heard somewhere that the maximum voltage swing at D is 2VDD and ranges from 0 to 2VDD.
Is that right?
For example, there is a circuit as in the picture. I removed coupling capacitor and arrange it as below to make it simple, though I know it is only in theory not practical.
VGS: Bias voltage to make M operate in saturation region.
vgs: Small signal voltage that need to be amplified.
Let's assume that VGS= constant so that the transistor is in saturation region.
And the small signal:
\(v_{gs} = V_{0} sin( \omega t)\)
The transconductance of the transistor, gm, is:
\(g_{m} = \mu _{n} c_{ox} \frac{W}{L}( V_{GS} - V_{TH} ) = constant \)
Drain current, iD:
\(i_{D}= g_{m} v_{gs}= g_{m} V_{0}sin( \omega t)\)
The voltage across the inductor:
\(v_{L} = L \frac{d i_{L} }{dt} = L \frac{d}{dt} (g_{m} V_{0}sin( \omega t)) = \omega Lg_{m} V_{0}cos( \omega t)\)
The voltage at D:
\( v_{D}= -v_{L} + V_{DD} = - \omega Lg_{m} V_{0}cos( \omega t) + V_{DD}\)
If gm, ω, VGS are fixed, then the magnitude of vL is directly proportional with L. We can make it lager arbitrarily by choosing the value of L.
And therefore, the voltage swing at D can be very large beyond the range from 0 to 2VDD, right?
I have heard somewhere that the maximum voltage swing at D is 2VDD and ranges from 0 to 2VDD.
Is that right?
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