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How to realize a MOS resistor?

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cheenu_2002

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Hi,
I am trying to use a MOSFET in place of a resistor. I am new to ckt design and I am finding it very difficult to bias it. Can anyone help me to understand how to bias a MOS in linear region so that it can act as a resistor. I used the ckt attached but I dont know if its the correct way to connect the MOS... I also dont know how to measure the Vbe of the BJT in the ckt. Both base and emitter are showing the same voltages. I cudnt understand whats happening.
 

nschutten

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A MOSFET is in its linear region when VDS<VGS-VT. This is guaranteed by ensuring VD=VG as in your diagram. Note that you need to connect the bulk connection of qn0 to GND, not to its source. Also, you may be experiencing bias issues due to the use of your current source (since this is ideal, if the device dimensions don't allow for the current to go through that you want, unexpected simulation results may occur).
Lastly, in your circuit diagram the base node is tied to GND, as well as the emitter node. So VBE=0V. The transistor is connected as a diode, not as a transistor.
 

Raviprasad_K

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Hi,

I'm confused by the above statement. When VD=VG, how does it satisfy VDS<VGS-VT.
VD=VG is the condition to make sure that the transistor is in saturation.
 

cheenu_2002

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Vds>Vgs-Vt is the condition to maintian the MOS in saturation. By tie'ing gate to drain, the MOS can be switched ON but can be biased to be in linear region or saturation. Pls correct me if I am wrong.
 

erikl

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nschutten said:
Lastly, in your circuit diagram the base node is tied to GND, as well as the emitter node. So VBE=0V. The transistor is connected as a diode, not as a transistor.
You probably meant: "as well as the collector node. So VCB=0V" !?
 

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