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[SOLVED] transconductance measuring in the OTA simulation

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an_82

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How can I measure transconductance (gm) of a OTA ? or how may I draw transconductance of OTA in terms of input voltage variation?
I want to divide the input voltage difference signal by output current, but it doesn't allow because when vin=0, the gm is infinite !!
 

an_82 said:
How can I measure transconductance (gm) of a OTA ? or how may I draw transconductance of OTA in terms of input voltage variation?
I want to divide the input voltage difference signal by output current, but it doesn't allow because when vin=0, the gm is infinite !!
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The OTA transconductance should NOT vary with the input voltage (within the normal operating range). It is a property of the amplifier that can be determined by the quiescent dc current.
What do you expect by dividing the input by the output? I suppose, you mean the output-to-input ratio. For vin=0 the output should also be zero. Transconductance never can be "infinite" (it is a fixed value, see above).
 
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    an_82

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Dear LvW,
I want to draw the "gm" of the OTA circuit, since gm=Iout/(vin+ - vin-). Is it right ? If not, pls explain how I can draw the transcendence? In fact I want to demonstrate the linearity of the designed OTA by showing the gm curve.

LvW said:
The OTA transconductance should NOT vary with the input voltage (within the normal operating range). It is a property of the amplifier that can be determined by the quiescent dc current.
What do you expect by dividing the input by the output? I suppose, you mean the output-to-input ratio. For vin=0 the output should also be zero. Transconductance never can be "infinite" (it is a fixed value, see above).
Click to expand...
 

gm is usually understood as a small signal quantity respectively slope of the transfer characteristic, gm = dIout/dVin.

You can either plot the transfer characteristic Iout versus Vin or it's derivative gm versus Vin. You can run a .DC analysis, shorting the output node to mid supply.
 
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    megaic

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