# [SOLVED]transconductance measuring in the OTA simulation

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#### an_82

##### Member level 2
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 !!

any response ?

megaic

### megaic

Points: 2

#### LvW

##### Advanced Member level 5
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 !!

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).

an_82

### an_82

Points: 2

#### an_82

##### Member level 2
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.

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).

#### FvM

##### Super Moderator
Staff member
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.

megaic

Points: 2