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Transistor Channel Length

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When designing for analog circuit, it is advisable to make the length of the transistor longer then the width? Wanted to this to reduce the power consumption (increase resistance and reduce current).

Besides that, is there a limit to how long the transistor length can be?
 

if u hav increased the length of the device its ro will be better but gm is proportional to w/l of device hance u hav to increase w by the same amount. That will cause more parasitic capacitances in ur design and more area.
 
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There is always trade-offs in Analog design. Don't just go one direction
 

Basically in analog, no restrictions are placed on that L must be smaller than W. However, you must think about why you need to do that and consider all the parasitic effects (slow, low gain, large area, large para. cap. etc). Usually in analog, the situation that L > W appeared mostly in troide region devices.
 
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I think if your L is too long ,then the symmetry of your layout will be more difficult, because this transistor will be very different with others.
 

If you want to be lower power, it is better to change you bias circuit.
Long length will cause more cap, less gm. And it offen use in startup circuit.
 

Bias circuit? Like for a simple two stage amplifier... there is a bias circuit?
 

There is no limitation on the length except the process limitation and the model parameter accuracy limitation if you beyond certain value.

There is also no rule that length must be longer than width.

The length decision is solely because of the circuit requirement and process accuracy.
 

Change your current source. The bias circuit is generate your current source
 

Short channel will improve the velocity!
 

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