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MOSFET fT (Transit Frequency)

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esdeath_123

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Hi. I tried plotting the transit frequency on Cadence Virtuoso and I get a plot, with values in the GHz range. My question is, What does a transit frequency of a MOSFFET signify? How do I choose which transit frequency a transistor should operate if I want to, say, design an amplifier with a GBWP (gain bandwidth product) of 10MHz given that the plot has values in the GHz range?
 

Transition Frequency is-by definition- where the Short Circuit Current Gain drops down to unity.So, you should choose appropriate MOS dimension with associated Transition Frequency in order to get a suitable Gain.For 10MHz GBWP, a transistor GHz range of Transition Frequency is not necessary.
 
Meaning of fT that this is the absolute maximum frequency to create an oscillator or an amplifier on that process. If you design an amplifier with GBWP of 10MHz you don't have to care with fT, other parameters are more important like minimum width/length, supply voltage tolerance, threshold voltage, beta factor.
 
frankrose said:
Meaning of fT that this is the absolute maximum frequency to create an oscillator or an amplifier on that process. If you design an amplifier with GBWP of 10MHz you don't have to care with fT, other parameters are more important like minimum width/length, supply voltage tolerance, threshold voltage, beta factor.
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Thanks for the reply! I have a follow-up question. I'm now able to achieve the desired GBWP and gain for the common source transistor with active load that I'm designing but I found out that at different bias voltage and length/width values, I am able to get the same results. How do I choose which bias voltage and transistor length/width is the best?
 

If you posted your schematic it would help to answer this, because I don't know how you generate bias voltage for example, or what is the target you want to achieve?
 

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