ft and fmax of transistor

[i_am_here]

fmax transistor

What are exactly ft and fmax of transistor and upto what fraction of ft and fmax transistor can be used efficiently?

 

[elbadry]

transistor ft fmax

ft is the frequency at which the transistors current gain falls to unity ( with the drain and source AC short circuits )

fmax is the frequency at which the transistor's power gain falls to unity ( with its input and output matched )

ft is usally easier to simulate.

what fraction of ft to use depends on the apllication. for an amplifier 1/10 I think is enough. for VCO's/switches higher fractions can be used 1/2 or 1/3

Note that you can overcome the ft limitation by tuning out the parasitics; a technique that was used for high frequency circuits in older technologies. Also note that ft is misleading since the actual performance of the transistor will depend on the actual parasitics on its terminals in the real circuit