Definition of fT: Frequency at which current gain is dropped to 1 (0dB);
Definition of fmax: Frequency at which power gain is dropped to 1 (0dB)
3.1 Calculation of fT
Assumptions: 1) Drain is short to ground; 2) Input current source is ideal (no parallel resistance); 3) Ignore the feed-through current to output by Cgd; 4) Cgd is considered only in input impedance calculation
, à
à (3.01)
Fig 3.1 fT Calculation
3.2 Calculation of fmax
Assumptions: 1) Drain is short to ground; 2) Input and output are conjugate matching; 3) The series gate resistance of M1 is rg; 5) Ignore the feed-through current by Cgd; 6) Consider feedback current by Cgd in output impedance calculation.
Fig 3.2 fmax Calculation
Firstly, calculate the real part of output impedance according to Fig 3.2(c):
à à (3.02)
Input/output power and power gain is:
à (3.03)
Therefore, the maximum operation frequency of MOS transistor is:
(3.04)