Radio Frequency Integrated Circuit Design - Webs

150 Radio Frequency Integrated Circuit Design
where
The pole and zero are given by
if g m R E >> 1, then
and
f z1 ≈ g m
C� =−� T
f p1 ≈−
1
C� R A
R A = r� || R B + R E
1 + g m R E
R A ≈ R B + R E
g m R E
R E
f p1 ≈−
R E + R B
� T
(6.19)
(6.20)
(6.21)
(6.22)
(6.23)
The input impedance of this amplifier can also be determined. If g m R E
>> 1, then we can use the small-signal circuit to find Z in. The input impedance,
again ignoring C� , is given by
Z in = Z� + R E (1 + g m Z� ) (6.24)
Likewise, the output impedance can be found and is given by
Z out = r� + R B + sC� r� R B
1 + g m r� + sC� r�
(6.25)
Provided that r� > R B and �C� r� > r� at the frequency of interest, the
output impedance simplifies to
1 + g m R B j� /�T
Z out ≈ re
1 + j� /�T
(6.26)