Digital Signal Processing Reference
In-Depth Information
10
10
BGE
Zerovgs
Hybrid
Diode
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
Δ
V
t
[V]
Δ
V
t
[V]
(a)
(b)
Fig. 3.8
The effect of a
V
T
change on the common-mode output voltage and the gain of a
differential amplifier for four different load topologies
10
10
BGE
Zerovgs
Hybrid
Diode
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
−2
−1.5
−1
−0.5
0
0.5
1
1.5
2
Δ
V
t
[V]
Δ
V
t
[V]
(a)
(b)
Fig. 3.9
The effect of a
V
T
change on the common-mode output voltage and the gain of a
differential amplifier with ideal, yet finite loop gain, CMFB for four different load topologies
sensitive to
V
T
changes due to the input transistors that are biased with a small gate
overdrive. This is also the reason why the zero-
V
GS
load amplifier performs better in
this case where the high sensitivity of the input transistors is partly neutralized by an
inverse sensitivity in the load transistors. It is clearly visible that the BGE amplifier
and the diode load amplifier show an identical DC behavior as expected. As can be
seen in the right panel of Fig.
3.8
the diode load, the hybrid load and the zero-
V
GS
load amplifiers perform with a stable gain that is the lowest for the diode load and
the highest for the zero-
V
GS
load. Although the BGE amplifier performs with a high
gain, this gain is sensitive to
V
T
.
The explanation for this remarkable observation is given by the output resistance
r
sd
,
3
of the BGE load transistor that is inversely proportional to the squared gate
overdrive, as shown in Eq. (
3.14
).
V
T
changes and drops down for high values of
