Biomedical Engineering Reference
In-Depth Information
−5
−35
−10
−45
−15
−55
−20
−65
−25
−30
−75
3
4
5
6
7
8
9
10
11
3
4
5
6
7
8
9
10
11
Frequency (GHz)
Frequency (GHz)
−9
18
−11
16
14
−13
12
−15
10
−17
3
4
5
6
7
8
9
10
11
3
4
5
6
7
8
9
10
11
Frequency (GHz)
Frequency (GHz)
Fig. 17
Measured S-parameters of the designed low-noise amplifier (LNA)
as better linearity and higher transconductance of this stage. Clearly, this will cause
a trade-off between power and other performance parameters. One approach that
designers took is using current bleeding technique [
25
,
26
] to improve noise, gain,
and linearity without dramatic power increase. Using this method is typically limited
to narrowband applications; and for UWB, several inductors should be used which
results in dramatic area increase. A popular method to overcome this trade-off is to
use a folding architecture. There are a number of works [
27
,
28
] that have utilized
this topology and achieved good bandwidth without sacrificing area or power. Using
a folded architecture, DC current of RF and local oscillator (LO) stages do not have
to be equal which results in more degrees of freedom in biasing and consequently
a better trade-off. The constraint will be even more relaxed when the biasing of RF
and LO stages are isolated.
Figure
19
shows the designed UWB mixer. As shown, a folded architecture with p-
type metal-oxide semiconductor (PMOS) LO-stage and inverter RF stage is chosen.
In conventional Gilbert cell topology, where RF and LO stages are stacked on top
of each other, the bias currents of both stages are tightly correlated which results in
a trade-off in performance parameters. For Gilbert cell, the conversion gain can be
written
CG
∝
g
m,RF
R
L
,
(9)
where,
g
m,RF
represents transconductance of the RF stage and
R
L
is the load
resistance. Also the input referred intercept point (IIP3) can be expressed as
I
DC,RF
3
K
RF
IIP
3
=
4
,
(10)
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