IEEE Solid-State Circuits Magazine - Spring 2015 - 47
VLO
LNTA
VnSW
R
R/2
Cpar
IRF
2 I (ω - ω )
LO
π RF RF
VnTIA
=
+ -
VLO
+
Rout
(Mixer)
Vout
-+
VnTIA
-
4kTRout
(from VnSW)
Cpar
R/2
Vout
1
Rout =
2 Cpar fLO
VLO
Figure 13: Equivalent circuit for current-mode mixer.
an overall noise factor that is only
inversely proportional to A + 1.
This LNTA is wideband, and it can
have a noise factor smaller than 2.
However, it suffers from a very low
transconductance gain that is equal
to 1/R s .
noise-Cancelling LnTA
The last LNTA presented is the
noise-cancelling amplifier shown in
Figure 12(d) [7], [8]. In this case, the
input impedance of the LNTA is realized with a transistor in common
gate configuration. For a proper
matching its transconductance must
be set equal to 1/R s .
The common gate stage creates
an additional feed-forward path in
parallel to the main path given by
the transconductor g m . The noise
produced by the common gate stage
has an inverting transfer function
in both paths. On the contrary, the
input signal has an inverting transfer function only in the main path.
This makes it possible to cancel the
noise of the common gate stage by
subtracting the signal between the
two paths, after a proper renormalization of the gain. The noise factor
for this LNTA is given by
c
nfLNTA = 1 +
.
gm RS
compared to the inductive degenerated LNTA the noise of the transconductor is not reduced since there is
no signal amplification in front of it.
Current-Mode Passive Mixer
The key element in the design of a current-mode passive mixer [Figure 9(b)]
is the parasitic capacitance (C par)
present at its input. The value of
this capacitance is the main limitation in the noise performance and in
the output impedance provided by
the mixer.
C
R
R
+ -
lin
-+
+ -
lin
Vout
Vout
-+
R
R
C
(a)
(b)
C
C
R2
R
R
C1
+ -
lin
C
Vout
-+
R
R
(13)
As in the previous LNTA, the
noise factor can be smaller than 2
since the noise of the input termination has been cancelled. However,
To study the noise and the interaction of the mixer with the TIA, the
current-mode passive mixer can be
modeled with a Thevenin equivalent circuit having as current source
the down-converted signal normalized by the conversion gain 2/r
and as output impedance a resistor
R out = 1/2C par fLO (Figure 13) [9].
In the noise analysis, the resistance
R out plays two different roles. The
noise injected by the resistance R out
models the noise produced by the
switches [10]. In addition to that, the
C
C2
R1
+ -
lin
-+
C1
Vout
R1
R2
(c)
C2
(d)
Figure 14: (a) Simple TIA, (b) first-order TIA, (c) second-order TIA with real poles, and
(d) second-order TIA with complex poles.
IEEE SOLID-STATE CIRCUITS MAGAZINE
s p r I n g 2 0 15
47
Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Spring 2015
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