IEEE Solid-State Circuits Magazine - Winter 2015 - 11
where g mb ^. 0.2g mh denotes the
transconductance with respect to the
body terminal. The two transistors
are, of course, placed in separate
n -wells. Also, the drain voltages
must remain above approximately
VDD - 0.5 V to ensure the drain-well
forward bias negligibly affects the
transistors' performance.
Cross coupling through the bulk
proves useful in low-voltage designs.
Shown in Figure 3(b) is an amplifier
example [7], where VDD . 0.5 V and
the XCP raises the voltage gain. Here,
the input common-mode (CM) level
and Vb can be near zero while the four
transistors are accorded sufficient
VDS so as to operate in the saturation
region. (The CM feedback circuit [7] is
not shown.) We point out that, in the
presence of mismatches, the XCP can
regenerate and cause latch-up if its
loop gain is equal to unity or higher.
The design in [7] employs a replica
loop to avoid this situation.
VDD
Vb
VDD = 0.5 V
+
Vin
Vb
M1
M2
Vb
-
Vin
Vb
M3
M4
M2
M1
Vout
Req
(a)
(b)
Figure 3: (a) An XCP using the bulk terminal and (b) the first stage of an op amp using the
XCP.
VDD
L1
RL
VDD
L2
L1
+
+
-
Vin
Vin
M1
L2
RL
-
Vin
M2
Vin
M1
M3
(a)
M2
M4
(b)
Figure 4: (a) A simple PA output stage and (b) the use of XCP to reduce input capacitance.
Output Stage
In this article, we
study applications
of the cross-coupled
pair in analog and
RF circuits.
RD
RD
X
VDD
Vout
M2
M1
Y
Vin
M4
M3
CC
NIC
Figure 5: The use of NIC to increase bandwidth in a broadband transmitter.
M2
M1
ESD
Termination
Resistors
ESD
The XCP has also been used to
improve the performance of RF
power amplifiers (PAs). In the simple
output stage shown in Figure 4(a), M 1
and M 2 are wide enough to carry a
high current, thus presenting a large
input capacitance. The preceding
stage therefore tends to consume a
high power. This issue can be alleviated if some of the input capacitance is driven by the output port,
i.e., if a portion of W 1, 2 is reconfigured as an XCP [Figure 4(b)] [8]. In
the limit, M 3 and M 4 form an oscillator along with L 1 and L 2 that is
injection-locked to the input signal
[8]. Such an arrangement provides
efficient amplification for constantenvelope signals. Interestingly, the
XCP also helps create CM stability
here by presenting a CM impedance
equal to 1/ (2g m3, 4) to ground.
CC
Figure 6: The use of NIC to increase bandwidth of I/O interfaces.
IEEE SOLID-STATE CIRCUITS MAGAZINE
w i n t e r 2 0 15
11
Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Winter 2015
IEEE Solid-State Circuits Magazine - Winter 2015 - Cover1
IEEE Solid-State Circuits Magazine - Winter 2015 - Cover2
IEEE Solid-State Circuits Magazine - Winter 2015 - 1
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