IEEE Solid-States Circuits Magazine - Fall 2021 - 114

communication systems, such as fullduplex
(FD) wireless systems [16]-[19]
and radars [20]-[24] with the three
ports connected to the transmitter (Tx),
antenna (ANT), and receiver (Rx). The
signal generated by the Tx circulates
to the ANT, while an incoming signal
at the ANT circulates to the Rx. Circulators
are also used ubiquitously in
cryogenic quantum computing applications
to excite and read out qubits
[25], [26]. In the rest of this review, we
will introduce recent advances in nonmagnetic
nonreciprocal components
based on 1) active transistor, 2) nonlinearity,
and 3) spatiotemporal modulation
schemes.
Using Active-Biased Transistors
Transistors that are biased through a
dc current or voltage are inherently
nonreciprocal because of their unilateral
gain and can be used as isolators
since their gate-drain gain is larger
than their drain-gate transmission.
Active transistor-based circulators
became popular for low-frequency
applications, where ferrite circulators
did not exist (below 100s of
MHz), in applications with relatively
low power handling, in cost- and sizelimited
implementations, and in scenarios
where frequency tuning was
of importance.
Active three-port nonreciprocal
devices can be classified into two
groups: circulators and quasi-circulators
(QCs). QCs are three-port devices
in which the transmission only occurs
between two pairs of ports, and the
third pair of ports is isolated in both
directions. Fully symmetric active
circulators can raise stability concerns
and, hence, are usually realized
with some loss [27]. QCs are of interest
in transceivers, where Tx-to-ANT and
ANT-to-Rx transmissions are necessary
and the Tx and Rx should always
be isolated. They can also provide
gain in the two transmission directions
[27], [28]. Each of the circulator
and QC categories can either be implemented
using only transistors and
lumped components, as was shown in
[15] (Figure 3), or can combine active
and distributed microwave components,
such as hybrids and couplers
[27]. Some recent examples of active
circulators and QCs are described in
[29] and [30].
Another example of using active
devices to enable nonreciprocity is
demonstrated in [31] and [32]. These
nonreciprocal components are built
using transistor-loaded ring resonators
that only allow a unilateral current
to flow inside the loop. The
implementation of a nonreciprocal
isolator and circulator using this idea
is shown in Figure 4. While the basic
operating principle is no different from
that of the lumped active circulators
described earlier, the coupling of many
such transistor-loaded ring resonators
together can realize a nonreciprocal
metamaterial or synthetic medium.
Active approaches are compatible
with IC integration and have applications
in low-power communication
[30] and biomedical systems [21] but
eventually are limited by the noise
and nonlinearity introduced by the
active devices [27]. As a result, they
do not find utility at the front end of
transceivers for traditional wireless
communication and radar applications,
where Tx power handling and
Rx noise performance are paramount.
Using Nonlinearity
Another avenue to achieve nonreciprocity
is by exploiting nonlinearity
in a material or circuit. In a nonlinear
system, an asymmetric physical
geometry can translate into an
asymmetric wave propagation. The
operation of such nonlinear components
generally relies on the fact that
the response of a nonlinear medium
depends on the amplitude of the signal
propagating through it. In many
cases, nonlinearity within a media is
compressive, implying that signals
with larger amplitudes experience
more attenuation. For example, consider
the following intuitive example
taken from [33] (Figure 5). The spatial
asymmetry (expressed in terms of
different permittivities) causes the
signals incoming from one side of
the nonlinear region to have a greater
magnitude, and hence undergo more
attenuation. The signals incident from
the other side, however, are smaller in
magnitude and suffer less attenuation.
An RF implementation of an isolator
based on the third-order nonlinearity
of resonators has been demonstrated
by using a cascade of a nonlinear
Lorentzian and a nonlinear Fano
resonator [34] and is shown in Figure
6. The nonlinear resonators
are built by taking advantage of
90 mm
2 mm
w = 15 mm
Port #1
70 mm
(a)
(b)
g = 1 mm
14 mm
VDD
g = 1 mm
d = 7 mm
Port
Port #2
d = 8.25 mm
VDD
(c)
FIGURE 4: (a) The field-effect transistor-loaded ring resonator concept. (b) The isolator and (c) circulator prototypes developed in [31].
114
FALL 2021
IEEE SOLID-STATE CIRCUITS MAGAZINE
Port #3
Port #1
VDD
40 mm
IEEE Solid-States Circuits Magazine - Fall 2021

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