  In this paper, we present voltage- and current-mode differential magnetless
non-reciprocal devices obtained by pairing two single-ended (SE) circulators,
each consisting of three first-order bandpass or bandstop LC filters, connected
in either a wye or a delta topology. The resonant poles of each SE circulator
are modulated in time with 120 deg phase-shifted periodic signals, resulting in
synthetic angular-momentum biasing achieved through spatiotemporal modulation
(STM). We tailor the two SE circulators to exhibit a constant 180 deg phase
difference between their STM biases. Unlike conventional differential
time-variant circuits, for which only the even or odd spurs are rejected, we
show that the proposed configuration cancels out all intermodulation (IM)
products, thus making them operate alike linear time-invariant (LTI) circuits
for an external observer. In turn, this property enhances all metrics of the
resulting circulator, overcoming the limitations of SE architectures, and
improving insertion loss, impedance matching, bandwidth and noise figure. We
show that this differential architecture also significantly relaxes the
required modulation parameters, both in frequency and amplitude. We develop a
rigorous small-signal model to guide the design of the proposed circuits and to
get insights into their pseudo-LTI characteristics. Then, we validate the
theory with simulations and measurements showing remarkable performance
compared to the current state of the art of magnetless non-reciprocal devices.
