  Superparamagnetic tunnel junctions (SMTJs) are promising sources of
randomness for compact and energy efficient implementations of probabilistic
computing techniques. Augmenting an SMTJ with electronic circuits, to convert
the random telegraph fluctuations of its resistance state to stochastic digital
signals, gives a basic building block known as a probabilistic bit or $p$-bit.
Though scalable probabilistic computing methods connecting $p$-bits have been
proposed, practical implementations are limited by either minimal tunability or
energy inefficient microprocessors-in-the-loop. In this work, we experimentally
demonstrate the functionality of a scalable analog unit cell, namely a pair of
$p$-bits with programmable electrical coupling. This tunable coupling is
implemented with operational amplifier circuits that have a time constant of
approximately 1us, which is faster than the mean dwell times of the SMTJs over
most of the operating range. Programmability enables flexibility, allowing both
positive and negative couplings, as well as coupling devices with widely
varying device properties. These tunable coupling circuits can achieve the
whole range of correlations from $-1$ to $1$, for both devices with similar
timescales, and devices whose time scales vary by an order of magnitude. This
range of correlation allows such circuits to be used for scalable
implementations of simulated annealing with probabilistic computing.
