  The time dependent master equation from the seminal article by Ragazzini,
Randall and Russell [J. R. Ragazzini, R. H. Randall and F. A. Russell,
"Analysis of Problems in Dynamics by Electronic Circuits", Proc. of the I.R.E.,
Vol. 35, pp. 444--452, (1947)] is recovered as necessary tool for the analysis
of contemporary circuits with operational amplifiers. This equation gives the
relation between time dependent the output voltage $U_0(t)$ and the difference
between the input voltages ($U_{+}(t)$ and $U_{-}(t)$). The crossover frequency
$f_0$ is represented the time constant $\tau_{_0}$ of this equation. The work
of the master equation is illustrated by two typical examples: a) the stability
criterion of the devices with negative impedance converters, which we consider
as a new result b) the frequency dependence of the amplifiers with operational
amplifiers given in the technical specifications without citations of time
dependent equation. A simple circuit for determination of $f_0$ is suggested
and the method is illustrated by determination of crossover frequency for the
low-noise and high speed ADA4898 operational amplifier. It is concluded that
for an exact calculation of the pass bandwidth of amplifiers with active
filters the 70 years old master equation is a useful technique implicitly
included in the contemporary software. The frequency dependent formulae for the
amplification coefficient of inverting and non-inverting amplifiers are given
for the case of non-zero conductivity between the inputs of the operational
amplifiers.
