ISSN 1063-7397, Russian Microelectronics, 2008, Vol. 37, No. 1, pp. 55–61. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © T.M. Agakhanyan, 2008, published in Mikroelektronika, 2008, Vol. 37, No. 1, pp. 60–66.
Monolithic operational ampliﬁers (opamps) are
known to experience severe photocurrents when
exposed to pulsed ionizing radiation, as in satellite
communications, nuclear technology, etc. [1–5]. The
recovery time can be of order tens of microseconds ,
which is unacceptable with some types of equipment.
Generally speaking, there are different ways to
enhance opamp radiation hardness, such as changing
from junction to dielectric isolation of elements or from
diffused resistors to thin-ﬁlm ones in order to reduce
photocurrents . Yet another course of action is to add
diodes that will form bypass paths for photocurrents, as
was done with the
A 7444 opamp . These
approaches, however, cannot shorten the recovery time
to a signiﬁcant extent, still less eliminate the need for
The point is that the actual source of opamp mal-
function lies in the asymmetric circuit that converts a
differential signal to a single-ended one, delivering the
latter to the intermediate ampliﬁer . A radiation dis-
turbance to the single-ended signal is thus passed to the
other stages. This explains why the recovery time can-
not be reduced to zero by the circuit-level methods we
discussed previously .
The above problems can be solved by eliminating
the converter. An example is the current-feedback
opamp, which is a two-channel circuit that is based on
complementary bipolar transistors and uses a push–pull
voltage follower to combine the channel output volt-
It is advantageous to realize the complementary
transistors on a bicrystal substrate and to use a dielec-
tric for their isolation. The latter is considered the most
effective way to control photocurrent pulses ; it
should not lead to serious difﬁculties in manufacture.
Moreover, this approach should simplify the fabrication
of thin-ﬁlm resistors in place of diffused ones.
Complementary bipolar transistors provide bypass
paths for photocurrents, as was done with the
opamp . In addition to protecting the transistors, this
strategy makes for much lower peak photocurrents
through the external circuitry, since the respective pho-
tocurrents through complementary transistors are in
opposite directions. In fact, the photocurrents would be
suppressed completely if the complementary transis-
tors had exactly the same parameters.
Note also that a photocurrent generates noise while
crossing an isolation junction; therefore, changing to
dielectric isolation tends to improve the noise perfor-
mance of the circuit, as does using thin-ﬁlm resistors
instead of diffused ones.
2. MODELING THE TRANSIENT
Problems arising in the course of prediction of radi-
ation behavior of integrated circuits and, especially,
associated with increasing the radiation hardness of
electronic equipment based on integrated circuits, can
be most effectively resolved based on macromodels of
separate blocks of the circuit, which are formed by a
group of elements, whose radiation behavior deter-
mines that of the circuit under consideration . Fig-
ure 1 shows a simpliﬁed model of an npn transistor in
an analog circuit with dielectric isolation. The upper
part of the model describes the active action of the tran-
sistor, which is characterized by the parameters of the
base region: normal and inverse transfer coef-
ﬁcients of minority carriers, and diodes of emitter
junctions with thermal currents
, which are determined for electron components
of carrier ﬂows .
Current-Feedback Operational Amplifier: Some Features
of Its Transient Radiation Response
T. M. Agakhanyan
Specialized Electronic Systems (SPELS), Moscow, Russia
Received April 9, 2007
—The behavior is investigated of a current-feedback opamp exposed to pulsed ionizing radiation suf-
ﬁciently intense to produce voltage surges in the opamp. For a system using such opamps, the possibilities of
maintaining normal operation under the stated conditions are explored.
PACS numbers: 85.40-e