Abstract Discrete bipolar operational amplifiers were irradi- ated with neutrons in order to study the evolution of the short circuit currents. Also, this paper explores the effect of the reduc- tion of this current in devices based on operational amplifiers. Index Terms COTS, displacement damage, bipolar opera- tional amplifiers, radiation tolerance. I. INTRODUCTION HE harsh environment expected in the cryogenic system of the LHC of CERN will affect the devices present in the electronic instrumentation. Unlike other radiation environ- ments, as space or nuclear plants, displacement damage will be very important, so neutron tests on devices built in bipolar technologies must be carried out in order to select the most tolerant commercial devices. The purpose of this paper is to discuss the effect of the displacement damage on the short circuit current since we believe that it is an interesting parame- ter whose behaviour is not well known. Furthermore, the con- sequences of the degradation of this parameter will be deter- mined in feedback networks and other devices where opera- tional amplifiers are integrated. Operational amplifiers built in bipolar technologies usually have three main stages: Input, gain and output. The first stage is a differential pair, whether bipolar or JFET, and the gain stage can be another bipolar differential pair or a Darlington transistor loaded with a current source [1]. The output stage of low signal bipolar op amps (power op amps are not included in this work) should have the following characteristics: First of all, it must be very linear to minimise the distortion and, second, the consumption should not be very high. Therefore, class AB complementary output stages are widely used [2]-[3]. Fig. 1a shows the push-pull network, the most simple network belonging to this kind of output stages. This output stage is complementary since there are This work was supported by the cooperation agreement K476/LHC be- tween CERN & UCM, by the Spanish research agency CICYT (FPA2002- 00912), and partially supported by ITN. F. J. Franco and J. A. Agapito are with the Departamento de Fisica Apli- cada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid SPAIN (Phone : +34 91 394 4434; e-mail: monti@fis.ucm.es). Juan Casas-Cubillos and M.A. Rodríguez-Ruiz are with the CERN, AT Division, Geneva, SWITZERLAND. Phone: + 41(0)22 767 31 31, fax: + 41 (0)22 767 46 20, email: Juan.Casas-Cubillos@cern.ch.. Y. Zong is with the Departamento de Fisica Aplicada III, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid SPAIN (Phone : +34 91 394 4441; e-mail: yzong@fis.ucm.es), on leave of absence of Wuhan Institute of Chemical Technology (China). NPN & PNP transistors that work in turns depending on the sign of V IN . The couple of diodes is used as a DC level shifter so that the input-output function becomes linear. In fact, if the op amp is loaded and the input voltage is negative, the active transistor is Q 2 so V OUT = V IN + V BE, Q2 ≈ V IN +0.7 (V IN < 0). If the input voltage is positive, Q 1 is working and V OUT = V IN + V D1 +V D2 - V BE , Q1 ≈ V IN + 0.7 (V IN > 0). Since both expres- sions are equal, the input-output function stage is continuous and linear. However, the pure push-pull network is found in very few types of operational amplifiers, like LF351. Other amplifiers, as OP-27, have a modified output stage (fig. 1b), with distor- tion compensation [3]. In this stage, V IN ≈ V OUT . Moreover, several improvements are introduced in many output stages [2]: Use of a Darlington pair or false PNPs (fig. 2a) to improve the low current gain of PNP transistors; replacement of D 1 & D 2 by other DC level shifter based on the BE junctions of bi- Neutron Effects on Short Circuit Currents of Bipolar Op Amps and Consequences F. J. Franco, J. A. Agapito, J. Casas-Cubillos, M.A. Rodríguez-Ruiz and Y. Zong T (a) (b) Fig. 1: Different kinds of class AB output stages. Push-pull network (a), and improved Class AB output stage with complementary emitter follower crossover distortion compensation (b). (a) (b) Fig. 2: (a) False PNP transistor to increase the current gain of PNP transistors. Also, (b) shows the use of a feedback network to limit the output current. If Q1 emitter current is too high, Q2 base-emitter voltage increases, taking cur- rent from Q1 base. 213