References zyxwvutsrqpon BtIAsiN, K.B zyxwvutsrqponmlkjih , and CONNOLLY, D J : ‘Advances in gallium arsenide monolithic microwave integrated-circuit technology for space communications systems’, zyxwvutsrqpo IEEE Trans., 1986, MTT-34, (IO), pp. 994-1 001 NGUYEN, N.M , and MEYER. R.G.:‘Si IC-compatible inductors and LC passive filters’, IEEE J. Solid-State Circuits, 1990, 25, (4), pp. 1028-1031 NGUY~N. N M., and MEYER, R.G.: ‘A Si bipolar monolithic RF bandpass amplifier’, IEEE J. Solid-State Circziils, 1992, 27, (l), pp. 123-127 NGLJYEN, N M , and MEYER, R.G.: ‘A 1.8-GHz monolithic Lc voltage-controlled oscillator’, IEEE J. Solid-Stafe zyxwvutsrqp Circuit, 1992, 27, (3), pp. 444-450 KOULLIAS, I.A.: ‘High Q inductors for wireless applications in a complementary silicon bipolar process’. Proc. IEEE Bipolar/ BiCMOS Circuits and Technology Meeting, 1994, pp. 179-182 ASHBY, K.B., FINLEY, W.C, BASTHK, J.J.. MOINAN. zyxwvutsrqponm S, and zyxwvut Bipolar rail-to-rail constant-g, input stage for low voltage applications F. Alesii, A. D’Amico, M. Faccio, G. Ferri, C. Poduti and G. Stochino Inclc~xing lernis: Bipolur integrated circziits, Analo,pe circuits A new bipolar rail-to-rail input stage for low voltage operational amplifiers is presented. This simple circuit features a virtually constant transconductance against input common mode voltage, in the 0-100°C temperature range, and can operate at a supply voltage as low as 0.85V. A circuit description is given, and some SPICE simulation results arc reported, which demonstrate the effectiveness of the proposed technique. Introduction: One of the goals which have characterised the recent research in the analogue integrated circuit design is the develop- ment of technologies and architectures able to operate with low supply voltages [l, 21. The main reason for this interest has been the increasing need for reduced power consumption in portable electronics and in battery-operated systems. In some areas of low voltage applications (‘in vivo’ biochemical and biomedical sensors, hearing aids, wireless devices for environmental monitoring, etc.), there is often the demand for single cell voltage operated analogue processing electronics, capable of providing satisfactory perform- ance for the lifetime of the cell, i.e. for a supply voltage varying ftom 1.5V down to -0.9V. Low voltage operational amplifiers (LVOAs), which are fre- quently employed in analogue signal conditioning and preprocess- ing, play a key role in this context. Low noise, low offsets, high DC gain and common mode rejection ratio (CMRR) are the main requirements for LVOAs. The need for a wide output dynamic range (the ratio of output voltage swing over the total output error contribution, i.e. offset plus noise) in most applications is reflected in the additional requirement for a rail-to-rail output voltage range. Furthermore, a wide input common mode voltage range may be required when the amplifier is used in the voltage follower configuration or in front end circuits, where the useful input signal is superposed on comparatively high and variable common mode voltages, as occurs in some critical sensor applica- tions. The aim of this Letter is to present a new rail-to-rail bipolar input stage featuring a virtually constant transconductance (gm) within the input common mode voltage range, good noise and off- set performances and capable of operating at very low supply volt- ages. The use of a constant zyxwvutsrqp gm input stage in operational amplifiers, as is well known, is necessary to obtain the following: (i) a differential gain and bandwidth v;.) which is independent of the input common mode voltage; this results in stable phase mar- gin and, hence, improves the settling time (ii) simplified frequency compensation of the amplifier (iii) increased CMRR ELECTRONICS LETTERS 1st August 1996 Vol. 32 (iv) reduced voltage gain non-linearities and harmonic distortion (THD). The above features are particularly important in low voltage appli- cations where common mode voltage may be comparable with useful signals. Compared with previously reported [3, 41 implementations of rail-to-rail constant zyxwv g,, input stages, this new design offers improved performance in terms of: a 100-15OmV lower supply voltage extending down to 0.85V, a simplified architecture, reduced power requirements and comparable offsets and noise values. zyxwvu ’ D D zyxw IC1 IC2 intermediate Fig,, 1 Proposed circuit Circuit description und simulation results: The proposed circuit, designed in HF2CMOS 2pn BiCMOS technology, provided by SGS-Thomson Microelectronics, is shown in Fig. 1. In this Figure, two complementary pairs (Ql to a) have been placed in parallel in order to achieve rail-to-rail operation. A level shifting circuit, consisting of resistors zyxwv R,, and RsL2, and current sourceslsinks QsL, and Qs1,4, are introduced to ensure proper operation over the full common mode input voltage range. Transistors Q5-Q7, which operate as a switch, help to maintain at a constant value of input stage output current with respect to any variation in input com- mon mode voltage. In this way, the pnp stage is in the conduction state only for a limited range of input voltages. The limit is set by the bias voltage TI,,,. In this design the level shift network does not seriously affect the input bias current and the input offset of the amplifier. The shifting currents, provided by transistors and QSL4, are not constant here. On the contrary, they are prop- erly controlled by sensing the difference between the emitter volt- ages of the two complementary pairs, AV,,,, by means of transistor Q8, and by adjusting the level shifting currents to the 0.9 08 > - 50.7 2 w > .- 0.6 0.5 0.1 0.3 0.5 0.7 0.9 ‘CM*’ Fig. 2 AVkpfi ugainst input common mode voltage desired value by means of transistors Qg and Qi2. Fig. 2 shows AV,,, against the input common mode voltage. Resistor R+,, between the collectors of transistors Q? and a, reduces the dependence of the differential gain on the input common mode No. 16 1467