The influence of device selection on the performance of a simple BiCMOS operational amplifier

Abstract

Abstmct -BiCMOS technology offers both the analog and digital designer a wide range of transistar selection which can be utilized to improve the puformanw of in6egretecl circuits. The tradeoffs which occur when BiCMOS is applied 00 analog circuits are examined in this paper. A simple BiCMOS op amp is used to compare the performance of all possible OOmbiaPtioos of device selections that results. A BiCMOS i q d e y t a t i o n is persently in that I. INTROWCTIONOne unique aspect of BiCMOS integrated circuit design is the availability of two distinctly different devices the MOSFET' and BIT. From the digital Peaspective, BiCMOS technology offers many advantages which must becarefull yevaluatedin a d e r to gain a performance advantage [1-4]. A simlar situation exists for the analog designer. While BiCMOS offers many opportunities, the tradeoffs in performance must be understood in order to take Theobjective of this papex is to outline b e tra&offs available to the analog designer using BiClHOS technology. While general guidelines canand will bepropoeeQ theindividual applications will makethetrpdeoffsmaecomphx. ThiswillbeillustrPtedby+nga simple op amp to analyze the performance for various select" of transistors. It w i l l be showo that the cmect sektion of transistor make salfie. This perfcnmancecomgarism isbased on simulation a l l o w e x~v e a f i c o h a r O f k d t 8~~l n thlspaper. advantage of these opportunities [J-71. ~~U~t h e~p e € f~. IL TRANSISTOR LEVEL COMPARISON Not onl daes the combination of CMOS and bipolar technology into Bi&OS give the designer a wider selection of devices, it also permits the improved perhmmx in the devices themselves [8]. An example is the lightlydoped drains of MOSFETS Probably the best way to illustrate the co * n of MOSF'ITs and bipolar devices is expressed by T * T k i s table, we have compared various Categoriesof d e v i c e p d " c e fa each type of transistm. Table 1 consists of information that is obvious and some whose impact is not so obvious. olre of the striking differences between CMOS and bipolar devices is the value of small signal t raug , Also, we note that while the turn on voltage of bipolar transistorsare less than CMOS transistors, the continual shtinkiog of MOSFETs with s d e r geometry can result in turn on volta&esanaoaching zero whiletheturnoa voltapof BJTs cannot bechaoged. ThiswouldimplythptCMosgsturptron voltagescould approach that of BJT devices. Two areas w e CMOS devices excel over bipolar devices is in the impn of an analog switch and capacitance. Si/Poly collectma Vertical and lateral PNP BJTs were available although only the lateral PNP was used in this study. Typical fT and Br for the npn wem 2GHz and 200, respeaively, while for the lateral PNP, fT was at best 25MHz and & was approximately 150 at low collector current (-LFA) with rapid rolloff at higher currents. 0 -7 8 0 3 4 5 1 0~3 .~ e1992IEEE Bipolar 05v 4 m V 4 mS 0.2-0.3V Offs&s,asymmetric 3 Gmd Table 1 -Comparison of performance categories between CMOS IIL A SIMPLE BiCMOS OP AMP In or...