Analogue IC Design: The Current-Mode Approach

by Dirty Chocolate, F.J. Lidgey, D.G. Haigh, Fj.J Lidgey

Overview

Analogue IC Design has become the essential title covering the current-mode approach to integrated circuit design. The approach has sparked much interest in analogue electronics and is linked to important advances in integrated circuit technology, such as CMOS VLSI which allows mixed analogue and digital circuits and high-speed GaAs processing.

Product Details

ISBN-13:	9780863412974
Publisher:	The Institution of Engineering and Technology
Publication date:	09/01/1993
Series:	Circuits and Systems Series
Edition description:	REPRINT
Pages:	666
Product dimensions:	6.12(w) x 9.25(h) x 1.60(d)

Read an Excerpt

Chapter 6: Bipolar Current Mirrors

Barrie Gilbert

6.1 Introduction - The Ideal Current Mirror

Few readers of this book will need to be introduced to the concept of the current mirror; it has become a familiar icon of modem analog design. Accordingly, we will deal only briefly with the well-established foundations of the subject, already adequately presented in many excellent texts ^[1], and concentrate instead on developments of the basic current-mirror forms, having properties suited to special, though not uncommon, applications. Some of these developments have been included only to illustrate the wide variety of possibilities, and may not have any immediate practical value; this is true of the high-ratio forms presented in Section 6.5.2.

In principle, the basic function could have been realized before the advent of the bipolar junction transistor (BJT), but the unique properties of this device opened the door to efficient and eminently practical forms of mirrors, now to be found in a large proportion of analog ICs (and in many digital ones, too), The design approach used here is based strongly on the translinear view of the BJT. In many cases, this will mean the invocation of strict-TL forms of which the classical current mirror is the simplest possible example. Just as probable, however, will be a strong dependence on the basic translinearity of the BJT, that is, the predictable behavior of many special-purpose current mirrors will frequently hinge on the unique relationship between collector current, I_C, and base-emitter voltage V_BE; they are "TN" circuits, although not always "TL" (see Chapter 2).

Current mirrors findendless uses not only in biasing applications of low to moderate accuracy, where their high output impedance makes them valuable as good approximations to ideal current sources. More complex mirrors provide special capabilities, such as high accuracy over many decades of current, exceptionally high output resistance, very low or high transfer ratios, and so on. Depending on how broad is one's definition of a "current mirror", the term can also embrace various types of useful nonlinear behavior and temperature shaping; we will certainly want to examine these important aspects of current-mirror design. Mirrors are also employed as broadband signal conveyors (as, for example, in current-mode amplifiers, described in other chapters of this book). We shall therefore spend some time examining the noise performance of common mirrors. A thorough discussion of dynamic behavior (AC gain and phase, and large-signal transient response) is not included here, since it is a large topic in itself, and dependent on numerous details which lie beyond the scope of this brief survey of useful topologies. However, where appropriate, some aspects of dynamic behavior will be mentioned.

The simplest current mirror is a three-terminal device, Figure 6.1a, having an input node, N1, capable of accepting a current, I₁, of only one polarity, an output node, N2, into which a replication, I₂, of the input current flows in the same direction, and a common node, N0, in which the sum of the input and output currents flow. For now, we will assume the basic mirror to be built with NPN transistors; obviously, the polarity of bias voltages and direction of current flow will be reversed for PNP types. A practically useful mirror is characterized by three desiderata, listed roughly in order of importance:

1) The current I₂ in the output branch should be essentially independent of the voltage V₂ on node N₂ which may be biased at any potential from a few hundred millivolts above the common node to many volts above it; !hat is the incremental output resistance¹, r_o (more generally, impedance, z_o should be relatively high, ideally infinite.

2) Both the large-signal mirror ratio, M=I₂/I₁, and the small-signal ratio G=DI₂/DI₁, should be essentially independent of the magnitude of the currents over many decades; that is, the ideal mirror is a linear element (so G/M) In signal-path applications, the gain magnitude and phase response should be benign functions of frequency, ideally, completely independent of it.

3) The DC voltage, V₁ at the input node N1 should be small (say, within a few hundred millivolts of the common node) and it, and any AC voltage generated at this node, should be essentially independent of the input current, I₁; that is, the input resistance, r_i (more generally, impedance, z_i should be relatively low. In a formal context, it might be convenient to define an ideal mirror as one having an input impedance which approaches zero, but in practice this is rarely as important as achieving a near-zero output admittance (item 1).

We shall see that it is possible to design mirrors in which one or more of these characteristics can be highly refined. Frequently, such optimization is at the expense of other properties. For example, in directing attention to improving output resistance, the input resistance often has to increase. The particular way in which a mirror is optimized depends, of course, on the application. In many bias applications, for example, the emphasis will be on maintaining the highest possible output impedance (both the conductance and capacitance may need to be minimized): the input characteristics are less important here. On the other hand, in signal-path applications, it will often be found that quite high output conductances can be tolerated, while the input impedance (the reactive part often containing both capacitive and inductive components) must be minimized.

6.1.1 Mirrors, Reflectors, Conveyors, Sources

Before proceeding, it will be useful to compare the general properties of the current mirror with other cells having similar properties. The term "current reflector", for example, is sometimes applied to a three terminal network (Figure 6.1b) which satisfies all of the basic criteria for a mirror, but in which the direction of the output current is reversed². However, this terminology is not standardized. As signal-path cells, the mirror is inverting stage while the reflector is non-inverting; a folded cascode can viewed as a reflector. Current conveyors, dealt with at length in chapter 3, can be viewed as a special kind of "double-mirror" capable of both, & and sourcing currents at input and output. Figure 6.1c depicts an id current conveyor. Nodes N1 and N2 can now accept and deliver currents either polarity. The voltage at N1 closely follows that at NO; little (ideal zero) current flows in N0. In practice, two extra terminals, NP and NN are required to provide biasing and a source of positive and negative current to the output. Many of the techniques for improving the performance of current mirrors to be described in this chapter can be applied to current conveyors.

Finally, Figure 6.1d shows a floating current source, which requires only two terminals. Since there is no external control of the current magnitude Igc applications sources of this kind are generally only used in DC brasing applications. While it is perfectly possible to design two-terminal current-source circuits, it is customary to utilize three-terminal controlled sources because of the simplicity (particularly, as is often the case, when multiple sources art required) and flexibility they afford. Current mirrors are widely used in such applications, although where the emphasis is on current generation on rather than replication even simpler circuits often suffice. Some transducers generate current-mode signals directly (such as photodiodes), but most signals, and all accurate fixed references, are in the form of voltages es requiring voltage-to-current (V/I) conversion. This is a large topic, and there have been some interesting developments, particularly in the field of wideband V/I conversion, in recent years. However, it is beyond the scope of the present discussion, which for the most part assumes that we are dealing with variables (bias levels and signals) which are already in the form of currents. Section 6.5.3 will show some simple techniques for V/I conversion. . . .

Table of Contents

1 Introduction Chris Toumazou, John Lidgey and David Haigh

2 Current-mode Circuits From A Translinear Viewpoint A Tutorial Barrie Gilbert

3 Current Conveyor Theory And Practice Adel S. Sedra and Gordon W Roberts

4 Universal Current-Mode Analogue Amplifiers Chris Toumazou and John Lidgey

5 High Frequency CMOS Transconductors Scott T. Dupuie and Mohammed Ismail

6 Bipolar Current Mirrors Barrie Gilbert

7 Dynamic Current Mirrors Eric A.Vittoz and George Wegmann

8 Gallium Arsenide Analogue Integrated Circuit Design Techniques Chris Toumazou and David Haieh

9 Continuous-Time Filters Rolf Schaumann and Mehmet Ali Tan

10 Continuous-time and Switched Capacitor Monolithic Filters Based on LCR Filter Simulation using Current and Charge Variables David Haigh

11 Switched-Current Filters John.B.Hughes

12 Analog Interface Circuits For VLSI Evert Seevinck

13 Current Mode A/D and D/A Converters C. Andre T. Salama, David G Nairn and Henry W Si

14 Applications of current-copier circuits David Vallancourt and Steven J. Daubert

15 Integrated Current Conveyor Doug C.Wadsworth

16 Applying "Current Feedback" to Volt Amplifiers Derek F. Bowers

17 Neural Network Building Blocks for Anolog MMOS VLSI Steven Bibyk and Mohammed Ismail

18 Future of Anologue Intergrated Circuit Design

Index