Stanford EE 214 - Voltage References and Biasing (15 pages)

Previewing pages 1, 2, 3, 4, 5 of 15 page document View the full content.
View Full Document

Voltage References and Biasing



Previewing pages 1, 2, 3, 4, 5 of actual document.

View the full content.
View Full Document
View Full Document

Voltage References and Biasing

31 views


Pages:
15
School:
Stanford University
Course:
Ee 214 - Advanced Analog Integrated Circuit Design
Advanced Analog Integrated Circuit Design Documents
Unformatted text preview:

Handout 20 EE214 Fall 2002 Voltage References and Biasing 1 0 Introduction In this set of notes we take up the study of an important topic How to generate voltages and currents that are relatively independent of supply voltage and or temperature Because CMOS offers relatively limited options for realizing bias circuits we ll see that some of the most useful biasing idioms are actually those based on bipolar circuits A parasitic bipolar device exists in every CMOS technology and may be used in a bandgap voltage reference for example Even though the characteristics of parasitic transistors are far from ideal the performance of bias circuits made with such devices is frequently vastly superior to that of pure CMOS bias circuits In what follows it is worthwhile to keep in mind that any voltage we produce must depend on some collection of parameters that ultimately have the dimensions of a voltage such as kT q for example Similarly any current we produce must depend on parameters that ultimately have the dimensions of current such as V R Although seemingly obvious and trivial statements we ll see that they are extremely useful guides for the design of stable references 2 0 Review of Behavior of Diodes While the voltage across a forward biased diode is relatively insensitive to current because of the logarithmic dependence of diode current on diode voltage its variation with temperature is significant To understand the precise nature of the temperature dependence recall that the diode voltage may be expressed as I VD nVT l n D IS 1 where VT is the thermal voltage kT q and n the ideality factor is typically between 1 and 1 5 in diodes A transistor s VBE conforms more closely to the ideal diode law than do ordinary diodes so we will assign n a unity value in all that follows It is frequently inferred incorrectly from Eqn 1 that VD has a positive temperature coefficient TC because of its proportionality to VT The fly in the ointment is that I D itself has an exponential temperature dependence and this alters the situation considerably To clarify matters consider the following quasi empirical expression for I S V G0 I S I 0 exp VT Voltage References and Biasing 1993 Thomas H Lee rev November 27 2002 All rights reserved 2 Page 1 of 15 Handout 20 EE214 Fall 2002 where IO is some process and geometry dependent current1 I0 is typically around 20 orders of magnitude larger than IS at room temperature so I 0 is much larger than typical values of ID and VG0 is the bandgap voltage about 1 2 volts extrapolated to absolute zero Using this detailed expression for IS we can expand the equation for VD as follows 2 I VD VGO VTl n 0 I D 3 Thus we see that the junction voltage decreases linearly if I0 were constant from a value of VG0 as seen in the following plot of VD vs temperature at constant diode current FIGURE 1 Approximate behavior of VD vs temperature VD about 2mV K VG0 1 2V higher I D lower ID 600K typ T kelvins Note that this equation tells us that VD always equals VG0 at absolute zero 3 Furthermore it s easy to see that if I 0 were constant the temperature coefficient at any temperature is simply dVD VG 0 VD dT T 4 1 It also depends weakly on temperature but we ll defer a detailed discussion about the behavior of I0 until the section on bandgap voltage references 2 The minus sign is not an error Just remember that the argument of the log here is typically much larger than unity 3 Again this value is an extrapolated one It must be stressed that the behavior of real junctions at both extremes of temperature will differ from that shown the equations presented lose validity at extremely cold temperatures say 100K because of carrier freeze out i e failure of dopants to ionize and because of bandgap variation with temperature and at high temperatures 450 500K because the silicon goes intrinsic Voltage References and Biasing 1993 Thomas H Lee rev November 27 2002 All rights reserved Page 2 of 15 Handout 20 EE214 Fall 2002 With the assumption of constant I0 the temperature coefficient is independent of temperature and equal to about 2mV K This linearly decreasing behavior is known as CTAT for complementary to absolute temperature Note that the voltage does depend logarithmically on diode current so the temperature coefficient also depends somewhat on the diode current with lower currents associated with higher temperature coefficients Although a VD based reference can provide an output that depends very little on supply voltage the CTAT behavior may or may not be acceptable depending on the application However we shall see that the CTAT behavior of a VD is particularly valuable for use in a class of references based on the bandgap voltage VG0 We ll take up the detailed study of bandgap references in Section5 0 3 0 Diodes and Bipolar Transistors in CMOS Technology The most flexible option for realizing diodes and bipolar transistors in standard CMOS technology derives from the parasitic substrate pnp transistor available in n well processes The p source drain diffusions serve as the emitter the n well as the base and the substrate as the collector FIGURE 2 Parasitic substrate PNP in n well CMOS not drawn to scale emitter n p base collector n n well p substrate In applications where it is important to reduce series base resistance it is advisable to surround completely the emitter with n diffusions placed as close to the emitter as the design rules allow as suggested by Fig 2 Just as its counterpart in inexpensive bipolar processes the substrate pnp in CMOS technology can only be used in circuits that allow the collector to be at substrate potential Fortunately there are numerous circuits that satisfy this condition For example a simple voltage reference can be constructed with this device connected as a grounded diode in which the emitter is the anode and the cathode is the base and collector substrate tied together Voltage References and Biasing 1993 Thomas H Lee rev November 27 2002 All rights reserved Page 3 of 15 Handout 20 EE214 Fall 2002 4 0 Supply Independent Bias Circuits To minimize sensitivity to power supply variations it is desirable to derive the bias currents for reference voltages from the reference voltages themselves rather than directly from the power supply Although it may seem a violation of some fundamental law the no free lunch principle it is possible to arrange for this condition To illustrate how one may accomplish this feat consider the following circuit FIGURE 3 Self biased


View Full Document

Access the best Study Guides, Lecture Notes and Practice Exams

Loading Unlocking...
Login

Join to view Voltage References and Biasing and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Voltage References and Biasing and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?