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Lect2UP170_(100420)

# Lect2UP170_(100420) - Lecture 170 Temperature Stable...

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Lecture 170 – Temperature Stable References (4/20/10) Page 170-1 CMOS Analog Circuit Design © P.E. Allen - 2010 LECTURE 170 – TEMPERATURE STABLE REFERENCES LECTURE ORGANIZATION Outline • Principles of temperature stable references • Examples of temperature stable references • Design of bias voltages for a chip • Summary CMOS Analog Circuit Design, 2 nd Edition Reference Pages 153-159 Lecture 170 – Temperature Stable References (4/20/10) Page 170-2 CMOS Analog Circuit Design © P.E. Allen - 2010 PRINCIPLES OF TEMPERATURE STABLE REFERENCES Temperature Stable References • The previous reference circuits failed to provide small values of temperature coefficient although sufficient power supply independence was achieved. • This section introduces a temperature stable reference that cancels a positive temperature coefficient with a negative temperature coefficient. The technique is sometimes called the bandgap reference although it has nothing to do with the bandgap voltage. Principle V REF ( T ) = V PTAT ( T ) + K · V CTAT ( T ) where V PTAT ( T ) is a voltage that is proportional to absolute temperature (PTAT) V CTAT ( T ) is a voltage that is complimentary to absolute temperature (CTAT) and K is a temperature independent constant that makes V REF ( T ) independent of temperature

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Lecture 170 – Temperature Stable References (4/20/10) Page 170-3 CMOS Analog Circuit Design © P.E. Allen - 2010 PTAT Voltage The principle illustrated on the last slide requires perfectly linear positive and negative temperature coefficients to work properly. We will now show a technique of generating PTAT voltages that are linear with respect to temperature. Implementation of a PTAT voltage: V PTAT = ± V D = V D 1 V D 2 = V t ln ² ³ ³ ´ µ · I 1 I s 1 - V t ln ² ³ ³ ´ µ · I 2 I s 2 = V t ln ² ³ ³ ´ µ · I 1 I 2 I s 2 I s 1 = V t ln ² ³ ³ ´ µ · I s 2 I s 1 = V t ln ² ³ ³ ´ µ · A 2 A 1 = kT q ln ² ³ ³ ´ µ · A 2 A 1 if I 1 = I 2 . Therefore, if A 2 = 10 A 1 , V D at room temperature becomes, V D = ¸ ¹ ¹ º » ¼ ¼ ½ k q ln ² ³ ³ ´ µ · A 2 A 1 T = ¸ ¹ ¹ º » ¼ ¼ ½ 1.381x10 -23 J/°K 1.6x10 -19 Coul ln (10) T = (+ 0.086mV/°C) T ¾ V PTAT = V t ² ³ ³ ´ µ · A 2 A 1 Lecture 170 – Temperature Stable References (4/20/10) Page 170-4 CMOS Analog Circuit Design © P.E. Allen - 2010 Psuedo-PTAT Currents In developing temperature independent voltages, it is useful to show how to generate PTAT currents. A straight-forward method is to superimpose V PTAT across a resistor as shown: Because R is always dependent on temperature, this current is called a pseudo-PTAT current and is designated by I PTAT ’. When a pseudo-PTAT current flows through a second resistor with the same temperature characteristics as the first, it creates a new V PTAT voltage.
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Lect2UP170_(100420) - Lecture 170 Temperature Stable...

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