EE 435 Lect 41 Spring 2010

EE 435 Lect 41 Spring 2010 - EE 435 Lecture 41 References...

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EE 435 Lecture 41 References
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Voltage References V BIAS V REF Voltage Reference Circuit Observation – Variables with units Volts needed to build any voltage reference What variables available in a process have units volts? V DD , V T , V BE (diode) ,V Z ,V BE ,V t , V G0 ??? What variables which have units volts satisfy the desired properties of a voltage reference? V G0 and ?? How can a circuit be designed that “expresses” the desired variables? V G0 is deeply embedded in a device model with horrible temperature effects ! Good diodes are not widely available in most MOS processes ! .• •• Review from last lecture .•
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Voltage References Good diodes are not widely available in most MOS processes ! Substrate pnp transistor Diode-connected substrate pnp .• •• Review from last lecture .•
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Voltage References = t G0 V -V m SX S e T J J ~ t BE t G0 V (T) V V -V m SX C e e T A J (T) I = ~ t BE V V S C Ae J I = Bandgap Voltage Appears in BJT Model Equation as well .• •• Review from last lecture .•
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Standard Approach to Building Voltage References Negative Temperature Coefficient (NTC) K Positive Temperature Coefficient (PTC) X OUT X N X P Pick K so that at some temperature T 0 , ( ) 0 T KX X 0 T T P N = + = P N OUT KX X X + = .• •• Review from last lecture .•
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Q 1 V BE1 Bandgap Voltage References Consider two BJTs (or diodes) Q 2 V BE2 If I C is independent of temperature, it follows that C 2.1mV/ 25mV 1.2 0.65 2.3 8.6x10 T V o 5 K 300 T T BE o 0 + = = If ln(I C2 /I C1 )=1 ( ) C V/ 86 8.6x10 T V V o 5 K 300 T T BE1 BE2 o 0 μ = = = = Magnitude of TC of PTAT source is much smaller than that of V BE source P N OUT KX X X + = ( ) 0 T KX X 0 T T P N = + = If K will be large .• •• Review from last lecture .•
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V BE and Δ V BE with constant I C 0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 250 300 350 400 Temperature Volts Δ V BE V BE .• •• Review from last lecture .•
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Comparison of V BE with constant current and PTAT current 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 100 200 300 400 500 600 Temperature Volts Constant Current PTAT Current .• •• Review from last lecture .•
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First Bandgap Reference (and still widely used!) V DD V REF R 1 R 2 R 3 R 4 Q 1 Q 2 P.Brokaw, “A Simple Three-Terminal IC Bandgap Reference”, IEEE Journal of Solid State Circuits, Vol. 9, pp. 388-393, Dec. 1974. .• •• Review from last lecture .•
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Thanks to Ziyue
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First Bandgap Reference (and still widely used!) V DD V REF R 1 R 2 R 3 R 4 Q 1 Q 2 V BE2 BE 2 1 ΔV R R Current ratios is R 4 /R 3 Current not highly dependent upon T [ ] BE2 BE1 2 1 BE2 REF V V R R V V + =
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First Bandgap Reference (and still widely used!) 2 1 2 1 BE BE E V V R I = + ( ) 1 2 1 2 R I I V V E E BE REF + + = 3 OS C2 DD C1 R V V V I = 4 C2 DD C2 R V V I = E1 1 C1 I α I = E2 2 C2 I α I = β 1 β α + = V DD V REF R 1 R 2 R 3 R 4 Q 1 Q 2 ( ) + + = 3 1 1 OS 4 3
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EE 435 Lect 41 Spring 2010 - EE 435 Lecture 41 References...

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