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Common Customer Problem: Op-Amp Output Voltage Swing

By Marek LisSr Application Engineer

Texas Instruments -Tucson

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Op Amp Output Range Consideration

V3 5

Vout2-

+ +

U2 OPA140

VF2

V1 5

Vout1-

+ +

U1 OPA140

Vos

RF 1kRIN 1k

V2 -2.5

A Valid Op Amp Configuration (Input and Output within the linear range)

An Invalid Op Amp Configuration (Output outside of the linear range)

26.7uV2.5V 171.4mV

171.4mV

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REAL WORLD Outputs

V+

V+ V+

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Classic Output Stage

• Common-emitter output• Current source driver• Headroom set by

VBE+VCESAT

• Unity Gain

Q 1

Q 2

V O UT

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OPA827 – Classic Output

OPA827

OPA827

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R-to-R Output Stage

• Common Collector or Common Drain

• Headroom set by VDSsat or VCEsat

+V s

R LO AD R LO AD

+V s

-Vs-Vs

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R-to-R Output Stage

• Common Collector or Common Drain

• Headroom set by VCESAT

• Value of RLOAD affects AOL and Output Swing

+V s

R LO AD R LO AD

+V s

-Vs-Vs

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MOSFET Characteristic Curves

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Achieving Output Swing to the Negative Rail

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SS Pull Down Cheat Sheet

Part # Resistor Value

OPA335 40 kΩ

OPA340 7.5 kΩ

OPA343 7.5 kΩ

OPA348 250 kΩ

OPA350 2 kΩ

OPA353 2 kΩ

OPA333 20 kΩ

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Thank you

Marek Lis

lis_marek@ti.com