09_op-amps

# 00 v reference across precision 10k make 100 ma

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Unformatted text preview: ( °C) • Goal: put 1.00 mA across RTD and present output voltage proportional to temperature: V out = V 0 + T • First stage: – put precision 10.00 V reference across precision 10k make 1.00 mA, sending across RTD – output is 1 V at 0°C; 1.385 V at 100°C resistor to • Second stage: – resistor network produces 0.25 mA of source through R9 – R6 slurps 0.25 mA when stage 1 output is 1 V • so no current through feedback output is zero volts – At 100°C, R6 slurps 0.346 mA, leaving net 0.096 that must come through feedback – If R7 + R8 = 10389 ohms, output is 1.0 V at 100° C • Tuning resistors R11, R7 allows control over offset and gain, respectively: this config set up for Vout = 0.01T 0.01 Winter 2008 Lecture 9 15 Winter 2008 16 4 Op-Amps 02/14/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Stick it in the feedback loop! Hiding Distortion V+ • Consider the “ push-pull” transistor push-pull” arrangement to the right – an npn transistor (top) and a pnp (bot) – wimpy input can drive big load (speaker?) – base-emitter voltage differs by 0.6V in each transistor (emitter has arrow) – input has to be higher than ~0.6 V for the npn to become active – input has to be lower than 0.6 V for the pnp to be active out V+ Vin + out in V input and output now the same V • There is a no-man’s land in between no-man’ where neither transistor conducts, so one would get “ crossover distortion” distortion” crossover distortion • By sticking the push-pull into an op-amp’s feedback loop, we op-amp’ guarantee that the output faithfully follows the input! – after all, the golden rule demands that + input = – output is zero while input signal is between 0.6 and 0.6 V input • Op-amp jerks up to 0.6 and down to 0.6 at the crossover 0 .6 – it’s almost magic: it figures out the vagaries/nonlinearities of the thing in the loop • Now get advantages of push-pull drive capability, without the mess Winter 2008 17 Winter 2008 18 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Dogs in the Feedback Reading “there is no dog” Vin • Read 6.4.2, 6.4.3 • Pay special attention to Figure 6.59 + inverse dog dog • The op-amp is obligated to contrive the inverse dog so that the ultimate output may be as tidy as the input. • Lesson: you can hide nasty nonlinearities in the feedback loop and the op-amp will “do the right thing” thing” We owe thanks to Hayes & Horowitz, p. 173 of the student manual companion to the A rt of Electronics for this priceless metaphor. Winter 2008 Lecture 9 19 Winter 2008 20 5...
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