Op-Amp Imperfections and Compensation Techniques
Explore the imperfections in op-amps such as input offset voltage and bias current, and learn about compensation techniques to mitigate these issues. Discover how to tackle challenges like effective resistance calculation and bias cancellation.
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Presentation Transcript
Lecture 5 Opamp imperfections Reference: Analog Devices, Opamp basics tutorial: http://www.analog.com/static/imported-files/tutorials/MT-038.pdf Pradeep Sarin, EP212 Spring 2014 Slide 1/11
Consider opamp internals OpAmp integrates precision matched actives+passives in one IC Intermediate gain stages Output Stage Input Stage Pradeep Sarin, EP212 Spring 2014 Slide 2/11
Simplified input stage IERC Ibias- Ibias+ IE Pradeep Sarin, EP212 Spring 2014 Slide 3/11
Imperfection 1: Vos Input offset voltage Vos If IE +and IE are different, voltages at the collector terminals will be different IE+RC IE-RC Ibias- Ibias+ IE IE IE+ Pradeep Sarin, EP212 Spring 2014 Slide 4/11
You can compensate Vos Input Stage Adjust IE+ & IE- Pradeep Sarin, EP212 Spring 2014 Slide 5/11
Imperfection 2: Ibias Ibias+ Ibias- Sets the two transistors at two different Q points Ibias- Ibias+ Pradeep Sarin, EP212 Spring 2014 Slide 6/11
Cannot compensate for Ibias Good news: Ibias is usually very very small Ibias- Ibias+ Pradeep Sarin, EP212 Spring 2014 Slide 7/11
Ibias can be bad in a poor circuit Bad news: Ibias multiplies Rin Ibias- Ibias+ Depending on Rin,will cause a fake Ibias+Rinappear as v+ even if vin=0 Pradeep Sarin, EP212 Spring 2014 Slide 8/11
Must bias compensate a circuit 1. At DC, with Vin= 0, what is the effective R looking out from - ? 2. If there is Ibias-, what is effective Vin ? 3. If Ibias+ Ibias- what must you do at the + terminal to cancel (2) Pradeep Sarin, EP212 Spring 2014 Slide 9/11
Must have equal DC resistance looking out of each input terminal 1. Calculate effective R looking out from - terminal 2. Put in equivalent Reffin the other + terminal 3. Reff*Ibias+ voltage drop cancels Reff* Ibias-atVin = 0 Pradeep Sarin, EP212 Spring 2014 Slide 10/11
Preparation for Lab 5 Read up on background of Vos & Ibias Techniques for measuring Vos & Ibias not trivial! - measure an intrinsic property of the device without applying Vin Pradeep Sarin, EP212 Spring 2014 Slide 11/11
