Circuit Stories

This is  my answer  to SE EE question  Why would I want to use a differential op amp configuration while amplifying differential signals? My answer Your question is "philosophical" and therefore requires a "philosophical" answer:-) You are actually asking, "What is the idea behind the classic op-amp instrumentation amplifier?" The best way to show it is by reinventing the circuit step by step. Here is my story... Step 1: Single-ended amplifier It is assumed that the two basic circuits of op-amp amplifiers with negative feedback have already been "invented":  1. Non-inverting amplifier  2. Inverting amplifier This is another (no less interesting) story, but now let's just mention the trick with which this was done - in the basic circuits of a follower (K = 1) and inverter (K = -1) an attenuation is introduced in the negative feedback. Compensating for it, they have become amplifiers. Step 2: Imperfect differential amplifier We need a device th...

My answer to SE EE question  What does this op-amp circuit do? (part of an ECG) I have realized that my meeting with TL Fong somehow changed the course of my life... The fact that still there are such curious, enterprising and, above all, well-meaning and honest people on the web seems to multiply my creative abilities. Every day he asks me more and more questions to discuss... which makes me think, over and over again, about great circuit ideas... understanding them better... and finding better explanations... Three days ago, he put the topic of ECG amplifiers and thus he brought me back to my answer above in SE EE from a year ago. I temporarily stopped thinking about his previous question about  Tunnel Diode I-V Curve Plotting  and begin thinking again about this ingenious circuit idea implemented in the so-called "right leg driven circuit" (RDL). So I realized even more deeply the great idea behind it and expanded my answer in EE. Here it is... Answer I saw...