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Dennis Feucht’s “Designing High Performance Amplifiers” FAIL

December 21, 2014

Author Dennis Feucht’s bio:   …40 years of designing amplifiers, oscilloscopes, function generators, etc, and I figure, this guy has been around the block.  I ordered this text and the accompanying series from Amazon.

To be clear, I am an analog design engineer and amplifiers are my expertise.  It didn’t take me long to find errors in Feucht’s text…..one particularly botched analysis has to do with opamp noise.  This error is somewhat embarrassing, and shows the author is not quite as competent as his bio suggests.  I was surprised considering he has 40 years of experience in test and measurement….a background not too different from mine….I have 10 years of experience in test and measurement and 16 years in amplifier design at the IC level.

The circuit discussed is found on page 95.  It is an inverting opamp circuit of a gain of -1 with 100k input R (rin) and 100k feedback (Rfb).

His equation:

eno=sqrt(eni^2 + 2(enR)^2+ (in*rin||Rfb)^2) = 61.1nV/rtHz ….which is All Wrong!

The proper equation:

eno=sqrt( ((1+Rfb/rin)*eni)^2 + (in*Rfb)^2 + 2(enR)^2 ) = 70.8nV/rtHz

To verify my analysis, I modeled the opamp on LTspice with an ideal VCVS (no noise) with 100dB of gain.  I modeled the input referred noise of the opamp voltage and current noise (20nV/rtHz), and 0.1pA /rtHz, I used a 24.7k resistor in series with the VCVS, and a 1.6Meg resistor whose current noise is sensed by a CCCS tied between the summing node and ground respectively, being careful to not allow the current noise of the 1.6Meg resistor to flow through the 24.7k resistor.   I then added the 100k input and feedback resistors around the loop to set the gain.  Simulating the noise, the output noise came out to be about 70.8nV/rtHz at 27C (300K).

See the linked LTspice schematic:  Schematic for Simulation Verification

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