Monday, February 11, 2013

Op-amp applications

Today's lecture is a review of op-amp applications. There are several good references:
Here is a list of circuits you should know (and other interesting circuits):
  • Inverting and noninverting amplifiers
  • Summing amplifier (multiple inputs)
  • Differencing amplifier (two inputs)
  • Instrumentation amplifier (three op amps)
  • Integrator
  • Noninverting integrator
  • Single-pole low-pass filter
  • Single-pole high-pass filter
  • Differentiators don't work (*)
  • Lag and lead transfer functions
  • Negative impedance converters and gyrators
  • Low-pass and high-pass Sallen-Key filters
  • Analog computer block diagrams, which lead to...
  • Kerwin-Huelsman-Newcomb (KHN) biquad circuit
  • Tow-Thomas biquad circuit
  • Full-wave rectifier
  • Logarithm and exponential circuits
  • Temperature-compensated logarithm and exponential circuits
  • Multipliers (log/expo, light-bulb, servomotor, quarter-square, and PWM)
(*) Note that the simple differentiator circuit (input capacitor and feedback resistor) doesn't work because the op-amp feedback loop is unstable (there isn't enough phase margin). The TI Handbook says:
It should be mentioned that of all the circuits presented in this section, the differentiator is the one that will operate least successfully with real components. The capacitive input makes it particularly susceptible to random noise...
The first sentence is correct, but the second sentence is wrong. It's not a "random noise" problem, it's a loop-stability problem. The resistor and the capacitor introduce a low-pass filter in the feedback path, which reduces the phase margin (close to, or even below, zero degrees) of the op-amp loop.


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