Bad Circuit Design 6 - Breaking the Feedback Loop around an Op-Amp
Suppose we want to implement the simple op-amp circuit seen below using switched-capacitors (SCs).
The schematic seen below is a direct SC implementation, see page 843. The bad design issue in
this circuit occurs because there isn’t a continuous path from the output back to the op-amp’s
input. When either the phi1- or phi2-controlled switches are off (which is always) there isn’t
a feedback path so the op-amp’s outputs will go to the power supply rails.
The fix is to add a capacitor from the output back to the input as seen below. The added capacitor
provides a continuous-time feedback path at the cost of speed.
Question, have we made the bad design mistake discussed in “bad design 5” and not provided a DC
path to the input of the op-amp? Answer no. The DC output voltage will be fed back through the
SC feedback resistor. It will just take several clock cycles for the circuit to settle (the DC signal
to get fed back).
In simple terms if the only thing connected to the output of the op-amp is a switch (and a load) then
the design is likely a “bad design”.