The Circuit Designer's Companion - diagramas.diagram...

160 The Circuit Designer’s Companionamplifier that oscillates (and its converse, the oscillator that doesn’t) at some time orother. There are really only a few fundamental causes of unwanted oscillations, they areall curable, and they can be listed as follows:• feedback-loop instability• incorrect grounding• power supply coupling• output stage instability• parasitic coupling.The most important clue in tracking down instability is the frequency of oscillation.If this is near the unity-gain bandwidth of the device then you are most probablysuffering feedback-induced instability. This can be checked by temporarily increasingthe closed-loop gain. If feedback is the problem, then the oscillation should stop or atleast decrease in frequency. If it doesn’t, look elsewhere.Feedback-loop instability is caused by too much feedback at or near the unity-gainfrequency, where the op-amp’s phase margin is approaching a critical value. (Manybooks on feedback circuit theory deal with the question of stability, gain and phasemargin, using tools such as the Bode plot and the Nyquist diagram, so this isn’t coveredhere.)Ground couplingGround loops or other types of incorrect grounding cause coupling from output back toinput of the circuit via a common impedance in its grounded segment. This effect hasbeen covered in Chapter 1 but the circuit topology is repeated here, in Figure 5.12. IfInput = V in + I out · Z CMV inI outR LV = I out · Z CMZ CMTrue groundFigure 5.12 Common-impedance ground couplingthe resulting feedback sense gives an output component in-phase with the input thenpositive feedback occurs, and if this overrides the intended negative feedback you willhave oscillation. The frequency will depend on the phase contribution of the commonimpedance, which will normally be inductive, and can vary over a wide range.Power supply couplingPower supplies should be properly bypassed to avoid similar coupling through thecommon mode power supply impedance. Power supply rejection ratio falls withfrequency, and typical 0.01−0.1µF decoupling capacitors may resonate with theparasitic inductance of long power leads in the MHz region, so these problems usuallyshow up in the 1−10MHz range. Using 1−10µF tantalum capacitors for power railbypassing will drop the resonant frequency and stray circuit Q to the level at whichproblems are unlikely (compare Figure 3.19 for capacitor resonances).