Page 427 - Op Amps Design, Applications, and Troubleshooting

Review Questions 403

closed-loop gain reaches unity and has a phase shift of -180°. This is the dividing
line between oscillation and stable operation.
Gain margin is defined as the difference between unity and the actual
closed-loop voltage gain at the point where a -180° phase shift occurs. To insure
stable operation and to allow for variances in component values, the loop gain
should fall to about one-third or -10 dB by the time the phase shift has reached
-180°. Similarly, the phase margin is the number of degrees between the actual
phase shift and -180° at the time the loop gain reaches unity. A safety margin of
about 45° is recommended. If these safety margins are maintained and the capaci-
tive loading on the output of the amplifier is light, then the circuit should be stable
and perform as expected.

REVIEW QUESTIONS

1. The value of the difference between the two input bias currents of an op amp is
provided by the manufacturer in the data sheet and is called .
2. Drift is a nonideal characteristic that primarily affects (AC-coupled/DC-coupled) op
amp circuits.
3. What type of op amp would you select if a very high value of input impedance was
required?
4. Noise generation in op amp circuits can be reduced by selecting very large values of
resistance. (True or False)
5. If the intersection of the closed-loop gain curve and the open-loop gain curve for an
op amp amplifier occurs with a net slope of 60 dB per decade, will the amplifier be
stable? Explain your answer.