Page 426 - Op Amps Design, Applications, and Troubleshooting

402 NONIDEAL OP AMP CHARACTERISTICS

as to present problems (e.g., saturation on peak signals) in the present stage, it can
probably remain uncompensated.
Frequency response and slew rate, on the other hand, are important in
nearly every AC-coupled application. These parameters should be fully evaluated
before a particular amplifier is selected for a given application.
Noise characteristics can often be ignored, but it depends on the application
and on the amplitude of the desired signal relative to the noise signal. If the noise
signal has an amplitude that is comparable to the desired signal, then the designer
should take steps (discussed in an earlier section) to nrurtimize the circuit noise
response. On the other hand, if the primary signal is many times greater than the
noise signal, the design may not require any special considerations with regard to
noise reduction.

10.3.2 DC-Coupled Amplifiers

DC-coupled amplifiers seem to present some of the more formidable design chal-
lenges. Depending on the specific application, a DC amplifier may be affected by
literally all of the nonideal op amp characteristics. This is certainly the case for a
DC-coupled, low-level, wideband amplifier.
If, however, the input frequency is always very low (e.g., the output of a tem-
perature transducer), then considerations regarding slew rate and bandwidth can
often be disregarded. In these cases, the emphasis needs to be on the DC param-
eters such as DC offsets and drifts.

10.3.3 Relative Magnitude Rule

A good rule of thumb that is applicable to all classes of amplifiers and to all of the
nonideal characteristics of op amps involves the relative magnitude of the nonideal
quantity compared to the desired signal. If the nonideal value is less than 10 percent
of the desired signal quantity, then ignoring it will cause less than a 10 percent error.
Similarly, keeping the nonideal value below 1 percent of the desired signal will gen-
erally keep errors within 1 percent even if the nonideal quantity is disregarded.
Consider, for example, the case of input bias current. If the input bias current
is approximately 700 microamperes and the input signal current is expected to be
1.2 milliamperes, then to ignore bias current would be to make a significant error
because the bias current is comparable in magnitude to the desired input current
Now suppose that the cumulative effects of bias current and input offset
voltage for a particular amplifier are expected to produce a 75-miUivolt offset at
the output of the op amp. If the normal output signal is a 1-volt sinewave riding
on a 5-volt DC level, then the undesired 75 millivolts offset can probably go unad-
dressed without producing any serious effects on circuit operation.

10.3.4 Safety Margins on Frequency Compensation
It is common to speak of phase margin and gain margin with reference to op amp
frequency compensation. The terms describe the amount of safety margin
between the designed operating point of the op amp and the point where oscilla-
tions will likely occur. The absolute limits (i.e., zero safety margin) occur when the