Page 418 - Op Amps Design, Applications, and Troubleshooting

394 NONIDEAL OP AMP CHARACTERISTICS

Another important aspect of noise gain is apparent when capacitance is used
in the input circuit (e.g., the differentiator circuit). By having a capacitance in
series with the input terminal (-), we cause the effective K/ to decrease at higher
frequencies, thus increasing the noise gain of the circuit.
The data sheet for the standard 741 provided in Appendix 1 shows one man-
ufacturer's method of providing noise specifications for an op amp. The graph
labeled "Output Noise versus Source Resistance" is particularly useful. Here the
manufacturer is indicating the total RMS output noise for various gains and vari-
ous source resistances. The source resistor always generates thermal noise that
increases with resistance and temperature. For low values of source resistance
(below 1.0 kQ), the op amp noise is the primary contributor to overall output
noise. Thus, the curves remain fairly flat for various source resistance values. As
the source resistance is increased beyond 10 kilohms, its noise begins to swamp
out the internal op amp noise and we begin to see a steady rise in overall noise
with source resistance increases.
The graph labeled "Spectral Noise Density" also provides us with greater
insight into the noise characteristics of the 741. This graph indicates the relative
magnitudes of noise signals at various frequencies. In particular, notice that at fre-
quencies above 1.0 kilohertz, the distribution of noise voltage is fairly constant.
This flat region is largely caused by the noise generated in the source resistance.
Much of the internal op amp noise decreases with increasing frequency. By the
time we reach 1.0 kilohertz, this internal noise contributes little to the overall noise
signal, but below 1.0 kilohertz the overall noise amplitude increases sharply as fre-
quency is decreased. This increase is largely caused by increased internal op amp
noise and can present problems in DC amplifiers.
There are several things we can do to minimize the effects caused by noise
voltages. First, we can take steps to minimize the noise gain of the circuit. This
means avoiding the use of large values of feedback resistance (R F) and small values
of input resistance (R/). Unfortunately, the gain of the circuit for normal signals
determines the ratio of these two components, but if we bypass R F with a small
capacitor, we can cause the noise gain to decrease at frequencies beyond the normal
operating range of the circuit. In other words, the normal input signals will see the
bypass capacitor as an open and will be unaffected, but the noise signals will see the
bypass capacitor as a low impedance and reduce the overall noise gain of the circuit.
A second way to minimize the effects of noise in an op amp circuit is to
ensure that the resistance between the inverting input and ground, and between
the noninverting input and ground, are equal. You will recall that this same proce-
dure helped us minimize the effects of input offset current.
_ Third, since the noise generated by resistors increases with resistance (actually
A/K), we should avoid large values of resistance when noise is potentially a problem.
Finally, we can reduce the op amp's contribution to overall output noise by
selecting an op amp that is optimized for low-noise operation. The OP-27 op amp
manufactured by Motorola, Inc., for example, is called an "ultra-low" noise device.
It generates only 3.0 nanovolts of RMS noise at 1.0 kilohertz.

10.2.4 Frequency Compensation
We know from the study of oscillators that the two primary criteria for oscillation
are in-phase (or 360°) feedback and a gain of at least unity at the feedback frequency.