Page 34 - Op Amps Design, Applications, and Troubleshooting

Basic Characteristics of Ideal Op Amps 17

FIGURE 1.16 The output impedance of an ideal op amp is 0.

age gain. Regardless of the absolute value of the internal source, the equivalent
circuit shows that this voltage is divided between the external load (R L) and the
internal series resistance r 0. In order to get the most voltage out of the op amp and
to minimize the loading effects of external loads, we would want the internal out-
put resistance to be as low as possible. Thus, the output impedance of an ideal op
amp is zero. Under these ideal conditions, the output voltage will remain constant
regardless of the load applied. In other words, the op amp can supply any
required amount of current without its output voltage changing. A practical op
amp will have limitations, but the output impedance will still be quite low.
NOTE: For most purposes throughout this text, we do not distinguish
between input/output resistance and input/output impedance.

1.3.7 Temperature Effects

Because the op amp is constructed from semiconductor material, its behavior is sub-
ject to the same temperature effects that plague transistors, diodes, and other semi-
conductors. Reverse leakage currents, forward voltage drops, and the gain of
internal transistors all vary with temperature. For now we will ignore these effects
and conclude that an ideal op amp is unaffected by temperature changes. Whether
we can ignore temperature effects in practice depends on the particular op amp, the
application, and the operating environment.
1.3.8 Noise Generation

Anytime current flows through a semiconductor device, electrical noise is gener-
ated. There are several mechanisms that can be responsible for the creation of the
noise, but in any case it is generally considered undesirable. In many applications
the noise levels generated are so small as to be insignificant. In other cases, we
must take precautions to minimize the effects of the noise generation. For now,
however, we will consider that an ideal op amp does not generate internal noise.
If we apply a noise-free signal on the input, then we can expect to see a noise-free,
high-fidelity signal reproduced at the output.