Gain and Bandwidth

It is important to note that, since the product of gain and bandwidth is a constant for any particular operational amplifier. Hence, an increase in gain can only be achieved at the expense of bandwidth, and vice versa.

Figure 11.5 shows the relationship between voltage gain and bandwidth for a typical operational amplifier (note that the axes use logarithmic, rather than linear scales). The open-loop voltage gain (i.e., that obtained with no feedback applied) is 100,000 (or 100 dB) and the bandwidth obtained in this condition is a mere 10 Hz. The effect of applying increasing amounts of negative feedback (and consequently reducing the gain to a more manageable amount) is that the bandwidth increases in direct proportion.

Figure 11.5 Frequency response curves for an operational amplifier

The frequency response curves in Figure 11.5 show the effect on the bandwidth of making the closed-loop gains equal to 10,000, 1,000, 100, and 10. Table 11.3 summarizes these results. You should also note that the (gain×bandwidth) product for this amplifier is 1×10⁶ Hz (i.e., 1 MHz).

Table 11.3

Corresponding values of voltage gain and bandwidth for an operational amplifier with a gain×bandwidth product of 1×10⁶

We can determine the bandwidth of the amplifier when the closed-loop voltage gain is set to 46 dB by constructing a line and noting the intercept point on the response curve. This shows that the bandwidth will be 10 kHz. Note that, for this operational amplifier, the (gain×bandwidth) product is 2×10⁶ Hz (or 2 MHz).