Unlike the ideal operational amplifier, a typical operational amplifier has a finite differential gain and bandwidth. Because many of the ideal operational amplifier characteristics cannot be achieved, the characteristics of a typical amplifier differ significantly from those of the ideal amplifier. The open-loop gain of a TL321 operational amplifier is shown in Figure 2-3. At low frequencies, open-loop gain is constant. However, at approximately 6 Hz it begins to roll off at the rate of – 6 dB/octave (an octave is a doubling in frequency and decibels are a measure of gain
calculated by 20 log 10 VO/VI). The frequency at which the gain reaches unity is called unity gain bandwidth and referred to as BI.
When a portion of the output signal is fed back to the input of the operational amplifier, the ratio of the output to input voltage is called closed-loop gain. Closed-loop gain is always less than the open-loop gain. Because gain error is proportional to the ratio of closed-loop gain to open-loop gain, a very high value of open loop gain is desirable.