The bandwidth of any circuit is of paramount importance. Thus the op amp bandwidth is a particularly important factor in op-amp circuits.
Op amp bandwidth, gain and gain bandwidth product are all tightly related.
As any op amp has limited bandwidth, it is necessary to carefully consider the gain, bandwidth, and frequency response at the outset of the design of any circuit.
Op amp bandwidth
Although op amps have a very high gain, this level of gain starts to fall at a low frequency. The open loop breakpoint, i.e. the frequency at which the gain has fallen by 3 dB is often only a few Hz.
The long lived and still very popular 741 op amp has an open loop breakpoint around 6Hz. Beyond this the response falls at a rate of -6dB/octave or -20dB/decade. Note: an octave is a doubling in frequency, and a decade is a ten-fold increase in frequency and therefore these two figures are two ways of expressing the same characteristic.
Op amp gain, bandwidth & compensation
One of the main reasons why op amps generally have low break points is that a feature called compensation is incorporated into virtually all op amps.
This frequency compensation is used to ensure that the op amp remains stable under all operating conditions. The very earliest op amps were prone to instability and as a result, compensation was introduced into virtually all op amp IC designs as a matter of course.
The effect of the compensation on the op-amp bandwidth is to reduce the break point. This means that if no compensation had been incorporated the breakpoint and bandwidth would be greater, but at the cost of instability.
Effect of feedback on op amp bandwidth
When designing a real circuit using an op amp, negative feedback is used to give controlled levels of gain. Applying this feedback enables the very high gain to be traded for bandwidth.
In this way, very flat frequency response curves can be achieved within the required bandwidth.
Op amp gain bandwidth product
When designing an op amp circuit, a figure known as the op amp gain bandwidth product is important.
The op amp gain bandwidth product is generally specified for a particular op amp type an open loop configuration and the output loaded:
GBP = op amp gain bandwidth product
Av = voltage gain
f = cutoff frequency (Hz)
The op amp gain bandwidth product is constant for voltage-feedback amplifiers. However it is not applicable for current feedback amplifiers because relationship between gain and bandwidth is not linear.
Therefore decreasing the gain by a factor of ten will increase the bandwidth by the same factor.
By Ian Poole