Op-Amp Bandwidth, Gain Bandwidth Product & Frequency Response

Op-amp Tutorial Includes:
Introduction     Op amp gain     Bandwidth     Op amp slew rate     Offset null     Input impedance     Output impedance     Current feedback op amp     Understanding specifications     How to choose an op amp     Op amp circuits summary    

The bandwidth of any circuit is of paramount importance that needs to be considered during the electronic circuit design process.

When designing the electronic circuit, it will be seen that the bandwidth of the circuit is related to the gain. Too much gain and the bandwidth will be low, less gain and the bandwidth that can be achieved is much higher.

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.

Operational amplifiers themselves when operating in open loop mode only have a very limited limited bandwidth before the gain starts to fall. However by using negative feedback, the huge gain of the amplifier can be used to ensure that a flat response with sufficient bandwidth is available.

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.

By adopting the correct approach during the electronic circuit design, the flat bandwidth of the complete circuit, i.e. the op amp and the additional electronic components can be made to be very flat over the required bandwidth.

Voltage vs current feedback operational amplifiers

The most common type of operational amplifier is one that is known as a voltage feedback op-amp. However there are also current feedback op amps as well.

Although in essence they are very similar in that they both have differential inputs and many of the standard circuit designs operate in the same way, there are some major differences between them.

One of the differences is in the gain bandwith product occurs and how it manifests itself. A current feedback op amp does not conform to the constant gain bandwidth product relationship that voltage amplifiers follow.

This can be difficult to grasp at some times, because it is a factor that is built in to all voltage feedback amplifiers that are far more prevalent, so everyone is more used to this.

Note: the equations and theory on this web page refer to voltage voltage feedback operational amplifiers.

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:

Where:
    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.

When designing any circuit using a voltage feedback operational amplifier, the aspects of the gain bandwidth product must form a key element of the design. Although the op amps have exceedingly high levels of gain, it is not possible to have a flat bandwidth over a large frequency range with high gain. There is a balance that msut be made withint he early stages of the design, allowing a suitable margin for variations in gain between differnet op-amps as wella s allowing for th etolerance of other components, etc.

Accordingly it is very important to accommodate the gain bandwidth product in any circuit design using an op amp and not trying to squeeze too much gain out of a single stage if a reasonable bandwidth is needed.

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