Op Amp Bandwidth & Frequency Response
- op-amp bandwidth, gain-bandwidth product, open and closed loop frequency response: what they are and how they affect operational amplifier circuit design.
Op-amp circuit design tutorial includes:
• Operational amplifier basics • Op-amp circuit gain • Op-amp bandwidth • Op-amp slew rate • Op-amp offset null • Op-amp input impedance • Op-amp output impedance
• Op amp circuits
Operational amplifier bandwidth and frequency response are key parameters for any circuit.
Typically operational amplifiers are used for comparatively low frequency circuits, but with the performance of these chips is improving all the time, much higher bandwidth op amps and op amp circuits are available.
The bandwidth of the op-amp itself obviously has a bearing on the design of the op amp circuit and the frequency response or bandwidth available for the circuit.
Op amp bandwidth basics
The frequency response of a typical op amp chip will often start to fall at a very low frequency when operated in its open loop mode.
Open loop op amp frequency response
The point at which the frequency starts to roll off is known as the break point [ typically the -3dB point is known as the break point].
Most op amps have their bandwidth limited as a result of what is called compensation which is added.
Op amp frequency compensation
Most op amp chips have frequency compensation added to them. It is introduced to ensure they remain stable and do not produce unwanted high frequency spurious oscillations.
It is required because stray capacitances in the chip can cause unwanted phase shifts at high frequencies, e.g. 1 MHz and more. While the stray capacitance levels may not be significant at low frequencies, they can cause significant problems at higher frequencies, and they are almost impossible to eliminate in the chip.
Most op amp IC manufacturers solve this problem by intentionally reducing the open-loop gain at high frequencies. This is called compensation and it is normally implemented by bypassing one of the internal amplifier stages with a high-pass filter. The aim is to reduce the gain to less than unity at frequencies where there could be a possibility of oscillation.
Very early op amps did not have this frequency compensation built into the chip and external compensation components were required on the pins provided - the 709 was a prime example of this. Later chips such as the 741 had internal compensation making the chips much easier to use. However they also had a low open loop break point. In the case of the 741 it was just 10 Hz. This compensation is now standard in all general purpose op amp chips. Modern chips continue to have it built in a standard.
Op amp frequency response with and without frequency compensation
Frequency compensation is the major reason why op-amps are not very fast devices - the higher frequency components of the signals are intentionally attenuated. The frequency at which the Op amp open loop gain falls to unity, is called fT - as for bipolar transistors. This frequency gives a good indication of the speed of the op-amp.
However, comparators, do not use negative feedback and as a result they are designed without compensation and their speed of operation is typically much faster than that of op amps.
Feedback vs bandwidth
In view of the very high gain of the operational amplifier it is possible to, in effect, exchange some of the open loop gain for bandwidth.
For a circuit like this, applying feedback will reduce the gain but increase the bandwidth.
Closed loop op amp frequency response
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:
GB = op amp gain bandwidth product
Av = Voltage gain
f = cut off 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
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