Frequency response of OP-AMP

OP-AMP Bandwidth, frequency response & gain bandwidth product are key parameters for any circuit. Typically, OP-AMPs are used for comparatively low frequency circuit but with the performance of these chips is improving all the time, much higher bandwidth OP-AMPs are available.

The frequency response response of a typical OP-AMP chip will often start to fall at a very low frequency when operated in its open loop mode. The point at which the frequency starts to roll off is known as break point.

Frequency compensation added in most of OP-AMP chips. It is introduced to ensure that they remain stable& do not produce unwanted high frequency spurious oscillations.

The frequency response of open loop OP-AMP is shown below: –

(open loop c frequency response)

Frequency compensation is required because stay capacitances in the chip can cause unwanted phase shifts at high frequencies i.e. 1MHZ & more. While the stay capacitance levels may not be significant ay low frequencies, they can cause significant problems at higher frequencies & they are almost impossible to eliminate in the chip. This problem can be solved by reducing open loop gain at high frequencies. This is called compensation & is normally implemented by passing one of the internal amplifier stages with a high pass filter. The objectives is to reduce the gain to less than unity at frequencies where there could be possibility of oscillation.

The figure represents a comparison between frequency compensation & without frequency compensation.

(OP-AMP frequency Response With & Without Frequency Compensation)

Frequency compensation OP-AMPs are not very fast devices. The frequencies at which the c open loop again falls to unity is called unity-Gain frequency (ft ).

In view of the very high gain of the operational amplifier it is possible to exchange some of the open loop gain for Bandwidth.

(Close Loop OP-AMP Frequency Response)

OP-AMP gain Bandwidth product is generally specified for a particular OP-AMP type on open loop configuration & the output loaded.

It is expressed as: –

Gain Bandwidth product = AV * f

Where, AV = Voltage Gain

               f = Cut-off frequency (HZ )

Also Read- Construction of depletion type MOSFET

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