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For a buffer amplifier using op-amp, what happens if input voltage is greater than supply voltage?
we know op-amp saturates at supply voltage when product of gain and input is greater than supply i.e +/-12V
i want to know output of buffer amplifier when input is greater than saturation voltage
The input voltage range is usually specified by the input common mode range and should be within the power supply range or sometimes considerable less (as much as half the supply in some). It is a good idea to have series current limiting resistors too if the inputs can be widely separated in voltage, so that high current doesn't pass through the clamp diodes. See the internal circuit of a bipolar op-amp like a 741 on its data sheet for an understanding. Some JFET input op-amps can have the inputs outside the power supply within reason, and also have no clamp diodes.
The effect of driving the input beyond the specified range depends on the individual device type, considering there are different input circuits, but the sort of effects are latchup (can be catastropic in some older designs), reversal of input polarity, loss of open loop gain, loss of common mode and power supply rejection. The simple point is that a circuit will not behave as expected.
For a buffer amplifier the output is driven to saturation when the closed loop gain x input signal exceeds the output voltage limits, which are often specified one or two volts below the supply rails. This is just overdriving the amplifier as long as the inputs remain within their specified range. This is a different specification to the input voltage specification. When the output is saturated the loop cannot operate as a closed feedback loop - the loop is broken. However if the output is just driven into saturation by exceeding the input drive for that gain, the output signal is just clipped. The tops and bottoms are cut off AC signals. As soon as the input comes back in range the loop closes again and all works as expected.
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