Consider an op-amp with negative feedback which uses a feedback resistor network which may be used as an inverting amplifier for V_{2} = 0 and non-inverting amplifier V_{1} = 0

The closed loop transfer function is given by A_{CL} = A/1+Aβ

Where; A : open loop voltage gain

B : Feedback ratio

If (1+Aβ) = 0 the circuit will become unstable and gives sustained oscillations.

Re-writing 1+Aβ = 0 as 1-(-Aβ) = 0 leads to –Aβ = 1

Aβ is a complex quality

Therefore, │Aβ│ = 1 and phase condition L^{le}– Aβ =0 or (multiple of 2π)

L^{le}– Aβ =1 or (multiple of π)

Since, resistor is the present in feedback network it does’t provide any phase shift.

When op-amp is used in inverting mode it provides phase shift

Of 180^{o} and at low frequencies.

However at high frequencies due to each corner frequency an additional phase shift of maximum -90^{o} can take place in open loop gain phase. So, for two corner frequencies maximum phase shift is added

Hence at high frequencies magnitude Aβ becomes unity when A has an addition phase shift of 180^{o} whic make total phase shift is ‘0’.

This causes the amplifier to begin to oscillates and oscillations are the starting point for in stability.