Electrical – Inverting buffer – simple circuit, weird problem

bufferoperational-amplifier

i want to make a unity gain inverting buffer. It's usually done by placing 2 equal resistors – in this case R1 = R2 = 47k. I wanted to simulate it in LTSpice and something is definitely wrong. Is this simple circuit designed bad?

thanks

invertingbuffer

Best Answer

The LT1115 is not intended for unity gain (+ or - gains) and will oscillate when used this way. Luckily the sim model responded as the real device does. Read the following extract from the data sheet on page 7 under applications information: -

The LT1115 is a very high performance op amp, but not necessarily one which is optimized for universal application. Because of very low voltage noise and the resulting high gain-bandwidth product, the device is most applicable to relatively high gain applications. Thus, while the LT1115 will provide notably superior performance to the 5534 in most applications, the device may require circuit modifications to be used at very low noise gains. The part is not generally applicable for unity gain followers or inverters.

Note also the phase margin falling to zero (oscillatory condition) before open-loop gain drops to unity: -

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