A really basic question about the inverting LM3886

[lhwidget]

I've been curious about this for some time; Why does the inverting 3886 design not need a cap on the feedback loop to block amplification of DC signals?

I thought this was due to the simple fact that the inverted output would be fed back to the input which would cancel any DC in the input signal or generated by the chip. Is this correct?

[AndrewT]

Originally Posted by lhwidget

Why does the inverting 3886 design not need a cap on the feedback loop to block amplification of DC signals?

the input feed to an inverting amplifier is part of the NFB loop.
A DC blocking capacitor on the input of an inverting amplifier does both jobs. It blocks DC coming from the source and it reduces the DC gain down to below 1times. Yes, an inverting topology can have a gain of less than +0dB. It is not limited to gains of greater than or equal to +0dB, as are non-inverting topologies.

[ratza]

The downside of this configuration is that you need to drive it with a low output impedance source. If the LM's inverting input is left in the air, the chip will start oscillating.

[lhwidget]

Thanks Andrew, I had a feeling it was something I was overlooking (I still find it hard to see all the legitimate circuit paths in even simple schematics).