Phase Margin, stability of Opamp inverting amplifier with |gain| < 1 (attenuator)

[ikansw]

I am considering to build an inverting attenuator based on the classic OpAmp inverting amplifier with purely resistive feedback, with gain = -R2/R1. Except that I will have R2<R1 to get a gain magnitude less than 1, ie. it is an attenuator. I'm interested in R1=2R2 i.e gain = -0.5. The load is taken as pure resistive.

Now considering the phase margin and stability of this circuit, the OpAmp chosen (OPA209 Texas Instruments) has about 80 degrees phase margin at its unity gain point (0 dB open loop gain). But should I be looking at the unity gain point, or the point further down where open loop gain is -6dB ? It somehow does not feel intuitive (not that it should!).

Thanks for comments.

 

[Harald Kapp]

You should be looking at the operating point, i.e. at gain <1.
You can avoid all the hassle by using a resistive divider (gain <1) in combination with an OpAmp (gain >=1), Whether you put the divider in front of the OpAmp or after the OpAmp depends on the environment (source impedance, load impedance).

Harald

 

[ikansw]

Phase margin wins, SNR slightly poorer

Thanks Harald, I gather that even though |gain|<1 there could be some ringing if the phase margin is poor.

I'll take your method to escape the hassle then, and place a resistor divider in front as I need to offer low output impedance to the following stage.

The Boltzmann and other noise will now be riding on the attenuated signal dragging down the SNR, but I guess there is no choice about that, stability is more important for my situation.

Vielen Dank!

 

[Albertisthebest]

I'll take your method to escape the hassle then, and place a resistor divider in front as I need to offer low output impedance to the following stage.

The Boltzmann and other noise will now be riding on the attenuated signal dragging down the SNR, but I guess there is no choice about that, stability is more important for my situation. Biodegradable Pallet Wrap