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If a datasheet (like AD828) says that an opamp is stable at Gain >2 (or reccomends to work with G>2, hence it is clearly not unity gain stable), what can we deduct about its stability in the inverting configuration at G=-1; G=-2 or G<<-2 (like in any transimpedance amplifier configuration)? Is it always instable in the three above cases if not compensated?

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  • \$\begingroup\$ Good question. The dynamic performance is also specified at G= -1, so it would seem that it is also stable below -1, but im not sure. \$\endgroup\$ – Linkyyy Mar 28 at 12:34
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    \$\begingroup\$ @Linkyyy Are you sure that you do not mean: so it would seem that it is also INstable at G = -1 The loopgain does not change for G = 1 vs G = -1. It is also the loopgain that determines (in)stability. G= -1 vs G = +1 only differs in the way where the input signal is applied. \$\endgroup\$ – Bimpelrekkie Mar 28 at 12:40
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    \$\begingroup\$ transimpedance amplifier configuration I think that the transimpedance amplifier is a bad example here as the ones I know all apply the input (current) at the - input so basically they're all inverting. I think we should only consider voltage amplifiers instead as these can be inverting and non inverting. \$\endgroup\$ – Bimpelrekkie Mar 28 at 12:43
  • \$\begingroup\$ It's a video amplifier so why are you even considering as a TIA? \$\endgroup\$ – Andy aka Mar 28 at 12:45
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    \$\begingroup\$ @Linkyyy the bandwidth at -1 is substantially lower than what is given at G=+2 You're comparing apples to pears. It is only fair to compare G = -1 vs G = 1 or G= 2 vs G = -2. The BW will be different between G = +/-1 and G=+/-2 because GBW product is constant. \$\endgroup\$ – Bimpelrekkie Mar 28 at 12:52
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Stability is a function of NOISE GAIN, not strictly the same thing as gain...

Noise gain follows the formula for the gain of a non inverting stage $$NG = 1 + Rf/Rg$$

For an inverting unity gain stage this will be 2, making the part stable in this configuration.

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    \$\begingroup\$ Although I've been an analog designer for 25 years, I didn't know about "noise gain" but looking up what it is, it is strongly related to loop gain which is what I use to evaluate loop stability. I like the term "noise gain" though as it emphasizes that there is no relation between stability and the input signal of the circuit. Good reading material: analog.com/media/en/training-seminars/tutorials/MT-033.pdf \$\endgroup\$ – Bimpelrekkie Mar 28 at 13:11
  • \$\begingroup\$ The classics are by Tobey, Graeme, Huelsman; two good books \$\endgroup\$ – analogsystemsrf Mar 28 at 16:23
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    \$\begingroup\$ What is NG for a TIA? Infinite (Rg=0)? \$\endgroup\$ – Gianluca G Mar 28 at 20:28
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Loop gain is the stability determining factor.

Loop Gain = Beta * Ao where Beta = feedback fraction = R1/(R1+R2) and Ao = open loop gain.

1/Beta = Noise Gain.

So a non inverting amplifier with a closed loop gain of 2 (R1=R2, Beta = 0.5 and Noise Gain=2) has the same Beta and therefore the same noise gain as an inverting amplifier with a closed loop gain of -1 (R1=R2, Beta = 0.5 and Noise Gain = 2).

This means that an inverting amplifier with a gain of -1 is as stable as a non-inverting amplifier with a gain of 2.

In addition to Noise Gain being the stability determining factor, Noise Gain also determines the bandwidth of an amplifier.

Bandwidth = GBW/Noise Gain.

So a non-inverting amplifier with a gain of 2 (R1=R2) has the same bandwidth as an inverting amplifier with a gain of -1 (R1=R2). If you make the closed loop gains of the two amplifiers both equal to 2 then the inverting amplifier will have a bandwidth equal to 2/3 the bandwidth of the non-inverting amplifier.

Non-Inverting amplifier with a closed loop gain of 2 has R1=R2 and a noise gain of 2. Inverting amplifier with a closed loop gain of 2 has R2=2*R1 and a noise gain of 3.

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    \$\begingroup\$ Take a look at the data sheet for the AD744 op amp which is stable for non-inverting gains of +2 or greater and also for inverting gains of -1 or greater. To be used as a unity gain follower this op amp requires extra compensation. \$\endgroup\$ – James Mar 28 at 15:56
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Stabilty is a function of the total feedback phaseshift.

1) Rout + Cload: 100 ohms and 100pf are 10,000 picosecond time constant, producing 45 degrees phaseshift at 100 MegaRadians/second of 16MHz. Many opamps have Rout (internal output resistance) near 100 ohms; some have Rout >>> 1Kohms.

2) phase margin beyond 90 degrees: a 60 degree phase margin opamp (Unity Gain phase margin) has 90+30 = 120 degrees phase shift

3) phase shift at the virtual_ground node: assume 10pF on that node, and resistive equivalent (Rin || Rfb, or Rg || Rfb) of 1,000 ohms; this produces 10,000 picosecond tme constant, or 45 degrees at 16MHz.

What rescues a feedback network? Usually the parasitic feedback capacitance in parallel with the feedback resistor. IMHO

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