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?

  • \$\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
  • 1
    \$\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
  • 1
    \$\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
  • 1
    \$\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

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.

  • 3
    \$\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
  • 1
    \$\begingroup\$ What is NG for a TIA? Infinite (Rg=0)? \$\endgroup\$ – Gianluca G Mar 28 at 20:28

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.

  • 1
    \$\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

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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.