Capacitance on the output of an op amp

Question

I have read a lot about capacitive loads on the output of an op-amp and the possibility of unwanted oscillations and instability. my knowledge on phase margin and frequency domain analysis is a little weak and I can't figure out if my circuit bellow is gonna be unstable or not:

The Op-amp supply is connected to 5vdc and GND. and here is it's datasheet:

I read in some application note that placing R19 would help the stability issue by increasing phase margin or something like that and fortunately I already had that in my circuit. but as you can see C10 capacitor also has a very high capacitance value. is there an analysis or rule of thumb which would determine if this circuit is safe? should I change my op-amp IC? or should I use other methods as well to insure my circuit's safe operation?

I can provide additional information if you need it. Thanks.

Answer 1

Most OAs have limited capacitive load capabilities. The origin is quite intuitive. Consider the simple circuit below. On the left there is a real OA. On the right, the OA has the same characteristics (poles, etc.) but the output resistance is externally shown.

This will have the effect of creating a pole, with time constant \$R_{out}\cdot C\$.

Such pole will reduce your phase margin of your system.

Limiting case: C is so large that it can be considered a short. Then you're actually removing the feedback, and you get a comparator (assuming non zero feedback Z)!

Instead, if there is a "large enough" resistor in series to C (R19 in your circuit), the series R-C will add a pole (with time constant \$(R_{out}+R_{19})\cdot C\$ ) and a zero (with time constant \$R_{19}\cdot C\$ ). If, as said, R19 is large enough, the pole and zero time constants are close enough and the overall effect is negligible (limiting case: R19 is infinity).

Intuitively, after the zero frequency, the series R₁₉-C will act as a load, and it will not open the feedback (it will change the feedback attenuation factor, but if \$R_{19}>>R_{out}\$ then this variation will be negligible as well).

EDIT: In your case, the circuit will be stable.

EDIT2: the following consideration is no more valid with the new edits to the schematics made by the OP:

I assume that "Input Signal" in your schematics is a current signal (i.e. not a connected voltage source), otherwise the circuit won't work.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Lets cut through the hand waving, and provide a specific answer to Rseries, so as to dampen the ringing.

OpAmps have excellent Rout at low frequencies, with that Rout increasing continually above the open loop gain rolloff frequency, often 10Hz or 100Hz. The increase in Rout also shows a 90 degree phase shift. Result is the OpAmp has inductive Zout, above that 10Hz or 100Hz corner frequency.

That inductance, given a capacitance on Vout, will ring unless adequately dampened. Can we predict Lout (output inductance)? Some opamps provide a chart of Rout versus freq; if so, grab the Rout at UGBW (you may have to extrapolate), and compute the Rout = 2 * pi * Freq * L, solving for L.

An opamp with 100 ohm Rout at 1MHz (many ordinary opamps), has inductance of

L = 100 / ( 2 * pi * 1e+6) = 16 microHenry.

How to dampen? Use the formula Rdampen = sqrt(L / C), derived from considering the various equations for Q=1 for R and L, and R and C, and combining to eliminate Frequency as a variable.

Given 1uF Cload, and 16uH output inductance, the proper Rdamp is sqrt(16/1) = 4 ohms.

Answer 3

Some opamps can have an issue when driving a capacitor to ground or supply directly meaning, without any additional series resistance.

You do not have this issue as long as R19 is present and larger than 500 ohms or something in that order.

When an opamp has a large capacitor to ground or supply directly at the output and the output is also used in a feedback configuration then instability can occur or the circuit might even work as an oscillator.

Both opamps do have feedback loops but the large capacitor C10 is not directly on any opamp output so I do not see any potential stability issues with this circuit. Here R19 saves your day as it separates the opamp circuit from the large capacitor C10.

Answer 4

Tim Green wrote a quite nice and hands-on tutorial on stability of opamps. There are very little formulas and he tries to give an intuition by using lots of graphs and drawings.