# Stability measurement of Operational Amplifier

All,

I am trying to simulate as well as measure the stability of an op-amp with high gain (~50dB). The gain is necessary to boost a low level signal. I am struggling to understand if I am making the measurement right as well as if I am simulating it right.

I am using OrCAD for the simulations and an AP Instruments model 300 for the measurement. Page 78 in the user manual indicates the test setup, and you can find the manual at http://www.apinstruments.com/files/Model300.pdf. I have used the supplied transformer and a parallel leaded 10 ohm resistor for reference.

The Op-Amp is a texas instruments TLV316 component in a non-inverting configuration. I am having trouble taking screen shots for some reason so I will describe the circuit. It contains an AC coupled AC source to half supply voltage (say 2 volts), and the amplifier is configured as a non-inverting amplifier. The feedback is a 113k resistor in parallel with a 100pF capacitor. The inverting input is 3.3k in series with 0.036uF capacitor.

I've tried employing the 3 techniques outlined in the article below but I have convergence problems unless I AC couple the audio through.

http://electronicdesign.com/print/analog/use-pspice-verify-feedback-amplifier-stability

My question is: Is the stability measured by plotting the difference of the magnitude and phase of the output versus the negative input OR is it the difference between the positive input and negative input such as open loop gain? Further, if the former is true, am I supposed to measure the difference across the 10 ohm resistor in the lab and across the high 1000H inductor in my simulation, or do I measure from the output terminal and the negative input terminal? Also, what is the phase supposed to be at near DC? 180 degrees, -180 degrees, 0 degrees?

EDIT: The schematic is shown below. The non-inverting node has an AC source connected and is AC coupled as mentioned, however in the screen shot I removed the source and directly connected the left side of the capacitor to ground.

I think I may be getting hung up on the affect of the capacitors on the phase. I am going to retry the circuit but with just resistance. For reference, I don't have any reason to suspect the circuit is unstable and no, it is not oscillating.

• so I will describe the circuit Sorry, I'm far too lazy to "imagine" your circuit in my head from your words. Edit your question and draw the schematic, use the provided tool, start it with the small circuit icon. You also do not seem to have a full comprehension of what is needed for stability, therefore I suggest you study the Nyquist stability criterion: facstaff.bucknell.edu/mastascu/econtrolhtml/freq/opamp6.html maybe this is a good read as well: ti.com/lit/an/sloa020a/sloa020a.pdf – Bimpelrekkie Apr 14 '17 at 13:21
• 1) AC couple is necessary for single supply due to DC offset. Otherwise no. 2) Stability is the phase margin at unity gain or the gain margin when phase shifts from 0 to 180 deg.3) at DC phase depends on network, if there is DC gain then phase is 0 for non-inverting input and 180 deg for inverting input. If AC coupled then -90 deg.but attenuated to - infinity – Sunnyskyguy EE75 Apr 14 '17 at 14:31
• thus if testing Bode plot to DC you get a convergence problem if AC coupled doing ratios. – Sunnyskyguy EE75 Apr 14 '17 at 14:37
• Does the actual circuit oscillate? What is the load capacitance on the OpAmp? scope probe? coax-cable (100pF/meter)? or what? – analogsystemsrf Apr 14 '17 at 17:06