How to check an OP-Amp is in negative feedback or not

Hi Hari,

Yes, there is a practical and easy way to make sure that the circuit has sufficient phase margin to remain stable. If the circuit is stable and exhibits good transient response characteristics, then that is a direct indicator that the feedback characteristics are correct and doing the job.

A method we use in the lab and for simulation is to drive the op amp circuit with a small-signal square-wave and observe its output waveform with a wideband DSO. The intended load needs to be connected because its impedance characteristics need to be included in the output response. The input square-wave is set to a level such that the output remains small-signal and doesn't go into large-signal operation. Slew rate limiting can become a factor and that will alter the results.

For example, apply a 10 or 20 mVp-p, 100 Hz square-wave (or applicable frequency) to the circuit input. Then, observe the output from the op amp circuit with the DSO and note the overshoot on the waveform leading edges, and any ringing resulting from the overshoot.

The TI Analog Engineer's Pocket Reference, on Pg. 49 of the 4th edition provides useful graphs that relate the phase margin to percentage overshoot. Figures 34 and 35 are the graphs you would apply to determine the approximate phase margin. The circuit would ideally have at least 45 degrees of phase margin, and you really don't want less than 30 degrees for most applications.

You can find the Analog Engineer's Pocket Reference here:

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Thank you very much.

I will restate my question in another way. Please find the attached image of Howland current pump.

I removed the signs of the opamp.

My question is I need to determine the signs of the opamp such that the system works in negative feedback.

Regards

hari

Hello Hari,

I am not sure that an analysis that doesn't include the operational amplifier input signs makes sense. You wouldn't know if the output is inverted, or in the same phase as the input, which is important to know with regard to negative or positive feedback.

I show below the transfer function for the conventional Howland Current Pump and you will note that there are minus signs in the equation. That indicates that sense of the feedback has been taken into account.

I have attached a PowerPoint slide set that provides the derivation of the transfer function for a dual-feedback system. It allows one to determine the output voltage for an input voltage that takes into account the effects introduced by the negative and positive feedback factors.

Regards, Thomas

Precision Amplifiers Applications Engineering

Dual Feedback Beta Derivation.pptx

Hello Hari,

For the complete story regarding dual-feedback stability analysis see the attached PowerPoint. Slides 100 through 114 are specifically about op amp dual-feedback stability analysis.

This stability slide content was developed by Tim Green, the Precision Amplifiers Applications Engineering Manager, and Collin Wells, the General Purpose Op Amps Applications Manger. There is a great deal of information about op amp stability issues covered by these slides.

Regards, Thomas

Precision Amplifiers Applications Engineering

Solving Op Amp Stability 2013 TG_RevB.pptx

Hi Thomas,

Thank you for your time and concern.

Could you please go through the attached document from page 3 on wards, and will tell me how to find the op amp signs for Figure-12.

Opamp Sign.pdf

nptel.ac.in/.../104

Regards

Hari

Hello Hari,

I must admit when I review the paper you reference I am confused about the purpose of establishing the "SIGNS OF OPAMP TO PROVIDE NEGATIVE FEEDBACK IN A LOOP" in the manner in which it is presented in the material. Conventional op amp practices based on control loop theory establish which input on the op amp results in negative feedback, and that one is indicated by the negative sign symbol. Certainly there can be enough phase delay in a feedback circuit such that the feedback is no longer negative and becomes positive, resulting in instability. But at least that can be fully analyzed using gain-phase plots with respect to the inverting, or non-inverting input which are established up front.

The problem in Fig. 12 is an academic exercise which doesn't seem to be complete. There isn't any input voltage (Vi) or current (Ii) source, only Vo; therefore, it is not clear what this particular circuit's function is and what it is supposed to do. You will note that all of the other figures in the article have input sources and an expectation of an output voltage or current. I do not think it is a good use of time trying to determine the signs on the op amp inputs when it is not known what function the circuit is supposed to be.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Hari,

Actually, the author provides the solution for the circuit shown in Figure 10 on pages 4 and 5:

"In fig(10),there are two opamps. First we have to determine the signs of one opamp assuming all other opamps are ideal. Let opamp(1),assuming opamp(2) as ideal, then:

Vid1 = −Vo1(R3 R4) [R1 / (R1+R2)]

So we connect this to positive terminal. Now we know the signs of opamp(1). To dermine the signs of opamp(2),

Vo1 = Ao(Vo2) [ R1 / (R1+R2)]

Vid,2 = Vo1 [R3 / (R3+R4)] + Vo2 [R3 / (R3+R4)]

So we connect this to negative terminal of the opamp(2)."

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Hari,

It appears that I am not communicating effectively regarding your negative feedback questions. Your may benefit more and improve your understanding from some on line resources that do a very good job of explaining the concepts of negative and positive feedback.

One resource that I find particularly good on the feedback subject is provided on the Electronics-Tutorials web site. You can find the link here:

This is Section 4, but be sure to view Section 5 as well. It builds on Section 4 and covers negative feedback systems.

Regards, Thomas

Precision Amplifiers Applications Engineering 

Hi Thomas,

Thank you very much .I analysed the problem with the support of your material.

Could you please check the attached document and tell me my approach is wrong or not.

Sign.pdf

Regards

Hari