# Questions tagged [loop-gain]

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### Why do some op amps' datasheet specify phase margin at open loop?

I am not able to understand what significance phase margin would hold in case of open loop. Shouldn't it be just phase? If it was loop gain I'd understand specifying phase margin to denote stability. ...
58 views

### Stability analysis - 0 dB crossover point

I have a question about Bode diagrams. For example if a Bode plot (gain) cross 2 time the 0 dB axis which point should we consider. Should we analyse both of them and keep them stable (good phase and ...
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### About stability in system with loop gain AB>1 in negative feedback

I have been reading about stability and feedback systems, and a very silly issue has appeared. We can describe a negative feedback system as what can be seen in the following picture: Image source: ...
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### Negative loop gain of LDO

I am trying to design an LDO with the following specification: Vout = 1.5V; Vin = 1.7V - 2V; Iload = 75uA to 1200uA. However, while I am simulating its loop gain, its gain is coming negative. Even ...
4k views

### What is the best way to simulate the open-loop gain of an op-amp in LTspice?

I have been trying to simulate open-loop gain to check the SPICE model in LTspice. From what I could find out so far, there appear to be two main approaches to achieve this. The example in this video ...
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### Changing the frequency response of an open loop circuit op amp

Usually on the frequency response of an open loop op amp, the gain decrease greatly in the higher frequencies. How can we increase the gain from 50 hz to 25 khz to remain high at, say, 120 dB? The ...
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### Instability of fully differential op-amp in LTspice

I tested the following circuit including a fully differential op-amp for stability in LTspice. The circuit has an inverting gain of 6 and a roll-off at 4 MHz. When exciting the circuit with a step, ...
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### Confused regarding gain margin and stability

I plotted the bode plots of the transfer function: G(s)= 2.123×10^6 ((0.105s+1.01))/(s^2 (s+12566.37) ) It is as follows: As can be seen, it shows the gain margin as negative infinity, however loop ...
244 views

### Closed Loop Gain with Phase Reversal in 741 Op-Amp

This is a typical example of negative feedback in an Op-Amp - For this case, the feedback of the output can be controlled using a resistor divider, and let's call the feedback fraction to be X. We ...
416 views

### Qualitative explanation of the high open-loop gain of a 741 op-amp using its internal circuit characteristics

Here is the internal circuit of a 741 op-amp: Using small signal analysis, I can claim the open loop voltage gain to be the product of the current gain of the transistors in the Darlington pair(Q15, ...
249 views

### Voltage gain VS power gain in an oscillator

I just have a general question about amplifiers for oscillators. Is it a necessary condition that the amplifier has to have overall power gain or just voltage gain (and decreasing the current so that ...
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### Why can you define closed loop gain when return ratio approaches infinity like this?

This is from this lecture video. Please check the video link if you're not familiar with the terms. I can explain it but I think the video lecture is pretty good at it. Because the feedback amplifier ...
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### Is there a formal definition of a loop (in stability context)?

The circuit below with multiple feedback loops is taken from this page. The page considers only two loops in order to calculate the overall loop gain T. Is that enough? Doesn't it have 4 loops ...
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106 views

### how to find gain of this complicated circuit?

I am having trouble with the following circuit. This circuit is AC model of Colpitts oscillator designed with NPN type BJT transistor. I am trying to find loop gain and check if it meets Barkhausen ...
1 vote
645 views

### How to break the loops properly here to get exact loop gains?

I got the circuit with two feedback loops from this page. I'm trying to calculate the exact loop gain of each loop. However, I'm not sure how to break the loop properly so you can account for loading ...
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797 views

### How to get exact loop gain expression for this circuit?

This is a voltage-current feedback circuit. If I break the loop at the gate of the transistor then excite a voltage at gate and measue the return signal at the other end I got the loop gain ...
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### How to guarantee the stability of a RF closed loop amplifier with inner loops?

I've a 3 stage closed loop amplifier with many loops inside of which some loops cannot be broken, to analyze the loop gain, as the parasitic capacitors are intrinsic to the device. What is the method ...
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139 views

### Modeling MOSFET Pole and Transconductance for use in Larger Overall Control System

Recently I began a project involving some active current limiting using a MOSFET and an Op Amp. It is part of a larger overall circuit involving other Amplifiers. I would like to perform a block-...
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### Gain-bandwidth product and open loop/closed loop gain

I'm reading the TI-precision lab of opamp. And I'm confused with the calculation of the BW_CL (bandwidth of th close-loop). I'm prettry sure the BW_CL= GBPW/GAIN_CL. This is very clear refer to enter ...
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### Converter stability - bode plot

I referred this YouTube video for buck converter loop stability. In the image we can see that the gain of the control loop we are talking about is Vout / Vin. Vout and Vin are both measured across ...
143 views

### Buck Converter Feedback Loop - Stability Criteria

Question 1 : I am having some trouble understanding the Stability criteria of the Buck Converter Loop Stability which says, "The cross over frequency should be less than 1/8th or 1/10th of the ...
1 vote
105 views

### Is it possible to find the voltage gain of the constant gain block of this control system?

I'm looking at the following circuit and I don't know how to find the gain of the constant gain block. The only information I'm given are the Bode plots (magnitude and phase) of the two transfer ...
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1k views

### Differentiator cut-off frequency

A practical differentiator has two cut-off frequency but can no longer perform differentiaiton after first cut-off. After first cut-off there is a 20db per decade gain reduction but why does this ...
519 views

### Open-loop frequency response for an op-amp

I want to know what is the ideal circuit to measure open-loop frequency response and why?I have seen the typical open-loop frequency response curve for an op-amp a lot of times. But when I try to plot ...
662 views

### Gain error calculation for op-amp circuit in inverting configuration

For an inverting op-amp, calculate minimum gain that is required for the op-amp to achieve a gain error of 1%, if the required closed-loop DC gain is -9. If the input resistance R1 is 1Kohm,find the ...
535 views

### What is the definition of "the loading of feedback network on the input/output side"?

There is the following example: simulate this circuit – Schematic created using CircuitLab Its loading of the feedback network on the input side is 40kohm in series. Its loading of feedback ...
304 views

### SMPS stability near and above switching frequency

I am simulating in LTSpice a switched-mode power supply (LT3845) at a frequency of 150kHz. I am measuring the loop gain at the feedback node according to this article. The way I understand it, the ...
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### Middlebrook's Method VS Tian's Method VS Middlebrook's General Feedback Theorem

For https://sites.google.com/site/frankwiedmann/loopgain , how is Tian's Method different compared to Middlebrook's Method and Middlebrook's General Feedback Theorem ? Tian's Method Middlebrook's ...
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### Dominant Pole Approximation for loop gain

I have the following loop transfer function for a unity feedback system: I understand that the $(s+2)$ term becomes $2$, but what happens to the $s$ ?
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