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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: ...
Taela Shanthi's user avatar
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There are two ways for calculating loop gain. Which one is the correct loop gain?

The following is a question from 8.18 in Design of Analog CMOS Integrated Circuit, page 342. Consider the circuit in this following figure. \$(\frac{W} {L})_{1-4} =\frac{50} {0.5} \$, \$|{I_D}_{1-4}| ...
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How can I find \$A \beta \$ step by step? [closed]

The following is a question from 8.18 in Design of Analog CMOS Integrated Circuit, page 342. Consider the circuit in this following figure. \$(\frac{W} {L})_{1-4} =\frac{50} {0.5} \$, \$|{I_D}_{1-4}| ...
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Is this loop gain infinite?

Here is the circuit from this youtube link Break the loop. For M1, because there is no current through M1, so we have $$\frac{V_y - V_F} {r_{o1}} - V_F g_{m1} = 0$$ $$\frac{V_y} {V_F} = \frac{r_{o1}}...
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Why am I getting a nonzero voltage out of this inverting amplifier op-amp when I ground the input? [closed]

The circuit in my figure is an inverting amplifier with a gain of -100. My instructor asked me to ground the inputs to see what would happen and discuss what should happen for an ideal op-amp. In an ...
nwsteg's user avatar
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Need to evaluate the loop gain of a corrector stage

I would want to simulate the loop gain of a corrector stage. I have no idea to add the static point at the output (Vout) of the opamp (3.6039875V). What is the method to simulate the loop gain of this ...
exolune's user avatar
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1 answer
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Why is my LTspice simulation so different from my calculation by hand?

This is the circuit: $$V_{in} g_{m1} + \frac{V_{in}} {r_{o1}} = (V_{in} - V_o) s C - \frac{V_o} {r_{o2}}$$ $$\Rightarrow \frac{V_o} {V_{in}} = \frac{s C - g_{m1} - 1/ r_{o1}} {s C + 1/r_{o2}}$$ The ...
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How to determine if the feedback is positive or negative?

How to know if I have to apply the formula Af=A/(1-AB) [positive feedback] or Af=A/(1+AB) [negative feedback]?
Aaryan Pugarja's user avatar
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135 views

Getting a square wave as a output of op-amp

I have the following op-amp circuit and i am trying to find out what is the open loop gain. In order to make this simulation properly, we need to make sure that the DC operation of the amplifier is ...
Aleat's user avatar
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Calculate Gain of TS922

Im trying to figure out the gain of this opamp inside a st board. Here is the schematic where this opamp is used. As you can see, the closed loop opamp with feedback has a voltage divider on the ...
Tom's user avatar
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Voltage divider in a compensator

In a compensator (could be any type), the lower resistor from the voltage divider (R4 in the image below) is not included into the transfer function of the compensator, meaning it doesn't impact the ...
PosFLo's user avatar
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Simulating the open loop gain

My questions regarding the op amp simulation are as follow : Why do we need closed loop for DC and open loop for AC ? Why this separation? The inductor will behave like short at DC so the op amp ...
Mhan's user avatar
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Measuring flyback SMPS feedback loop response

I'm trying to measure the feedback loop on a isolated flyback SMPS. My goal is to determine the phase margin and 0 dB cross-over frequency. The most common circuit I have seen is the one recommended ...
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Telling loop instability from ringing

In the application note https://www.analog.com/media/en/technical-documentation/application-notes/an149fa.pdf unwanted oscillations are said to be caused by either loop instability or other ...
Hyp's user avatar
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Plant gain measured with Bode100 dependent on compensator

I tried to find the correct compensation elements for a high voltage supply based on the application note https://www.analog.com/media/en/technical-documentation/application-notes/an118fb.pdf The ...
Hyp's user avatar
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Estimation of bode plot of closed loop system based on open loop characteristics [closed]

I just got an exam back in which I did the error of not drawing the phase part of the bode plot of a closed loop system. The question sparked, how does one estimate the shape of the phase plot of a ...
Luka Pacak's user avatar
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How could we predict the transient response of the multi-loop circuit?

Typically, when analysis the stability of a single loop circuit, we can use the open-loop method, just to break the loop and get the loop gain. Then, using the Bode diagram, we can easily get the PM(...
hideinink's user avatar
3 votes
2 answers
163 views

Power converter: Why we always focus on loop gain involving control to output transfer function?

While measuring loop gain using AP300 frequency analyzer, why we always focus on loop gain involving control to output transfer function? Is it because input to output transfer depends on power stage ...
Dynamic_equilibrium's user avatar
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3 answers
404 views

Frequency analyzer for loop gain measurement - Why should the injection point have low output impedance and high input impedance?

@5:08 The video mentions: "The insertion point should have a low output impedance and high input impedance" What is the background for this condition?
Dynamic_equilibrium's user avatar
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Double pole, double zero: incorrect plot?

I can't seem to find where I'm going wrong here. Suppose I want a double pole, double zero system that looks like this: $$ \dfrac{\left(1+\dfrac{s}{\omega_z}\right)\left(1+\dfrac{s}{\omega_z}\right)}{\...
SM32's user avatar
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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 ...
Noob Coder's user avatar
11 votes
3 answers
3k 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 ...
kenearl's user avatar
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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 ...
SnoopyKid's user avatar
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5 votes
2 answers
1k views

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, ...
FrüchteTony's user avatar
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2 answers
353 views

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 ...
needbrainscratched's user avatar
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2 answers
231 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 ...
electrovolt's user avatar
-1 votes
1 answer
380 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, ...
electrovolt's user avatar
-2 votes
2 answers
232 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 ...
Shocked's user avatar
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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 ...
emnha's user avatar
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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 ...
emnha's user avatar
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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 ...
onurkirmizi's user avatar
1 vote
2 answers
598 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 ...
emnha's user avatar
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1 vote
2 answers
763 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 ...
emnha's user avatar
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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 ...
Vysakh K's user avatar
1 vote
2 answers
137 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-...
banjoeschmoe's user avatar
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3 answers
2k views

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 ...
FNJU's user avatar
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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 ...
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1 answer
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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 ...
user avatar
1 vote
1 answer
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 ...
EErocks's user avatar
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1 vote
2 answers
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 ...
vikas sajanani's user avatar
2 votes
1 answer
515 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 ...
vikas sajanani's user avatar
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0 answers
650 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 ...
Mahesh Bhat k's user avatar
3 votes
4 answers
499 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 ...
Audra Jacot's user avatar
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2 answers
283 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 ...
aosborne's user avatar
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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 ...
kevin's user avatar
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1 answer
938 views

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\$ ?
Hector's user avatar
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2 answers
120 views

4ma-20ma loop from 0.97V to 2.75V

I'm trying to develop a 4ma-20ma loop from an 0.97(4ma)-2.75(20ma) input. This input comes from the output of a low pass filter of a PWM module. I've searched the web quite thoroughly but all the ...
HelpMeBro's user avatar
1 vote
0 answers
171 views

The Effect of Input and Feedback Capacitors on an Operational Amplifier Open-Loop Gain Response

I have the following circuit that I want to compensate: As shown from the step response, the circuit is unstable because of the high input capacitance (the green plot is without any additional input ...
fuad1502's user avatar
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1 answer
714 views

Improve the performance of Buck Converter Loop Stability

The following are the specifications of my Buck Converter. Input Voltage Range : 6V to 16V Output Voltage : 5V Switching Freq : 2.1MHz Output Inductor : 22uH Minimum Load Current : 200mA Typical ...
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1 answer
1k views

Why should 180 degrees of phase margin be considered?

We've had one feedback on the phase difference, with the gain as 1. At 300 to 60 degrees, there's just a phase difference, and at 61 to 299 degrees, the output is larger. Shouldn't we consider an ...
aockfskf's user avatar