Questions tagged [loop-gain]

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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 ...
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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 ...
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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, ...
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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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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 ...
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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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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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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 ...
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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 ...
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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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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 ...
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1 answer
278 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 ...
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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 ...
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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 ...
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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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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 ...
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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 ...
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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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Why 180 degrees of phase margin should 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 61 to 299 degrees, the output is larger. Shouldn't we consider an angle ...
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Negative loop gain?

I was dimensioning the CE capacitor. To do that I was looking at some formulas seen in class. The formula (freq_C=1/(2*piReqC)) involves the knowledge of the loop gain (open loop gain*beta). It ...
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Calculating the loop gain for an ideal op-amp with time delay

In the circuit below, \$\tau \$ represents a time delay. We are using a first-order Pade approximation for this time delay. This means that \$\ e^{-s\tau} = (1-s\tau)/(1+s\tau)\$. Below is the ...
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MATLAB model of an op-amp's Open Loop Gain

I'm using MATLAB to analyze an amplifier in a transimpedance circuit, and I'm a little lost in implementing the op amp's open-loop gain function with the following formula: $$A(s) = A_{OL} \cdot \...
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3 answers
426 views

Op-amp inductive output impedance oscillation

In Learning the Art of Electronics pg.362, the author presents an op-amp follower driving a long cable. I don't understand the explanation given for the cause of oscillation. In particular, the author ...
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Drawing the Bode Plot of a loop gain

Below I have the loop gain and the Bode plot for it, however I do not understand the phase plot. Why does it start at -90 degrees instead of 90 degrees since we have a zero? Please explain.
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Estimate the phase margin of a loop gain

The loop gain is \$10^6 /[(s+1)(1+s10^5)]\$. Below I have the Bode plot. How can I derive the unity gain frequency from the Bode Plot? How can I find the phase margin from the phase plot of the Bode?
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How to find voltage gain of below analog circuit without using Nodal analysis?

This is a theoretical circuit just made to see that if I can find voltage gain of circuit and it took me much calculations then anticipated to get voltage gain.I just want to find if there is better ...
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4 answers
642 views

Negative feedback and op-amp poles

I'm trying to get a better understanding of frequency compensation for op-amps. I'll start off with a very specific questions and then get into the context. For a certain op-amp, open loop gain is ...
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What's the largest practical stable gain possible?

I have a feedback application to set the voltage of a node in which there is considerable loss in the loop. Simulations tell me that a differential voltage gain of 10000, 80dB (roughly about what you ...
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opamp simulation

Is the following open loop transient simulation of opamp correct ?
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Shunt Shunt Feedback SImulations

I am trying to simulate the solution to an old text book example on a simple Shunt-Shunt feedback amplifier. simulate this circuit – Schematic created using CircuitLab The book only shows BF = ...
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Stability at loop gain <1 and 180 degree phase shift

I'm reading this article about stability of OpAmps: Link to article. It says: "If Aβ is less than unity at the high frequencies where phase shift reaches 180°, the high-frequency phase-shifted ...
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Amount of feedback in an amplifier

Sometimes I read things like "This amplifier has 30dB of feedback", what figure is that refering to? the \$\beta\$ of the amplifier? or the loop gain \$1+A\beta \$? or the negative feedback factor \$\...
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LT spice not predicting instability

I'm trying to make a linear regulator using an op amp and a pass transistor. Here is the circuit: From my limited understanding you can predict the stability of the circuit's feedback loop by ...
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General Feedback Theorem - Spice model and loop gain

I am implementing the General Feedback Theorem (Prof. Middlebrook, 2006) in LTSpice, using the example given in the Spice folder. I used it to calculate the loop gain in a simple voltage follower ...
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Find optimal gains transfer function

I have the following closed loop transfer function: $$G(s) = \frac{K(s^2 + 200^2)}{0.05s^3 + (1+K)s^2 + 200{K_v}s + 200K}$$ Where \$K\$ and \$K_v\$ are gains. My goal is to find the optimal gains ...
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Why gain crossover frequency must be lower than phase crossover frequency for stable systems?

As far as I understood from here, phase margin means the amount of phase that needs to be added in order to coincide with 0 db gain and -180 degrees phase. Likewise for the gain margin it means means ...
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Why we can consider inductor as a current source when the current loop gain infinite?

I am reading about current-mode control from this site. Please look at the excerpt below and especially the boldfaced part. Question: Why we can consider inductor as a current source when the ...
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Relation between crossover freqquency of loop gain and closed loop's corner frequency when feedback is not unity

In this Control Loop Cookbook from TI, in the red box the author said that the crossover frequency (indicated in figure 2) of the open-loop is same as 3dB down frequency or corner frequency of the ...
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Differential Amplifier's Unusual Behavior

This circuit is made of one differential BJT transistor pair and voltage-amplification stage with negative feedback. simulate this circuit – Schematic created using CircuitLab About this ...
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Benefits of higher order low pass in a negative feedback loop

Let \$G(s) = \frac{Y(s)}{R(s)}\$ be the loop gain of a negative feedback loop system. For example a system like this, where Y(s) and R(s) are the laplace transformed functions y(t) and r(t): ...
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Negative Feedback Portion of Amplifier

Here, in this question, I am referring to 3 stage Lin topology amplifier (differential stage - voltage amplification stage - output power stage + negative feedback). I am wondering about portion of ...
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Loop bandwith and open-, closed- loop gain in ADIsimPLL

I'm want to use ADIsimPll to calculate the loop filter properties for a PLL I want to build. I read some things in the programs help topics which I find somehow strange. Maybe you guys can help me out ...
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Any transfer function has the same denominator 1+T

Sometime ago, I came across some theory saying that in a control loop any transfer functions such as ...
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