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Questions tagged [feedback]

Feedback is a process whereby information from an output of a signal is used (intentionally or unintentionally) to alter the input of the same signal. Commonly used in operational amplifiers. An everyday example is the high-pitched audio frequency heard when a microphone picks up audio from its own amplified output.

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23
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6answers
7k views

What is noise gain, really? And how is it determined in the general case?

UPDATE: This question triggered what might fairly be called a research obsession for me. I've gotten pretty close to the bottom of it I think, I've posted my findings as an answer below. There was a ...
22
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5answers
56k views

Choosing resistor values for inverting amplifier and why?

The gain here is A = -Rf/Rin. However, lets say I want a gain of 10 V/V. Which resistor value would you choose and why? I know that you could have infinite number of combinations for these resistors ...
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2answers
3k views

Why would you stack a resistor and capacitor on top of each other?

I've inherited an charge amplifier/shaping circuit from my predecessor. When he wanted to make a low-pass filter with current-to-voltage conversion, he had a standard circuit like: simulate this ...
19
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5answers
7k views

Side effects of using large resistances

Are there any problems that can be caused by using resistors of large resistances (in the order of megaohms)? I'm designing a feedback network that is just a voltage divider, and I want the feedback ...
17
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6answers
58k views

Why is feedback required in op-amp circuits?

I understand that, in order for an op-amp to function correctly, a DC feedback loop from the output to either the inverting or the non-inverting input (depending on the external circuitry) is required....
15
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3answers
4k views

What does this circuit do? Op-amp with transistors in feedback

I'm an electrical engineering student and I find this circuit in a guitar amplifier schematic, and basically, I'm just wondering what it does, and how one would go about working out what it does. ...
15
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2answers
4k views

Stability (phase margin) analysis in real feedback circuits

So I had a bright idea of using negative feedback to control offset current in my data acquisition circuit. Sure, you could do this in software, but removing the offset at the input stage would ...
15
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5answers
1k views

Which CMOS logic families can safely be used to construct linear circuits?

I've just learned that digital CMOS inverters can be configured to perform analog functions (most notably oscillators and amplifiers). However, many of the examples tend to favor old CD4000-series ...
12
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6answers
35k views

How are positive and negative feedback of opamps so different? How to analyse a circuit where both are present?

In an opamp, feedback on the positive input places it in saturation mode and the output is of the same sign as V+ - V-; feedback on the negative input places it in "regulator mode" and ideally Vout is ...
12
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6answers
4k views

Step-by-step explanation of how voltage follower reaches steady state using negative feedback

Just one minute! I am not trying to understand what negative feedback does eventually, or why it should be used. I am trying to understand how the circuit reaches steady state, and how, step by step, ...
12
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6answers
2k views

Why do we get only one frequency as output in oscillators?

I am just into oscillators where I learned \$AB=1\$ for sustaining oscillations in positive feedback. Since \$A\$ and \$B\$ are both frequency-dependent, \$AB=1\$ is true only for a particular ...
10
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11answers
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Virtual Ground Paradox?

I'm unable to come to terms with something I think is a paradoxical situation relating to the virtual ground of an Operational Amplifier.Please pardon me if this is a really stupid question. When the ...
9
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1answer
9k views

How negative feedback increases the bandwidth

Negative voltage feedback affect all the frequencies equally. Hence by simple observation it is clear that, negative feedback doesn't affect the bandwidth of the circuit. But in many books, it is ...
9
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3answers
3k views

Op amp analysis: when are the “negative feedback rules” applicable?

When we build op amp circuits that use negative feedback, like so: ... we can analyze the circuit very easily, by assuming that $$v^- = v^+$$ due to negative feedback (when also assuming the op amp ...
9
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1answer
636 views

Diode in positive feedback?

I have a colleague who is off work for the next 2 weeks and has asked me to finish off one of his schematic designs. I have a list of operations it needs to be able to perform, seems simple enough. ...
9
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2answers
468 views

Chopper Amplifier feedback loop stability

I have a question regarding the stability of a circuit I plan on building. This is a voltage-controlled current source, with the IN-AMP being used to sense the current through Rsns and provide ...
8
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2answers
3k views

Why do we compensate PWM controllers?

I have been studying switch mode power supplies for a while. I have understood the working principle of several common topologies But there is a detail in the control circuit structures which I still ...
8
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6answers
24k views

How to identify the feedback topologies?

It is extremely important to identify the feedback topology first before starting analysis. However, I find it difficult and cannot get it right. Is there an accurate yet easy way for me to identify ...
7
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3answers
481 views

Block diagrams can be different and still works for the same system?

I am an telecommunications engineer who is working now as a programmer. At university we took a class on digital filters, Laplace transformation and working in the frequency and time domain. At the ...
7
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5answers
757 views

Is transistor in cutoff?

In the following circuit, consider the op-amp to be ideal (gain = ∞ , input resistance = ∞) and transistor beta is 20 A/A Initially I was using virtual ground concept here but in the given answer ...
7
votes
1answer
314 views

Have current feedback op amps gone out of fashion?

Or have they perhaps always been a bit of a niche product? I've been working with op amp circuits on nights and weekends for about a year now. It was a fair amount of time before I encountered a ...
7
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1answer
865 views

Bias analysis in a dual BJT circuit with feedback

I am building an Wooly Mammoth audio circuit, and I am really interested in finding how and why it works. You can check out and simulate the whole thing here. I must say I haven't been able to get ...
7
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1answer
228 views

Should I be concerned when loop gain rises back above 0dB on simulation?

Interim update Okay, I just realized I screwed up on the simulation. When I added \$C_{bp}\$ I upset the "break" in the feedback loop necessary to get the loop gain. The left-side lead has to go to ...
7
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1answer
663 views

Step-down with external mosfet - why heavy current surges?

I built this circuit, which is supposed to deliver currents up to 20 amps (when everything else works...). The purpose is (eventually) to build a constant current source (as opposed to a constant ...
7
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2answers
1k views

Would this JFET+op-amp circuit work?

This is Figure 9-3(b) from Motchenbacher - Low-Noise Electronic System Design, so I would expect it to work, but I don't understand it: The addition of a discrete common-source stage at the input ...
6
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3answers
3k views

What is causing the oscillation in the feedback loop of an SMPS?

I am trying to make a switching constant current regulator for a Ni-MH charger. It accepts a current set input from the microcontroller using a PWM DAC. I basically want to source 2000mA and 200mA for ...
6
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3answers
6k views

I don't understand how the resistor between GND and the source in a common-source amplifier stabilizes it

See this circuit: Applying KVL around the gate-source-GND-gate loop, I get Id = (Vg - Vgs)/Rs. I know the typical argument begins with, if Id increases then Vs increases, but I am not sure of how it ...
6
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2answers
512 views

What is this RC local feedback configuration?

I'm studying power converter control loops using Christophe Basso's book Designing Control Loops for Linear and Switching Power Supplies. A very common pattern in the compensator circuits is a ...
6
votes
3answers
2k views

Can someone explain this tube amplifier circuit for me?

I have a tube radio that has really low output (I mean the sound is barely audible at full volume). Probably some capacitor in the LF amp failed. However, after I downloaded the schematic diagram, I ...
6
votes
2answers
1k views

Calculating values for compensation network (boost converter RC+C network)

I'm trying to build a boost converter with current mode PWM control, but I'm stuck at selecting capacitor and resistor values for compensation. I'm using a LT1619 as the controller IC: simulate this ...
6
votes
3answers
631 views

Pass device selection for bench DC power supply

I'm designing a DC Bench Power Supply. This one will be 0-30V, 0-300mA, but I expect I'll try some heftier models once I've gotten this one worked out. I'm following the general design procedure ...
6
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4answers
1k views

Push-pull power amplifier on complementary darlingtons

I decided to make a power DC-amplifier. I used complementary darlington pair TIP142 and TIP147. Also TL082 used for negative feedback. The schematic is as below: I gave 100 kHz sine signal and under ...
5
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5answers
1k views

Reason for small-valued feedback resistors in low noise Op Amp

I am wondering why the feedback resistor values specified in the datasheet of the AD797 are so small. My understanding is that the low feedback resistances keep the noise small, but isn't it also not ...
5
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4answers
14k views

Bandwidth of inverting and non-inverting op-amps

If the same op-amp is used in both inverting and non-inverting modes (with same closed loop gain using appropriate resistors), will the closed loop bandwidth of the op-amp in both cases be the same? ...
5
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3answers
3k views

Stabilizing opamps - am I doing it right?

I'm designing a high current linear regulator and learning about real opamps as opposed to ideal ones described in tutorials all over the net as I go. My initial voltage regulation design was stable ...
5
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2answers
874 views

Negative feedback output question

What is the value of Vout if we know the value of Vin in this case? It is a buffer amplifier, but it has two extra resistors so I think that the gain is smaller than 1, but how small? (Vin-V3)/R1=0 &...
5
votes
2answers
623 views

Controlling MOSFET's resistance using negative feedback

I wanted to use LM317 in a power supply project but wanted to control voltage digitally or with voltage. One possible way was to change one of the voltage divider resistors on the schematic below, ...
5
votes
2answers
173 views

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 ...
5
votes
3answers
392 views

Understanding the negative feedback and stablizing of a current limiter

Here's a current limiter circuit that uses the negative feedback to limit the current to a certain point. simulate this circuit – Schematic created using CircuitLab I have not studied ...
5
votes
1answer
392 views

Effect of zeroes in system stability

Suppose we have two third order systems (a and b), with three poles and two zeroes,poles for both the systems are at the same positions in frequency domain but these systems differ in the placement of ...
5
votes
2answers
1k views

Adjustable 0-5A voltage controlled current source

I would like to build a high side current source that can source 0-5A with a 1mA resolution. The voltage source feeding into this current source is a variable 0-50V that is always ~3V above the ...
4
votes
3answers
2k views

Why 60% overshoot with 55° phase margin?

UPDATE: Based on the answers I received, I got this working beautifully. There's a full outcome report in one of the answers below. I'm working through the design of an electronic load, basically a ...
4
votes
2answers
2k views

Non-inverting op-amp configuration with capacitor

I don't really get what is the purpose of the capacitor C1 which is connected in parallel with the feedback resistor. After my knowledge, if we modify the input signal frequency the over all gain will ...
4
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4answers
711 views

Switch-mode power supply output voltage manipulation via feedback node

I'm trying to design a cheap way to pull current from the feedback node of a SMPS in order to dynamically control the output voltage, as explained in this application note:http://www.ti.com/lit/an/...
4
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2answers
1k views

Op-amp dominant pole bandwidth

I have been playing around with an op-amp design using TINA from Texas Instruments. I have selected the LM301A. TINA says that this op-amp has a dominant pole at 5 Hz. I have put the circuit ...
4
votes
3answers
909 views

How do I implement a feedback to keep the DC gain at zero for this conceptual passive filter?

Regarding an example passive LC ladder filter, when I sweep the source resistance, Rs, the filter characteristics changes as follows as expected: On the other hand, if I sweep the load resistance, ...
4
votes
2answers
3k views

Adding negative feedback to a common-emitter transistor amplifier

I am trying to learn how to implement negative feedback to a common-emitter configuration.The goal is to reduce distortion by sacrificing some of the gain. simulate this circuit – Schematic ...
4
votes
2answers
3k views

In an isolated SMPS feedback design, how could optocoupler latency be higher than the switching period?

I often see this in practical SMPS designs. An optocoupler & voltage reference pair is used for informing the controller section about the output voltage level. However, when I read the datasheet ...
4
votes
2answers
775 views

LF356 integrator simulation using LTspice

I used the LTpsice to simulate the feedback performance of a integrator circuit with op amp LF356 which is a JFET input op amp but I go strange results (see the image below). The model of the LF356 ...
4
votes
3answers
7k views

What is a typical example of a combinatorial feedback loop?

By looking at some of the details of the Altera timing analyzer TimeQuest, the concept of a "combinatorial feedback loop" was mentioned. The obvious search is not terribly helpful and there doesn't ...