32
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Side effects of using large resistances
There are many drawbacks to both low and high values alike.
The ideal values will fall in between very large and very small for most applications.
A larger resistor of same type will, for example, ...
- 18.5k
26
votes
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How negative feedback increases the bandwidth
You have to realize what Bandwidth actually means.
Bandwidth is the frequency at which the gain starts to drop when frequency increases. So if lowering the gain (using feedback) moves that point (...
- 81.4k
18
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Side effects of using large resistances
In addition to the issues that @Asmyldof mentions, when using high resistances in the megaohms (and especially at 10M and more) environmental contamination such as dust, skin oils, soldering flux ...
- 6,195
18
votes
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How does the compensation resistor in an inverting amplifier compensate for the input bias current?
Sample Bipolar Schematic and Behavioral Description
Let's look at the LM324. It's a bipolar opamp and it is also a lot easier to follow that some. But it is still fairly representative of the basic ...
- 78.7k
17
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Virtual Ground Paradox?
It's #2. For a "perfect" theoretical opamp, the open-loop gain is infinite, and this makes the difference at the inputs zero. When introducing opamp circuits, or when working out how things are ...
- 12.3k
14
votes
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Reason for small-valued feedback resistors in low noise Op Amp
This op-amp boasts input noise of 0.9 nV/√Hz, which is roughly equal to the Johnson noise of a 50 Ω resistor. If you aren't putting resistors smaller than that around it, you're wasting some ...
- 3,762
13
votes
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Why do we get only one frequency as output in oscillators?
Why do we get only one frequency as output in oscillators?
Oscillators work at one frequency by ensuring two things: -
The signal fed back to sustain oscillations is exactly in phase with the signal ...
- 473k
13
votes
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Where do zeroes come from in a circuit?
Q1a
There is a direct connection from input to output without passing through the dynamics of the system. Output is directly influenced by the input.
Q1b
It is a feedback path as as well as feed-...
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How does a unity gain buffer work?
You've applied the non-idealities of the op amp inconsistently, and have reached a false contradiction. In particular, you've assumed that the op amp has infinite open-loop gain when you concluded ...
- 20.6k
13
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Why are these voltage regulators seemingly using positive feedback?
You must consider the whole feedback, not just the error amplifier but also the transistor. It is the transistor that adds a 180° shift, which changes positive feedback into negative.
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12
votes
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Which CMOS logic families can safely be used to construct linear circuits?
All logic families like to use buffered inverters, because those are more reliable and use less power in digital applications. However, unbuffered inverters are useful to build crystal oscillators, so ...
- 20.1k
12
votes
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Sziklai pair feedback loop
I have illustrated much of what was said in the answers and comments above and expanded it by some of my thoughts.
1. Compound transistor. The n-p-n and p-n-p transistor connected in this way form a ...
- 19.5k
11
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Why would you use a feedback resistor on a logic buffer?
The resistor adds additional hysteresis beyond what the chip provides. With the nature of what the internet is these days quite possibly you may have found a circuit idea that was designed before the ...
- 57.9k
11
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Op Amp Feedback Resistors
To a first-order approximation, it's a matter of how much current the op-amp can provide (sets a lower bound on resistance), how much power you can stand to consume (sets a lower bound) how much error ...
- 38.5k
10
votes
How does this Push-Pull amplifier work?
Let as exam the BJT push-pull amplifier along first.
Any BJT's need at least 0.5V to 0.7V of forward base-emitter bias voltage before they will go into conduction. In push-pull amplifier both of the ...
- 15.7k
10
votes
Virtual Ground Paradox?
Let's just do the WHOLE shebang, start to finish, instead of doing this piecemeal. Let's start with the definition for the op amp.
$$ V_{out}= A_{OL}(V_+ - V_-)$$
As has been pointed out, \$A_{OL}\$...
- 30.6k
10
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How do I explain this regulation action in terms of control theory?
There is no closed loop feedback system evident
That's because you didn't spot it.
It's the base-emitter region of the 2N3904 (Q2) that acts like a comparator at around 0.5 volts to 0.7 volts (...
- 473k
9
votes
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Input stage of a current mirror circuit
There are many different BJT models, with varying degrees of usefulness in varying circumstances. (See SIDEBAR at bottom.) I'm not going to delve into any of that as it's not necessary in this case. A ...
- 78.7k
9
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Virtual Ground Paradox?
The problem is that you mix-up two different models of the op-amp.
A real, but somewhat idealized op-amp, is a differential amplifier whose output depends on the inputs as follows (neglecting ...
9
votes
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Voltage regulator frequency range
Neither. You should use a potential divider:
simulate this circuit – Schematic created using CircuitLab
edit: Just to add, this will give a 0-4V wave at 24Vin, and a 0-2V wave at 12Vin. This ...
- 5,611
9
votes
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Non-inverting op-amp configuration with capacitor
A lot of times I hear that it's useful for stability but I don't get
why and how to calculate it's value.
Consider that the non-inverting pin might have a parasitic capacitance of maybe 4 pF. That'...
- 473k
9
votes
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Changing output voltage of a buck converter by electronically swapping the feedback resistors
Two things. First, use two FETs and two wires to control them instead of 3, and second, use a resistor-ladder approach.
You have three low-side resistors, R2a,b,c. Wire them in series, then use the ...
- 58.4k
9
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What does "Brushless DC is an AC induction motor with built-in speed feedback" mean?
It means the poster doesn't know the difference between an induction machine and a synchronous machine, nor do they know the range of DC brushless motors out there.
Having waxed cynical: the term "...
- 47.1k
9
votes
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DC Gain of the System
What you need to stabilize your converter is the control-to-output transfer function: if a stimulus is applied to the duty ratio input, how does it propagate through the converter and create a ...
- 23.5k
9
votes
How does the compensation resistor in an inverting amplifier compensate for the input bias current?
In short, every op-amp has a differential amplifier at the input.
Thus, every transistor needs a "base" current (input bias current) to flow to work as the amplifier.
So for example in the ...
- 15.7k
9
votes
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Voltage regulation feedback loops
If we ignore the PWM and switching elements for the moment, here's what the system looks like:
simulate this circuit – Schematic created using CircuitLab
Your implication that the TL431 ...
- 47.2k
8
votes
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Block diagrams can be different and still works for the same system?
Your answer is incorrect in the sense that you produce an error signal from signals that have different physical dimensions. In your system, you subtract temperature units from volts, which is not a ...
- 2,674
8
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Non-inverting op-amp configuration with capacitor
Just to add a bit to Andy's answer (a mathematical approach). In order to understand why, you need to be familiar with the frequency response. An opamp has input and stray capacitance on the inputs, ...
- 5,784
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