30
votes
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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, ...
- 18.1k
26
votes
Accepted
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 (...
- 79k
18
votes
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 ...
- 6,057
17
votes
Accepted
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 ...
- 70.9k
16
votes
Accepted
Why would you stack a resistor and capacitor on top of each other?
Forget the 40kHz- this kind circuit really likes to oscillate at very high frequency- the feedback resistor is almost open (1M) at high frequencies in comparison to a few pF and the amplifier has a ...
- 337k
16
votes
Accepted
What is noise gain, really? And how is it determined in the general case?
Okay, after a lot more research, I think I've gotten to the bottom of this. Actually I'm certain it's only approaching the bottom, as I've found this topic area quite deep, but I think I've gotten ...
- 6,852
15
votes
Accepted
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 ...
- 12k
14
votes
Accepted
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 ...
- 3,702
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 ...
- 390k
12
votes
Accepted
Step-by-step explanation of how voltage follower reaches steady state using negative feedback
"Vin is 5V, so Vout should be 50,000V."
Why? The OpAmp amplifies the the difference between the + and - inputs, not just the value on the + input!
OK, you might start with: the output is at 0V, and ...
- 47.9k
12
votes
Negative feedback using op amps?
You are confusing 'inverting' with 'negative feedback'.
Open loop
simulate this circuit – Schematic created using CircuitLab
Figure 1: op-amp with open-loop inverting mode.
In Figure 1 the ...
- 161k
12
votes
Accepted
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 ...
- 14.9k
12
votes
Accepted
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-...
- 3,661
12
votes
Accepted
Can EEPROMs have feedback networks to make state machines?
Put a register in the feedback path and yes.
Common in the early 80s when TTL PROMs were cheaper than PALs (before FPGAs came along)
Use a clocked register rather than a latch, to hold the ROM output. ...
- 66.1k
11
votes
Step-by-step explanation of how voltage follower reaches steady state using negative feedback
Most Basic Interpretation:
Here is my intuitive way to understand a given op amp circuit by personification. Picture a little dude inside the op amp. The little dude has a display that indicates the ...
- 2,383
11
votes
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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 ...
- 55.4k
10
votes
Accepted
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 ...
- 4,826
10
votes
Accepted
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 ...
- 9,468
10
votes
Accepted
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 ...
- 14.5k
9
votes
Accepted
Why do we compensate PWM controllers?
It's fundamental control and feedback theory. Thank you Mr Lyapunov, Mr Black, and Mr Nyquist.
Consider that everyone everywhere always wants the output of their power supply to have just the right ...
- 6,933
9
votes
Feedback loop-gain(beta) derivation for the inverting amplifier?
Definition: The feedback factor beta is defined as the portion of the output voltage that is fed back to the differential opamp input (input source grounded). Therefore, regarding beta there is no ...
- 22.8k
9
votes
Accepted
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 ...
- 70.9k
9
votes
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
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}\$...
- 28.2k
9
votes
Accepted
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'...
- 390k
9
votes
Having hard time to identify feedback types
There is a clear way to differentiate between the two types of feedback (Current/Voltage).
If you short the output (making Vout = 0), and the feedback signal goes to zero, you've got Voltage ...
- 12.5k
9
votes
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 "...
- 37.7k
9
votes
Accepted
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 ...
- 15.2k
8
votes
How do I detect servo stall with an Arduino?
The cheapest dodgiest thing you can do is use a low-side current-sense resistor (0.1ohm will do) and use the Arduino's ADC input with a 1-10K series resistor, over-voltage clamp, and ensure the servo ...
- 6,198
8
votes
Accepted
Why are feedbacks connected to the negative terminal of op-amps? Example given
Let's take into account a "Basic Feedback System" (hereinafter BFS) block diagram first:
simulate this circuit – Schematic created using CircuitLab
We can write:
\$ V_{OUT}=A \cdot (V_{IN}+ \...
- 632
Only top scored, non community-wiki answers of a minimum length are eligible