48
votes
Accepted
What's the intuition for reading transistor/resistor circuits?
What's a step by step way of thinking about the flow of electrons in such a circuit? How can we tell it would provide constant current for instance?
The first step is to forget about flow of ...
- 180k
23
votes
None-inverting op-amp must zero output, why not?
There is a g missing in the equations:
\$c = (a-b)g\$
\$c = (a-c)g\$
\$c = ag - cg\$
\$c + cg = ag\$
\$\frac{c}{g} + c = a\$
As g...
- 9,638
14
votes
What's the intuition for reading transistor/resistor circuits?
What's the intuition for reading transistor/resistor circuits?
More precisely speaking, "reading" means "understanding" here. So, the question is, "How do we intuitively ...
- 19.5k
13
votes
Accepted
Non-inverting op-amp
Probably you need to review what negative feedback means to an op-amp, how it makes the op-amp behave, and how it's implemented in your circuit and in a non-inverting setup too. Negative feedback is ...
- 47.2k
13
votes
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.
- 1,029
12
votes
Accepted
Understanding of Dominant Pole Compensation
I add some comments about the phase shift.
The phase shift show here
is the phase shift between the input and the output terminal of an amplifier without any feedback.
Next what you should do is ...
- 15.7k
12
votes
Why is inverting op-amp behaving like a low pass filter?
can anyone explain why this circuit behaves like a low pass filter
despite having no capacitor in the feedback loop?
All op-amp amplifiers will behave like low pass filters due to parasitic ...
- 473k
12
votes
Accepted
Confusion regarding signal amplifier
Preface
In the following, I'll respond in kind to your question. That means I won't bother criticizing the specific values you used or talking about better (in some ways, at least) topologies. I'll ...
- 78.7k
11
votes
How does opamp open-loop gain affect closed-loop gain in a circuit?
From Texas Instruments "Stability Analysis of Voltage-Feedback Op Amps Including Compensation Techniques"
http://www.ti.com/lit/an/sloa020a/sloa020a.pdf
A is the open loop gain (the gain of the ...
- 57.3k
11
votes
None-inverting op-amp must zero output, why not?
in our case: c=(a-c)g -> c= ag-cg -> c(1+g)=a -> c=a/(1+g)
You have made a mistake in your math but your first statement is correct: -
$$c = g(a-c)$$
And here's where you went wrong because ...
- 473k
10
votes
Sensor datasheet shows op-amp circuit with capacitor to ground from inverting input. How does this work?
If you're referring to the 2.2 μF cap, it was probably placed for the purpose of not amplifying a DC signal (including the op-amp's own offset).
Think about it: at DC, that capacitor is an open, ...
- 4,708
10
votes
Accepted
What is the purpose of including (or excluding) the output resistor in the feedback loop of an op-amp buffer?
The second design has a constant output with resistive loading (until the op-amp saturates or current-limits). The first one would output only half the no-load voltage with a 1k ohm load.
There are ...
10
votes
What is the purpose of including (or excluding) the output resistor in the feedback loop of an op-amp buffer?
Both circuits are used when driving unknown loads which may be capacitive, such as long cable.
Op-amps do not tolerate much capacitance directly on their output, or the op-amp may become unstable and ...
- 172k
9
votes
What's the need for a small capacitor in the negative feedback parallel to a feedback resistor?
The input capacitance of this opamp is a few pF which, combined with the rather high value feedback resistor, creates a pole at a frequency that's low enough to add enough phase shift in the feedback ...
- 81.9k
9
votes
Non-inverting op-amp
How do we explain circuits?
To understand a circuit, it is not enough to explain what it does and how it does it; it must also be explained why exactly it does so. Only then can we say that we have ...
- 19.5k
8
votes
Accepted
How does the op-amp stabilize?
Initial answer
We can answer this extremely interesting (and fundamental for circuit theory) question by conducting thought, real, or simulation experiments with some negative feedback amplifying ...
- 19.5k
8
votes
How does opamp open-loop gain affect closed-loop gain in a circuit?
The formula you quote is not the gain of an opamp. It is the gain of a circuit containing an opamp and several resistors. That formula only holds when the open loop gain of the opamp is much larger ...
- 16.4k
8
votes
How does opamp open-loop gain affect closed-loop gain in a circuit?
Let's start with the definition of the op amp:
$$e_{out}= A_{OL}(e_+ -e_-)$$
This is true for every op amp. The device is a differential amplifier, with a very high gain.
Now, given that the ...
- 30.6k
8
votes
Accepted
Irregular common emitter amplifier design
The "simple" two-resistor bias circuit might be used where every component added to the parts list has a cost, or space is strictly limited. Comparing the simple-bias circuit against the 4-...
- 27.6k
8
votes
None-inverting op-amp must zero output, why not?
There is no absolute zero: the amplifier doesn't know what zero is.
The amplifier does only one thing: amplifies the difference between the (+) and (-) inputs by a large gain, and puts it on the ...
8
votes
Unity Gain Amplifier Purpose
Unity gain generally refers to the voltage gain of an amplifier, but there is also current and power gain to consider.
The typical use for a unity gain buffer is to amplify the current while keeping ...
- 27.9k
7
votes
Accepted
Feedback in LM2574
Look at the datasheet, section 7.2, the functional block diagram:
Note how the Feedback pin connects to a resistor voltage divider (R1, R2) which generates an error signal which goes into the Error ...
- 81.4k
7
votes
Why is inverting op-amp behaving like a low pass filter?
basically, every physical system in this world has a finite bandwidth and hence exposes some low-pass behaviour, so in the larger scheme of things, this is to be expected
You use an ancient (LM)741 ...
- 100k
7
votes
I don't understand how this current limiter works
"Don't all voltage regulators work the same?"
No. But many of the three-terminal devices such as 78xx and LM317 type are similar. The circuit you have posted will not work as a current ...
- 180k
7
votes
Irregular common emitter amplifier design
Descriptors
Common Emitter NPN
AC-coupled input and output for RF amplification
self-biased for DC
Negative-feedback high gain (30dB) voltage amplifier
Characteristics
low input impedance to ...
- 148k
7
votes
Why are these voltage regulators seemingly using positive feedback?
There's no positive feedback here. The output device is a PMOS, and thus, it is connected to the (+) input terminal to keep the loop negative.
To understand this better, on your left circuit (a).
...
- 4,708
7
votes
Why are these voltage regulators seemingly using positive feedback?
These two circuits look very similar, but the one on the left inverts, and the one on the right does not invert:
simulate this circuit – Schematic created using CircuitLab
On the left is a &...
- 47.2k
7
votes
Negative feedback amplifier
It seems that if the current \$I_s\$ increases a small amount in magnitude, voltage at the drain of the MOSFET \$Q_1\$ increases. Then the input differential voltage at the Opamp increases since \$V_{...
- 1,377
7
votes
How is this op amp in negative feedback?
If the output of OA1 goes up, the output of OA2 goes down (OA1 and R1/R2 form an inverting amplifier) and a fraction of that voltage goes to the non inverting input of OA1 (tending to counteract the ...
6
votes
What happens to output if OPAMP its supply is tweaked?
For an ideal op amp, which has zero input offsets and infinite gain, nothing happens.
For real op amps, the output will change slightly with changes in the power supply voltages. The ratio between ...
- 61k
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