17
votes
mV variations even with a simple voltage divider
There are plenty of possible sources for noise, and all of them are pretty much to be expected. Real circuits have to deal with reality, and reality has a lot of things that simulators often ignore.
...
- 73.6k
10
votes
Charge, Voltage, or Current? What can be measured with the highest SNR?
In your example, there is absolutely a correct answer, and that answer is current. Current provides the highest possible signal-to-noise ratio.
Here is why: Johnson–Nyquist noise, commonly referred ...
- 29.3k
8
votes
Why is the frequency response of an LED important?
how this bandwidth is related to communication side?
Since this deals with optical telecommunication, I think it is relatively easy to see why the frequency response would be important. If you have ...
- 5,257
7
votes
MOSFET amplifier mid-point bias
The properties of MOSFETs vary a lot making this setup very unpredictable meaning, the DC level at the output will vary over almost everything including temperature. So even if you would know the Vgs ...
- 81.4k
7
votes
How is the AC equivalent drawn?
T- and \$\pi\$- Models
Earlier, in a comment I've since deleted since I'm now writing this, I suggested to you to consider applying the T-model to \$Q_1\$ and the \$\pi\$-model to \$Q_2\$. Figure 15-8(...
- 78.7k
7
votes
Accepted
Flyback converter small-signal model
The answer given by Antonio51 is good and uses the auto-toggling CCM-DCM model built on the PWM switch in 2005. This is great to determine the small-signal response of many switching circuits and not ...
- 23.5k
7
votes
Why is the frequency response of an LED important?
Bandwidth of something is generally defined to be the frequency range where the AC waveform amplitude drops by 3dB. That's the half power point.
The larger bandwidth you have the faster you can turn ...
- 172k
6
votes
What is a small signal? What is a small signal equivalent circuit?
'Small signal' is where variations in voltage and current are so small that we don't explore the non-linear parts of the device V/I curve.
At any a given bias point, we forget about any curvature and ...
- 174k
6
votes
Accepted
Cascaded BJT Amplifier
That resistor is to set the AC gain of the second stage.
The AC gain of a common emitter amplifier is roughly the impedance in the collector circuit divided by the impedance in the emitter circuit. ...
- 27.9k
5
votes
Accepted
AC Analysis of common emitter circuit, why can I ignore resistance R1 but not Rc?
A resistor that comes in parallel with a voltage source has effectively nothing to do with the rest of the circuit and can safely be ignored. See the following circuits that are equivalent:
In your ...
- 1,804
5
votes
Accepted
Small signal Forward converter
We can certainly compare the ac response of the forward converter modeled with the CM PWM switch model by running a SIMPLIS simulation. For that purpose, one of the 60+ free templates I posted can be ...
- 23.5k
5
votes
Re model for Collector feedback bias with emitter resistance
\$r_e\$ is irrelevant to gain here with Re=1k.
The base resistor values are wrong.
Base to ground must be about 40x Re =40k
Rcb feedback must be much greater than 40k to bias collector near Vcc/2
...
- 148k
5
votes
Accepted
How to calculate small signal gain with LTspice?
You can use an AC analysis.
Edit your signal voltage source, under 'small signal analysis' set AC Amplitude to 1.
Edit your simulation command. Select AC Analysis, Octave, 200 points per octave, start ...
- 27.9k
5
votes
Why are capacitors treated as shorts in small signal models?
When you treat them as short circuits you are making the assumption the have negligible reactance at the frequencies you are interested in. This is usually true for the coupling capacitors in an ...
- 51
5
votes
Why are capacitors treated as shorts in small signal models?
There's a bit of confusion regarding the role of capacitors in small-signal models. So, let's clarify.
DC Bias Point and Linearization: Indeed, the small signal model is a linearized model about the ...
- 1,469
4
votes
Accepted
If a MOSFET has infinite inner resistance (ro), how does current flow through it?
An ideal current source will have an infinite output impedance. You could connect a resistor in parallel with the current source but that resistor would need to have an infinite value R = infinite ...
- 81.4k
4
votes
What is a small signal? What is a small signal equivalent circuit?
This is something that confused me for a long time as well. The more mathematical approach helped me a lot.
To solve a circuit, what you basically do is take all the equations of each component, ...
- 16.6k
4
votes
Proof of linearity of a transistor
You cannot prove the linearity of a transistor, because a transistor is non-linear.
However, it's convenient to make linear models to simplify calculations about its operation.
By definition, those ...
- 174k
4
votes
Accepted
Small signal model of a forward converter operating in current mode control
The forward converter, whether is it a single- or a two-switch version, is a buck-derived topology. It means it can be simulated as a simple buck whose input voltage \$V_{in}\$ is scaled by the ...
- 23.5k
4
votes
Accepted
What is DC small-signal transfer function?
A transfer function is a mathematical relationship linking an injected stimulus \$U \$ with a response \$Y\$ observed on the output of the network under study. The ratio Y/U defines the transfer ...
- 23.5k
4
votes
Re model for Collector feedback bias with emitter resistance
The way to see what the actual gain is, is to embrace something LvW absolutely got correct -- that there is no negative feedback to worry about. That's because the base is being directly driven by an ...
- 78.7k
4
votes
Accepted
Output Resistance BJT regarding Early Effect
For regarding several tips and good insights coming from @jonk and @G36, I find the solution using small signal analysis.
simulate this circuit – Schematic created using CircuitLab
DC bias:
$$...
- 73
4
votes
Accepted
Relation between number of RC networks and phase shift in RC phase shift oscillator
There are many ways to determine the transfer function of a cascaded \$CR\$ networks such as the one you've shown. I am using the fast analytical circuits techniques or FACTs as described in my book ...
- 23.5k
4
votes
Why is the frequency response of an LED important?
Also some people directly applies bitstream to LED while some modulating bitstream with AC and apply. What is the difference?
You don't say what sort of communication you're doing.
If it's across ...
- 20k
4
votes
Accepted
Why are capacitors treated as shorts in small signal models?
The stable DC operating point of a capacitor is characterized by zero current: in that respect it resembles an open circuit. But a momentary current does not change the voltage across the capacitor: ...
- 3,971
4
votes
Superposition principle and Small signal analysis
Quoting from this comment by OP Abdelrahman:
superposition States that I take every source independently and short the other one, if I took the small signal voltage and Short the Dc bias voltage, the ...
- 134k
3
votes
Is the hybrid-pi parameter hie really equivalent to (beta+1)*re?
To be more precise the \$h_{ie}\$ is not strictly equivalent to \$(\beta+1)r_e\$.
\$h_{ie}\$ also known as \$h_{11}\$ comes directly from a two-port network theory. Where they treat the transistor ...
- 15.7k
3
votes
Input resistance confusion
Okay, I think I figured it out.
$$i_x = \frac{v_x}{R_E} - \underbrace{(i_b+i_c)}_{i_e}$$
However \$i_b+i_c = (\beta + 1)i_b\$ and \$i_b = -v_x / r_{\pi}\$
Hence
$$\frac{v_x}{i_x} = \frac{1}{\...
- 171
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