Timeline for Why are collector resistors grounded in AC small signal analysis of a differential amplifier?
Current License: CC BY-SA 4.0
13 events
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| Oct 21, 2021 at 20:48 | comment | added | Learner103 | @Null But in small signal analysis there is no DC bias is it not? | |
| Oct 21, 2021 at 20:35 | comment | added | jonk | @Learner103 Spice always (unless you order it otherwise) computes the DC bias point before performing a transient analysis (.TRAN card.) This DC bias point step is where it goes through lots and lots of iterations trying to find the DC bias point given gobs of non-linear device models. This is step 0, so to speak. Getting an initial set of values for all the nodes and capacitors and inductors before starting a run. Once it gets there, all it has to do is find a sufficiently short time step and then simply do linear solutions for each time step. Nice approach. | |
| Oct 21, 2021 at 20:24 | comment | added | Learner103 | @jonk I get your point! Nice!! | |
| Oct 21, 2021 at 20:01 | comment | added | jonk | @Learner103 If you consider a piecewise approach where the length of each piece diminishes to an infinitesimal, then I suppose you are right. But only then. Otherwise it is not a tangent, but a secant line. Large signal (meaning not small signal) analysis is used for finding the DC bias point of a circuit. This gives you the "point" at which, for non-linear device curves, you can find the tangent and use it (for a short range only) as an approximation. But after moving a short distance along that assumed line, you stop, re-evaluate all the DC bias info, and do it again. | |
| Oct 21, 2021 at 19:59 | comment | added | Learner103 | @jonk Sounds like understanding nonlinear curve by Piecewise linear appro. Anyway thanks for your input!! | |
| Oct 21, 2021 at 19:50 | comment | added | jonk | @Learner103 The small signal goes a bit deeper than this discussion suggests (though it is good.) And it is a purely mathematical concept, not an experimental one. It's not that things behave linearly, by experimental result. It's that in mathematics all continuous portions of curves have a tangent line associated with every point. That tangent line is, by definition, linear. And finally, solving sets of non-linear equations is hard. Solving sets of linear equations is well understood and easy. That's why the concept is used. To simplify analysis. Not to meet experimental result. | |
| Oct 21, 2021 at 19:38 | comment | added | Null♦ | @Learner103 You can verify it experimentally. Build the circuit and bias it with the DC sources you used in your small signal analysis, then apply a small signal to the input and you should see an output signal that matches what the small signal model predicted (assuming the small signal model was correct, and that the signal you applied was small enough for the circuit to behave linearly). | |
| Oct 21, 2021 at 19:31 | comment | added | Learner103 | Got it! So small signal analysis is just a theoretical model or can be done experimentally? If it can be done experimentally, then the circuit I have posted will it work and produce any output voltage as per theory? | |
| Oct 21, 2021 at 19:18 | comment | added | Null♦ | @Learner103 "Small signals" are those that are small enough for the circuit in question that the circuit behaves linearly, which we generally want in an amplifier. Large signals determine the bias parameters of the circuit but have non-linear effects. The idea is to analyze the DC operating point of a circuit to determine its small signal parameters, then analyze its (small signal) linear behavior (e.g. signal gain). | |
| Oct 21, 2021 at 19:12 | comment | added | Learner103 | Thanks for your answer@Null! Your answer make me to ask another question, what the significance of small signal analysis? or in other words what we try to understand from doing small signal analysis? | |
| Oct 21, 2021 at 18:49 | history | edited | Null♦ | CC BY-SA 4.0 |
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| Oct 21, 2021 at 18:41 | history | edited | Null♦ | CC BY-SA 4.0 |
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| Oct 21, 2021 at 18:33 | history | answered | Null♦ | CC BY-SA 4.0 |