A fundamental you won't see in the textbooks - Page 1
Forum section: electronics / beginners

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 04:58:37 am
content: Here's a good lesson:The trichotomy of frequencies.For every component in every circuit, there are always three frequency ranges to consider:Low band: "DC", bias/quiescent, LF cutoff, etc.Mid band: "AC", signals, pass band, etc.High band: HF cutoff, etc.If "low" and "mid" are distinguished by theory rather than value, then one must allow that they may overlap. Example: DC coupled amplifiers must consume bias, but the signal can be as slow as any change in bias.One can also create oddball examples where the low band overlaps the high band: magnetic amplifiers are biased by AC (as well as DC!), with the signal bandwidth being limited to a modest fraction of the bias frequency.How could this possibly be useful?It's a shortcut.You can analyze a circuit in many ways. The DC / AC approach (taught in standard EE courses, and used by SPICE's AC Analysis) finds the steady-state DC condition, then superimposes a steady-state AC condition on top of that. The AC analysis might be at a single frequency, or it might derive the whole transfer function.By first ignoring all AC signals, we can jump straight to the lowest level of operation: whether the circuit will do anything at all. This has underlying justification in linear circuit theory, and shows how useful the DC/AC distinction is.A transfer function is a very heavy-handed way to go. Sometimes, a circuit is designed in such a way that the Bode plot can be drawn practically by eye. When this is the case, the value of the third condition, the HF cutoff band, shines. You can approximate the HF cutoff of a circuit by assuming capacitors are nearly shorts, and inductors are nearly opens (or, preferably: making a reasonable judgement on how much of each is the case, for the frequency range needed -- there is no ideal resistance, capacitance or inductance in real life, after all). And, furthermore, you can use this information to set the upper limit of a circuit, which is important for pretty much everything. Amplifiers need a well defined frequency range; radios need sharp cutoffs; digital logic needs clean waveforms; etc.Yes? No? Bollocks? More confusing than the traditional approach? It occurred to me that I think broadly in these terms, but a thought process isn't always the easiest thing to put down in words, accurately and succinctly.Tim

author: rs20
author_rank: Super Contributor
timestamp: August 31, 2017, 05:28:39 am
content: If I'm thinking about an audio amplifier, there are only two concepts in my mind:- DC/Bias- Operation passband (e.g. 20Hz to 20kHz)Of course it's important that the operation above the passband is kept in check somewhat (e.g., wouldn't want an audio amplifier to waste power or worse by amplifying faithfully up to 5 GHz); but I feel like that's normally handled by setting up the passband properly. Put another way, the cutoff/rolloff between your mid and high band could just as easily be conceptualized as the cutoff that terminates the passband; and by speaking in those terms you maybe don't need to talk about the "high band" at all?Another slight objection that I superficially have is that it seems a bit much to say that every single component has three frequencies to consider. I mean, a single component is nothing on its own; two components (RC, RL) typically form a system with only two ranges of interest (high/low); and a trichotomy of frequencies can only appear in a system with > 3 components (not sure how an LC tank fits in, but every LC tank is truly a RLC tank in reality).If I might try to identify the core concept that you're suggesting, it's that you think about frequency ranges, whereas the tradition is to think about frequency cutoffs. The three bands you mentioned aretypicallyseparated by only two cutoffs frequencies; so perhaps it's considered easier to think about those 2 cutoffs frequencies as separate, simple problems.Note that I've been perhaps deliberately dumb in writing my immediate knee-jerk reactions to your message; it's entirely plausible that I'm just regurgitating the same incorrect messages that you are questioning.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: August 31, 2017, 05:49:15 am
content: Quote from: T3sl4co1l on August 31, 2017, 04:58:37 amI remember this approach from one my undergrad. text books by Gray and Searle, in the context of multi-stage amplifier design (using discrete transistors). A simplified analysis of DC biasing; LF cutoff (if any); mid-band; and HF cutoff can be useful for a rough first-pass at the design. IIRC, Gray and Searle also presented some handy rules for calculating the pole values (approximately) that dominate the frequency response.You *will* find it in textbooks :-). I think I still have the book.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 09:17:40 am
content: Quote from: rs20 on August 31, 2017, 05:28:39 amWhich itself is something not given much weight. For a first pass, if it amplifiesat leastthe required bandwidth (or the logic switches at the data rate, or..), you're done, right?But it's only a small length further to go and ask,shouldit go that fast? And often, the answer is no, and often, it turns out there are good reasons underlying that decision!But this is why it's a rule of mine, to always have that upper cutoff in mind.And, well, to be fair I suppose, even if a course does address this, or even if it mentions these things in the same sentences: lazy people are lazy, and will stop working on a thing exactly as soon as it performs its basic functions. So, something something straw men. And then there's the people who won't even stop to look at his effigy.Anyway, for an audio amplifier example, one also needs to understand it's not just the overall response, but any response to any stimulus: if it's out of band, it shouldn't flinch. RF rectification in bipolar circuits is the all-too-familiar example.Shining light on a glass-body diode is a rather extreme example: the effect on the circuit is microamperes of leakage (~DC), while the unintended interference source is 500THz!The audio amp also gives justification for the statement that bandwidth is  about every component -- or, at least, more than just the whole. An op-amp might have a GBW of 10MHz, and might even be used in a low-pass filter for some, like, ~kHz of bandwidth. But then it just so happens, internal components of some op-amp exhibit rectification effects for signals in the 20-1000MHz range. You can be forgiven for not knowing this -- they literally don't put it anywhere in the datasheets! But if you adopt a conservative stance in general, you can avoid troubles like this from the get-go.(Or, if you have the time to play, not having to get the project done ASAP -- intentionally violating this rule, subjecting the DUT to signals outside its intended range, can be very informative indeed.)QuoteYeah, it is kind of weird to call it a band, when it's not a band (a region of relative flatness?) as we'd be more likely to think of it.That said, "stop band" is a thing, where the attenuation is not simply asymptotic, but has peaks and valleys that need to be considered, and controlled. A stop band can fall anywhere, though (say, specifically to stop a harmonic or mixer image), so I kind of don't want to use that term.QuoteNow this is one of the good parts, though -- real componentsdohave limits. A resistor is only a resistor in the low and mid bands, and goes complex in the high band. Capacitors are resistive at DC (leakage), capacitive in the mid band, and resistive and inductive in the high band (ESR + ESL, which themselves define a new mid and high band, for the reason you note..). Inductors have ESR and C. BJTs have low band (DC parameters), mid band (AC parameters) and cutoff band (around fT, due to C and recombination). MOSFETs don't really have cutoff, the AC parameters just kind of keep getting muddier until it's kind of useless.For a lot of parts, you can hand-wave over their limits -- a chip resistor that's good from DC to 5GHz is more than good enough to ignore in an audio amp! It's enough to pause and consider, just once: "is there anything suspicious in here?" A 10Meg feedback resistor, for example, is a rather dubious thing at audio frequencies!QuoteCould be.Cheers,Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 09:19:34 am
content: Quote from: thermistor-guy on August 31, 2017, 05:49:15 amAh, perhaps I should edit the title "you may not see".Was it a good textbook overall, easy to read? May be a convenient reference for anyone looking to read more here.Quote from: blueskull on August 31, 2017, 05:56:13 amSpeaking of oscillators and such: since they are nonlinear, they no longer obey the rules of superposition, and the DC can (and usually does) shift as a result of the AC operation! A good lesson for a beginner adapting to the real, nonlinear world.Tim

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: September 02, 2017, 05:55:31 am
content: Quote from: T3sl4co1l on August 31, 2017, 09:19:34 am"Electronic Principles", by Gray and Searle, 1969. It pre-dates UC Berkeley Spice (1972), so I doubt if it's of interest to anyone here. I keep it for historical reasons.Old technical books like this are worth checking out, if you come across them in a garage sale or similar, and they are on offer for a few bucks.

author: rstofer
author_rank: Super Contributor
timestamp: September 02, 2017, 12:32:56 pm
content: Quote from: thermistor-guy on September 02, 2017, 05:55:31 amI'm not exactly sure of the time frame but I was using the IBM Electronic Circuit Analysis Program (ECAP) on an IBM 1130 around the same time - 1972. I certainly wasn't a bleeding edge user so I have to believe it was released some years earlier.It would do DC, AC, Transient and Sensitivity analysis. Punched cards weren't nearly as productive as a GUI. I rewrote a portion of the output code such that I would capture the frequency and phase information to punched cards which I then processed to make a Bode' plot on the attached plotter. Very cool for homework problems.