Help! My lab manual makes no sense! - Page 1
Forum section: electronics / beginners

author: Chris42
author_rank: Contributor
timestamp: July 07, 2020, 08:38:16 pm
content: So I have to finish my reports from electronics laboratory (which I have to do in Spice because Covid). I managed to finish first 2 reports ( both about diod circuits).Unfortunately things started to go wrong once I moved to BJTs and Opamps.Here are three big problems that I encountered while writing those reports:1) Some circuits have components that don't seem to be necessary for the proper operation according to both my knowledge and SPICE simulations.2) Some question don't make any sense and attempts to google are futile.3) Tutor apparently lives close to a supermassive black hole and due to time dilation it takes him 3 weeks to respond to any questions by email.(I don't know where is that supermassive black hole and spaceship fuel is expensive so I won't be solving that one.)So in an attempt to make sense of this I decided to as some questions here and hope that someone can explain what is going on.Question 1:"What is the linear stabilization of the point of work in discrete circuit?"This is what manual asks me as a control question about "Common-emitter amplifier with positive feedback". I can't really come up with any answer other than "It is making an operating point more stable using linearcomponents." I'm also not sure what linear stabilization exactly is. Does it have something to do with Linearization? Linear stability? Unfortunately I have no idea. My friends just wrote some garbage about temperature changes buttemperature coefficients of semiconductor devices are usually nonlinear so that is for sure not a correct answer. I tried googling it but all I can find are some papers about robot arm control systems and nothing related to electronics.Question 2:"What is the nonlinear compensation of BJT in the integrated circuits?"Another weird question. I think it is possible to compensate for many things and compensating for frequency probably is different than compensating for temperature changes so I'm not really sure what I'm supposed to answer here.Weird circuit no 1 :First thing that confused me was the name. Why is it called "Common-emitter amplifier with positive feedback for DC"? This circuit does not work at low frequencies due to AC coupling on the input so why arewe talking about some feedback for DC?Second weird thing is voltage divider on the input (Rd1, Rd2). What is the purpose of this voltage divider? I think it is there to attenuate the input signal but that makes no sense because the input signal comes from the signal generator so I can adjust the amplitude without it.Weird circuit no 2:Why are they the 10k resistors (R4, R5) connected to the potentiometer? This is a 50k potentiometer and the worst case load will be 20k (through R1 and R2) so they are not there to limit the current.Weird circuit no 3:Why is there a 10k resistor (R2) on the input of the opamp? The only reason I can think of is that it is there to limit the current at high frequencies . But this circuit is only operating up to 10 MHz so I'm not sure whether this is required (and whether we need such a high resistor value).Weird circuit no 4:This circuit also makes no sense. Now each opamp input has 10k series resistor for some reason. And for some reason resistors around potentiometer are now 1k. What is going on here?Weird circuit no 5:Why is there a resistor on the output of this opamp? I presume to limit the current but the output is connected only to the scope input which won't be drawing much current at the operational frequency of this circuit (1KHz).That makes no sense to me.Please help because I'm not sure anymore whether this is my lack of knowledge or those circuits are just really stupid.

author: magic
author_rank: Super Contributor
timestamp: July 07, 2020, 08:54:26 pm
content: Resistors around a pot: limit adjustment range to, say, 40~60% or whatever. Give full adjustment "resolution" offered by the pot over that chosen range.Resistors on opamp inputs: help cancel offset voltages created by input bias currents (on oldschool bipolar opamps with no bias cancellation). This is particularly the case if one pin is required to have some non-zero resistance anyway for other reasons. They also reduce fault current if the input is overdriven or applied when the chip is powered down.editNote that in your examples impedances seen by the inputs aren't actually balanced very wellResistor on opamp output: limit current, prevent oscillation with capacitive load (but typically 50~100Ω would be used for that).Why somebody includes such components on a silly simulation

author: ataradov
author_rank: Super Contributor
timestamp: July 07, 2020, 08:59:48 pm
content: My assumption that all of that stuff came from lab manuals based on the real hardware with more rugged design for student abuse. So you want to limit all the currents and eliminate a possibility of connecting power supply rails to the inputs as much as possible.The questions make no sense to me. Is it possible that it is some sort of translation error?Question 1 may refer to setting of the DC bias point with R1, R2.Question 2 sounds like a typical question about application of the RE and C3 in this circuit. But it is not typically phrased as "what is".

author: magic
author_rank: Super Contributor
timestamp: July 07, 2020, 09:08:56 pm
content: "Point of work" probably means operating point, so the first Q would be: what's linear stabilization of the operating point of a discrete circuit.The second I can't imagine how it could be mistranslated. I suppose it's just, word for word, what is nonlinear compensation of a BJT in an IC.Not entirely sure how to answer that. Given the discrete vs IC distinction, perhaps the latter refers to the various biasing tricks used in ICs which don't involve simple resistor networks. Current mirrors, thermal compensation for gain, constant gm generators, sky is the limit.The million dollar question in such situations is: what was discussed on the lectures?

author: Chris42
author_rank: Contributor
timestamp: July 07, 2020, 09:31:24 pm
content: QuoteThe problem is that nothing apart from simple resistor networks was discussed.I guess I will just look up those biasing tricks and list some.QuoteThe problem is that they are not really necessary for the operation of those circuits. Those pots are only there to serve as a very simple "voltage source" so we can set the reference voltage for the comparator and adjust the bias added in the summing amplifier. The exact value of that voltage does not really matter.My guess is that those circuits were once a part of some bigger design and someone just copied them because he didn't want to spend much time preparing the manual.Also I checked in the datasheet and input bias voltage is only 10mV. Input bias current is 5nA (part number is TL072, this is a JFET opamp). So those input resistors are probably either from some old textbook or from an old design.

author: magic
author_rank: Super Contributor
timestamp: July 07, 2020, 09:58:16 pm
content: No way that input bias of TL072 is 5nA. Maybe worst case at 125°C.Resistors around pots are helpful as they limit the range to something sane (if you get them right) and increase resolution over the range you care about.Quote from: Chris42 on July 07, 2020, 09:31:24 pmMaybe it was only mentioned that some compensation is required.Look at your common emitter stage: what's the gain of that contraption at 1kHz? What's that Vt rubbish in your answer? What are you telling me, that it varies 80% over -55° to 125°?

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 07, 2020, 10:19:33 pm
content: Most part of answers may be found in literature, for example, "Art of electronics".I only don't get into "positive feedback" in transistor cascade. There is only unintended parasitic temperature positive feedback. All others are negative?

author: magic
author_rank: Super Contributor
timestamp: July 08, 2020, 06:02:04 am
content: Maybe that's what it is. I think if you set the base to 1.2V you will get roughly constant gain over temperature due to thermal drift of emitter bias current.But is it really positive feedback? Only if the increasing current causes the transistor to further heat up and enter thermal runawayeditMaybe that was a translation error and something other than "feedback" was originally meant?

author: gcewing
author_rank: Regular Contributor
timestamp: July 08, 2020, 07:13:34 am
content: Question 1 makes some kind of sense if you replace "positive" with "negative". This kind of amplifier uses negative feedback to stabilise the DC operating point. There is no (deliberate) positive feedback in it anywhere that I can see.Question 2 sounds like it might be talking about the frequency compensation that is built into many IC op amps. This is designed to roll off the gain with increasing frequency to ensure that it can't oscillate. But the question is really too garbled to answer with any confidence.If this is typical of the stuff you're getting from this course, you need to ask for your money back. Whoever wrote the course materials does not seem to be able to communicate technical matters clearly in English.

author: magic
author_rank: Super Contributor
timestamp: July 08, 2020, 07:54:08 am
content: It's almost certainly translation by the OP, so OP could ask money back for his English course but it was most likely at a public school and there is no getting money back from the government

author: Chris42
author_rank: Contributor
timestamp: July 08, 2020, 10:30:37 am
content: Quote from: magic on July 08, 2020, 07:54:08 amMore likely because someone didn't bother to check what is the correct term in english so instead they just translated it using google translate. This is probably what happened because all manuals for this lab are so lazy. All we do is measure a couple of parameters for each circuit, plot some waveforms in time domain and plot gain in frequency domain. There are no design tasks because who would bother to check them when you can just look at the plots and see if the shape is ok. Also no calculations and comparing them with reality. All we do is measure and plot. And they can't even get this right because I got a lower score when I exported simulation results to an external program for more detailed analysis instead of using LTspice directly. This lab is a joke. At least the lecture is quite decent.

author: Chris42
author_rank: Contributor
timestamp: July 08, 2020, 10:32:18 am
content: Anyway thank's for all the answers.They were very helpful in understanding this abomination of a manual. Especially the one about the feedback being negative and not positive. That was quite confusing.I really need to read some decent electronics textbook during the summer because all I learned during this labs was how to automate Spice simulations with python. I wrote a package to hack netlist files so I can manipulate the parameters from the script. Without this I would spend even more time on trying to get all the figures for the reports.That wasn't the goal of this subject but I guess it's better than nothing.

author: magic
author_rank: Super Contributor
timestamp: July 08, 2020, 03:26:07 pm
content: Wait, so they gave you those instructions which are some half-assed English translation? Why?Back in my time (different uni, different course) that would only happen if there were Erasmus students on board, and everybody hated them for that (plus for a host of other things, typically, but I'm digressing).Quote from: Chris42 on July 08, 2020, 10:32:18 amI wouldn't be so fast, maybe they meant not "positive feedback" but "positive thermal drift" of the emitter current, which does indeed compensate for the negative thermal drift of gain, under right conditions.Quote from: Chris42 on July 08, 2020, 10:32:18 amAre you sure you know how DC sweeps and the .step command work?

author: Chris42
author_rank: Contributor
timestamp: July 08, 2020, 09:14:56 pm
content: QuoteYes I can do that too.The whole point of using python is that I can write reports in jupyter notebook. It is an app that allows for creating documents with live code inside. Thanks to that code I can press one button and it runs all required simulations, generates all required plots, puts them in right places and even computes things like bandwidth. This is really convenient because I can then change some circuit parameter and everything will be updated (apart from descriptions).It also enables access to things like optimization algorithms. They are quite useful for problems like "find the value of something for which some circuit parameter is the best'. Instead of manually changing component values I can feed simulation results into the optimization algorithm. I just have to construct some utility function and sometimes tweak some parameters to get a convergence. But most of the time it works quite well.It doesn't do conclusions yet but I'm working on it.Of course I have to write the code first but since tasks don't vary much from report to report I can reuse quite a lot of it. Also playing with simulations and numerical methods == a lot of fun.QuoteI don't think that this is the case here. I double checked with lecture notes and the same circuit was marked as having negative feedback. I guess it is just a stupid mistake in the manual.QuoteIt's called "international mode of study". Basically I have all classes in English and I have to spend at least one semester on an student exchange program. It usually works quite well but occasionally I get things like this.

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2020, 09:35:04 pm
content: Quote from: Chris42 on July 08, 2020, 10:32:18 amNo need to hack netlists. In the sim, simply parametrise component values or whatever else needs changing by your script, and .include a text file containing SPICE .param and/or .step commands generated by your script.

author: IanB
author_rank: Super Contributor
timestamp: July 08, 2020, 09:58:54 pm
content: Quote from: Chris42 on July 07, 2020, 08:38:16 pmRegarding this question, here's a thought that I don't think anyone touched on above.Transistors by themselves are rather non-linear in operation, but when you incorporate them in a circuit you would usually like the response of the circuit to be linear around the operating point (for example in an amplifier). A common way to achieve this is to include some kind of negative feedback in the circuit so that the response of the combined transistor plus feedback arrangement becomes linear where otherwise it wouldn't be.This may shed some light on the background to the questions?

author: MarkF
author_rank: Super Contributor
timestamp: July 08, 2020, 11:10:53 pm
content: Circuit #1:- Common Emitter Amplifier:- Detailed Walkthrough:(Pay particular attention starting at time 17:20)