What operating point to use when a diode has no DC bias ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: January 30, 2025, 01:40:45 am
content: Maybe I'm missing something, but what quiescent current do people use, like for instance for R_D or r_d = nVt/I_d , when there is no DC biasing. So for instance calculating by hand something like a peak detector, w/ no DC, what are you supposed to do, to get decent results on a calculator ? How did people accurately calculate op-amp circuits, or design them by hand, w/ diodes, if their impedance can change by so much?If the PieceWise Linear Model model does ok as a linear- line model, aren't there more detailed models like with a parabola or something closer than a straight line ? I don't mind doing a few more calculations, but I know these problems don't have easy analytic solutions.I haven't seen anyone break-up the analysis of such things into sections for different operating points (besides 1-2 diodes in a PWM model.) Even with common power supply rectifier circuits, what operating point do people use ? The one you'd get from the RMS, or some other statistic ?

author: PGPG
author_rank: Frequent Contributor
timestamp: January 30, 2025, 03:11:19 am
content: Quote from: MathWizard on January 30, 2025, 01:40:45 amPractically there are no something like you describe.You don't want to get the peak from infinite time in past. When amplitude drops down you want also to see it at peak detector so you need a way to discharge capacitor used in peak detector.Such peak detector for example was used in detector radio receiver (the first circuit I have ever done). The amplitude of signal plus modulation depth plus maximum modulation frequency you want to hear gives you minimum discharge speed you have to ensure so from that you can calculate the needed load current (will depend also on capacitor used. From that you can calculate what will be voltage drop between pulses when there is no modulation (based on that current and carrier frequency) so you can estimate how long will be the pulses recharging the capacitor back to peak value. From that you can estimate the current in pulses (much higher than discharge current you decided to use). This pulse current you can use to estimate diode voltage drop.All these are estimations. Current during pulses changes. No calculations in such circuits are precise and need not to be.Circuits you want to be able to calculate exact parameters you design that way that parameters you want depend only on resistors. OpAmp has DC gain of may be 100 000 and if you reduce it with resistors to 10 than you get real something like 9.9999 and you don't care that it is little less that 10 because resistors you used have probably 1% tolerance so give you bigger error than the fact that OpAmp gain is not infinity.Here you can see schematics (electronics speak schematics so understanding text is not needed) of oscilloscope I have made in 1982 when I was student (it was the only way for me to have oscilloscope those time):https://archive.org/details/Re0184OCR/Re_04_84_OCR/page/n7/mode/2upI still have this oscilloscope. I have written this article at the end of 1982 and it was published in April 1984. Nowadays time from writing something to publishing is much shorterYou asked for calculations by hand. I didn't had a computer those time. Everything was calculated by me 'by hand'.Rys 3 and Rys 4 (Fig 3 and Fig 4) are the Y amplifier. Oscilloscope Y amplifier have to have flat gain starting from DC. As it works also for DC, calculations are relatively simple. Instead of thinking of it like of gain you can think of it like of calculating DC polarization. You insert 10mV DC at input (first transistor is the voltage follower, while the other symmetrical input is kept at constant voltage) and see what are the (all the time DC) consequences in following stages.Here stage by stage is designed that way that transistors ensure several times higher gain than stage gives because gain is set by resistors. The deflection plates of the oscilloscope tube should be controlled symmetrically (in my case they needed about 30V difference to shift the point 1cm at screen), therefore from the very input the signal is converted into two symmetrical and all subsequent stages work symmetrically (the voltage drops as much in one place as it increases in the symmetrical point).Understanding this amplifier (down to calculating all voltages) can be in my opinion very good electronic lesson.At Rys 9 you can see how I made so called oscilloscope Z-axis. This circuit transmits DC signal (digital pulses, but you can look at them as just two DC states) from 0V level down to -1100V level to modulate the brightness of the line (you can have something like cursor (break in line)).Nowadays with digital scopes we can have much, much more information shown at screen.

author: jwet
author_rank: Frequent Contributor
timestamp: January 30, 2025, 05:02:35 pm
content: Mathwizard- you're not missing anything really. Back in the day and still today, you would look at a circuit like this in separate "modes". There is a mode where you're sub threshold essentially- <.7 where you could use the diode equation or some piece of it or just call the current zero. There is a light conduction mode where you might use a model like .6v plus 60 mV/decade and heavy conduction mode, etc. Reverse mode might be high value resistor. AC stuff and reverse recovery dynamic stuff get messy. Spice is good for this stuff. LTSpice is especially good for this stuff- they added some tricks that make SMPS analysis more accurate.One exercise that you might find enlightening is to look at a good SPICE model for a diode. (the 1N4148 is a good one). It seems like these days we're standing on the shoulders of giants in all things. This low level stuff gets lost and its easy to lose the overall logic of how things are. Spice analysis is only as good as the underlying models and assumption made in developing them.Its good to stop every now and then for these existential moments but after some brief reflection, get back to work lest we get sucked into our Navel.

author: MathWizard
author_rank: Super Contributor
timestamp: February 04, 2025, 10:24:57 pm
content: I checked a bunch of pdf's, and most of them just use a fixed forward voltage model, for DC and AC. But yeah I never noticed the DC Rseries in the LTSpice diode models, so that was throwing me off too.I have a couple of old CRT scopes, one of which needs a new power supply. So some day I'll have to try designing that.Yeah I've started digging into numerical techniques for solving differential equations, like circuits in the time domain. And looking up SPICE3. And I like using a spreadsheet, so it's not too hard to automate and calculate lots of points and graph it, without feeling like cheating by using the sim every time I want to do time analysis. I also started reading semi-conductor physics again, I want to get into the deeper levels of all that, including my quantum mechanics books. I haven't really looked at them, since I've learned a lot of the required math.What got me going on this tho, was I want to make an Avalanche Pulse generator, probably with some homebrew boost-converter, and voltage doubler's. And I never derived how voltage doubler's really work yet, because I get too hung up on the diode details.