Calculate Bjt Values - Page 1
Forum section: electronics / projects

author: Gerelectronic
author_rank: Contributor
timestamp: January 20, 2021, 10:14:19 pm
content: Hello everyone.I have a problem, I can't calculate a values of a bjt npn 2N3904,in particular I cannot calculate the value of the output voltage to the collector (which must correspond to 6.061V).Attached I send the photo of the circuit with the transistor.Could you please give me some advice or give me a suitable formula.For any questions about it I am available.Thanks a lot to everyone.

author: Marco
author_rank: Super Contributor
timestamp: January 21, 2021, 12:16:21 am
content: Current through R5 is (3-0.6)/2.7k, voltage over R3 is that current times 10K, voltage at collector is 15-10*(3-0.6)/2.7 ... all approximately, but close nuff.

author: Gerelectronic
author_rank: Contributor
timestamp: January 21, 2021, 07:34:18 am
content: Thank you Marco.But if i know that the Vb (voltage a t the base) is 2.94V, there is another method to calculate Vcollector (6.61V)?.Thank a lot again.

author: Bud
author_rank: Super Contributor
timestamp: January 21, 2021, 07:45:48 am
content: The exact 6.61V will depend on the transistor model used. Marco gave you a basic formula.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2021, 09:00:32 am
content: Certainly *IF* you had good parameters for your transistor you could grind through analyzing the circuit using theEbers-Moll modelequations for your transistor, but an analytic solution of simultanious equations including non-integer variable exponents is a P.I.T.A, and you'd need the Ebers-Moll parameters for your transistor which *WON'T* be in its datasheet. If you are lucky, the manufacturer may offer a SPICE3 BJT model for the transistor that you may be able to manually extract parameters from.Several pages of heavy math later, *IF* you don't make any mistakes you'll probably get a result other than 6.61V, but hopefully very very close to it!Also note that if you built ten of the circuit on breadboards, you'd expect to see several percent variation between them due to component tolerances, even if you hand selected the 2N3904 transistors for best matched gain and Vbe from a much larger batch.TLDR: Marco's simple 'back of an envelope' calculation is good enough to check if your SPICE package is telling you porkies and is certainly good enough for real-world use.

author: ledtester
author_rank: Super Contributor
timestamp: January 21, 2021, 09:08:47 am
content: QuoteWhat Marco is doing is using two rules (approximations):1. voltage at the emitter is 0.6 volts lower than at the base2. current out of the emitter is the same as current into the collectorNow, for the situation where Vb = 2.94V using these rules you should be able to compute the collector voltage.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2021, 12:04:13 pm
content: Quote from: Gerelectronic on January 20, 2021, 10:14:19 pmWhat are you trying to do?What does U1 do?It seems like you're doing this the wrong/long way.Please post the entire schematic. You probably don't need the BJT.

author: Marco
author_rank: Super Contributor
timestamp: January 21, 2021, 03:32:32 pm
content: Quote from: Gerelectronic on January 21, 2021, 07:34:18 amJust change the three in my formula to 2.94 ... but that was a meaningless level of precision, so I rounded it

author: Gerelectronic
author_rank: Contributor
timestamp: January 21, 2021, 04:43:32 pm
content: OK sorry I didn't understand.Thank you very much.I hope that one day I can help you too.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2021, 05:02:59 pm
content: Quote from: Marco on January 21, 2021, 03:32:32 pmYep. Everything past the first decimal place is pureGIGO, and even that first decimal place is dubious. The resistors are given to only two significant figures and the value '2k7' specifically implies E12 or E24 series parts. Although the E series used no longer defines the tolerance it still commonly indicates the accepted tolerances in a design if not otherwise specified so we can assume the resistors are likely to be +/-5% or worse. R2 has insignificant effect due to the action of 'D1' the LM335. Without the A suffix we must assume the LM335 accuracy is only +/-2%, so if the errors stack up unfavorably, worst case the collector voltage could be +/-12% before you've even considered the variation of Vbe with Q1 gain and the accuracy of the nom. 15V rail.I *ASS*U*ME* the designer has allowed for the BJT's decrease of Vbe with temperature of approx -2 mV/°K, which must be combined with the 10 mV/°K characteristic of the LM335 sensor to give a final approx. 8 mV/°K characteristic at the -in of the OPAMP. However that may be over-optimistic as the LM741 OPAMP is apparently open loop, and if so is being used as a comparator, and no-one in their right minds would use a 50 year old type of OPAMP as a comparator with an uncompensated BJT to invert and level shift its input in a precision design!@Marco: It *appears* to be a fairly crappy temperature limit sensing circuit from the dark ages . . .

author: Gerelectronic
author_rank: Contributor
timestamp: January 21, 2021, 05:43:16 pm
content: i know that this circuit is very orrible for everyone, but if we consider all the components ideal, i can calculate V collector more accurate?if this requires a lot of math I'm willing to understand it.Thanks you again.

author: Ian.M
author_rank: Super Contributor
timestamp: January 21, 2021, 06:01:05 pm
content: You can manually calculate Vc to greater precision but its GIGO that will *NEVER* improve accuracy.Anyone with aspirations to be a scientist or engineer *MUST* understand the difference between accuracy and precision. Seehttps://en.wikipedia.org/wiki/False_precisionEither let your SPICE simulator solve it numerically using the Gummel-Poon BJT model, then discard the unjustifiable extra significant digits, or waste a day or two manually grinding through the problem analytically then recognize that the precision of the numbers you are putting into the resulting equations doesn't justify having spent more than five minutes approximating the result.

author: ledtester
author_rank: Super Contributor
timestamp: January 21, 2021, 07:37:52 pm
content: Quote from: Gerelectronic on January 21, 2021, 05:43:16 pmI'm curious why do you want to compute it with more precision. Why isn't this approximation not good enough?No real-life component is ideal. All components are made to tolerances -- 1%, 5%, 10%, so you can't know precisely how a component will perform.

author: Bud
author_rank: Super Contributor
timestamp: January 21, 2021, 08:08:22 pm
content: I guess OP is a student who was given a task by his professor to prove the simulation result.

author: Bud
author_rank: Super Contributor
timestamp: January 21, 2021, 08:12:06 pm
content: @OP: what you need is to buy yourself "The Art of Electronics" book by Horowitz/Hill. You are welcome.