process for generating transistor curves - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: August 25, 2018, 11:01:25 pm
content: I understand the curves, just not the process of generating them. I know the Ibis set and then Icand Vceare measured. The curve tracer I have seems to set the Iband ramps the Vce. So starting at the bottom of the chart, I can see how the Vceprogressively measures lower as the transistor conducts more with higher Iband Ic. But this is confusing if Vceis being measured. Or do the typical curve tracers use a constant current for Vcand then measure Vce?I'm assume that at the top of the chart, if it was extended with a high enough Ib, the Vcewould approach and maintain a low value, something around .2V to .6V as the transistor saturates. I see this on my tracer.So I guess I just need to understand how a typical curve tracer works. Does it set and ramp Vc(assuming E is grounded this would be equivalent to setting Vce) after setting Ibor use constant current for Vc?ThanksJerry

author: tpowell1830
author_rank: Frequent Contributor
timestamp: August 25, 2018, 11:17:22 pm
content: W2AEW explains this very well in a recent video.Hope this helps...

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: August 26, 2018, 01:51:44 am
content: Quote from: cncjerry on August 25, 2018, 11:01:25 pmTypically a curve tracer generates two out of three quantities and measures the other. For the measurement you are thinking of, both the base current and the collector voltage are generated and the collector current is measured. The base current is generated as a constant, the collector voltage is generated as a ramp and the collector current is measured at many points during that ramp. The collector I-V characteristic is plotted on screen as the ramp progresses.Then the base current is changed to a new value and the process is repeated.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 26, 2018, 01:59:14 pm
content: If you download the manual for the old 575 curve tracer, it covers circuitry and operation very well, including a lot of measurements that aren't as common.

author: MarkF
author_rank: Super Contributor
timestamp: August 26, 2018, 05:03:00 pm
content: Alan made several videos on curve tracing that might help you.Here is one using a stair step generator that I think will give you better insight as to what is going on:

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2018, 05:07:25 pm
content: what is a advanced AC curve tracer called? Do they just fall under SMU's ?

author: Chris56000
author_rank: Frequent Contributor
timestamp: August 26, 2018, 06:35:33 pm
content: Hi!As well as the superb Tek 575 & 576 equipment, there's also "Paul's Electronic D.I.Y. Blog" that also covered the construction of a simple econimical Curve Tracer based on an ELV design, that you might like to study as well!Chris Williams

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2018, 07:06:13 pm
content: It is usually considered easier to sweep the collector voltage with a voltage source and series resistance but the same result could be produced by sweeping the collector current with a current source and shunt resistance.

author: cncjerry
author_rank: Unknown
timestamp: August 26, 2018, 09:37:22 pm
content: Thanks for all the input. Very helpful. I watched all the videos and yes, they make sense. My tracer, though limited in voltage to +/ 18V, can do a number of test signals including sine, triangle/ramp, stepped, positive sine, negative sine and others if I care to modify the DSP.So sweeping the Vcwould make sense in that as the Ibincreases the maximum Vcewould drop. So this all gives me a better understanding. My tracer ramps the Vcand measures both V and I.Thinking back, I'm glad I built this curve tracer. The guy that designed it put over 5K lines of code into the PC software and it is very complete. I then programmed an STM32F7 instead of the 8051 he used. I was in the process of converting and using the STM32 board when I ran out of interest two years ago. I think if someone picked this project up and used good 24bit ADC and DACs as well as a STM32F7 board, it would be quite a tracer for voltages of +/-18 as used in most analog projects (and others).thanks again,Jerry