Transistor Vbe variation - Page 1
Forum section: electronics / projects

author: JasperNL
author_rank: Contributor
timestamp: June 20, 2013, 02:24:55 pm
content: Hi All,I have a question about the Base-Emitter voltage of bipolar transistors. I'm using a BC857 PNP transistor to create a simple 100uA current source. I doesn't need to be very accurate but i would like to spec it.The accuracy is mostly determined by the base emitter voltage. And Vbe is depending on Ic, Ib and temperature. These variations I can take into account.The thing that confuses me is that the data sheet always give a typical spec for the above ratings. There are no min/max values. That would mean that when i test 1000 different transistors under the same conditions (Ib, Ic and Temp) that the base emitter voltages are all equal. And that is the part i don't believe. There must be production tolerances. But how far off would they be?Is it a few mV? is it in the order of 10mV or even 100mV?Thanks in advance!Jasper

author: AndyC_772
author_rank: Super Contributor
timestamp: June 20, 2013, 02:36:08 pm
content: It's not a parameter which is well qualified or specified. It'll vary from sample to sample, with current and with temperature. It's not good design practice to develop a circuit which depends on the precise characteristics of a particular transistor - can you find another way to make your current source which is more inherently predictable?

author: JasperNL
author_rank: Contributor
timestamp: June 20, 2013, 02:48:07 pm
content: Thanks for your reply. The accuracy of the current source is not that important. I don't care if it is 5% off or even 10%. And it will meet the 10% easy. But just for fun i would like to spec it for the documentation. But if the production tolerances are not known like you said it is not possible to do the calculations. Or i have to use a rule of thumb figure.

author: croberts
author_rank: Regular Contributor
timestamp: June 20, 2013, 03:27:01 pm
content: Hello JasperNLThe transistor you are using is not very expensive so you could determine what you want to know by setting up a test circuit to measure the parameters you are interested in and log the results.

author: JasperNL
author_rank: Contributor
timestamp: June 20, 2013, 05:18:00 pm
content: This is the actual schematic, very basic. I took a measurement and the current is set to 110,6uA.Calculating back gives us Vbe: 594mV (R = ideal).Let's assume the temperature is constant. Ic and Ib are constant. So theoretical there should be no drift in resistance and base-emitter voltage.Now I'm replacing the resistor with 1000 others from the same manufacturer from different production dates. Theoretical the current will be 110,6uA for all samples.But we all know that isn't the case. My guess is that when you plot the offset voltages (Vbe) in bins it will create of Gaussian curve. That leaves the question what will be the min and max values of that curve?

author: c4757p
author_rank: Super Contributor
timestamp: June 20, 2013, 05:24:51 pm
content: Min and max? RTFM!It's right there. -600mV to -750mV. That's for 2mA, but it's going to follow a similar pattern at 100uA.

author: JasperNL
author_rank: Contributor
timestamp: June 20, 2013, 05:35:35 pm
content: Shame on me!!!It i right there+100/-50mV indeed. Convert that to 100uA --> 530-680mVCalculated accuracy including base emitter tempco:Temp range: 0-60Typ 112uA +10,7%/-13,4%This does not include Rbias drift!!Thanks!