Consistency of Vbe(on) across a 2N222A2 Production Lot - Page 1
Forum section: electronics / projects

author: beaker353
author_rank: Unknown
timestamp: August 25, 2014, 03:47:44 am
content: I'm in the process of building a rather simple current limiting circuit to allow a modern switching power supply to replace an linear one in order to keep some rather old equipment breathing. My prototype seems to work quite well and is stable. Part of my design relies on the Vbe(on) voltage on the rather humble 2N2222A (im using an On Semi). I've carefully tweaked some low value resistors so I hit that Vbe(on) voltage at just the right point. When I get the parts in to build the other 12 units, am I likely to find a decent variance in the Vbe(on) voltage between individual transistors, or is this one of the specs of a transistor production lot that should be very consistent? (These type of topics are ones I realty wished my Solid State II teacher covered...) I'm operating these transistors completely with DC, so any type of high frequency effects can be written off. Thanks!-EM

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2014, 06:21:09 am
content: Measure a bunch and find out. I usually find a variation of at least 100 millivolts when grading transistors.

author: EEVblog
author_rank: Unknown
timestamp: August 25, 2014, 07:38:49 am
content: You are probably hard pressed to get data on this, best to measure yourself on actual parts.Be careful there will be about a 10mV/C temperature dependence and handling may warm them up.

author: mij59
author_rank: Frequent Contributor
timestamp: August 25, 2014, 08:31:27 am
content: Quote from: EEVblog on August 25, 2014, 07:38:49 amAre referring to Vbe temperature dependence ?

author: dannyf
author_rank: Super Contributor
timestamp: August 25, 2014, 10:11:42 am
content: Try to see if you can use a voltage reference instead, like tl431.

author: Andreas
author_rank: Super Contributor
timestamp: August 25, 2014, 12:46:41 pm
content: Quote from: EEVblog on August 25, 2014, 07:38:49 amIt will be more or less -2mV/K which gives about 200mV difference between "cold" and "hot".With best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2014, 03:12:33 pm
content: For fault current limiting, the Vbe voltage spread between units and over temperature is not usually a problem. If it is, then there are better circuits for handling it which also reduce the current shunt sense voltage.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2014, 05:23:34 pm
content: Quote from: AcHmed99 on August 25, 2014, 04:29:24 pmIntegrated circuits have other easy options if precision current limiting is needed because their Vbe voltages are inherently matched between devices on the same chip. Where a Vbe and shunt resistor are used which is common in small signal devices like most operational amplifiers, the current limit protection is expect to vary over a 2:1 range anyway without problems and the output devices are derated accordingly. This is especially the case since the absolute value of the current sense resistor probably varies significantly as well which is not a problem in a discrete design.