BJT with open emitter ? - Page 1
Forum section: electronics / beginners

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 23, 2020, 02:29:37 am
content: What are the advantages of using the B-C junction this way?(Oscilloscope frontend amp. Q318, Q319)How about this example with open collectors and B-E junctions back to back, on the same substrate though (I think)?(Keithley 616 electrometer.)

author: james_s
author_rank: Super Contributor
timestamp: January 23, 2020, 02:46:16 am
content: I've seen that done a few times, my assumption was that it was to get diodes with thermal characteristics that match the transistors in the circuit. I'm sure one of our resident analog experts can correct me or elaborate though.

author: MasterT
author_rank: Frequent Contributor
timestamp: January 23, 2020, 03:57:25 am
content: Generally, B-E junction has higher capacitance. Both C-B & B-E works as protection/ overvoltage, difference in frequency on the line. DC and low end ether option is o'k, but for high speed only C-B.

author: wraper
author_rank: Unknown
timestamp: January 23, 2020, 04:09:05 am
content: In first case seem to be used simply as high frequency diode. In second case as bidirectional Zener diode.

author: hsn93
author_rank: Regular Contributor
timestamp: January 23, 2020, 05:59:39 am
content: Quote from: wraper on January 23, 2020, 04:09:05 amis it same if they shorted C-B / EB instead of keeping it Not connected?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 23, 2020, 02:46:26 pm
content: Quote from: wraper on January 23, 2020, 04:09:05 amIn first case I think I get the idea, BF599 C-B capacitance is about half of E-B capacitance, with a tiny 0.35pF it's hard to beat even with a good diode. I suppose linking the Emitter anywhere would only add (unwanted in this case) capacitance.In second case I'm not convinced... Would that bidirectional "zener" rely on one of the transistors having to avalanche while the other is in forward conduction? If so, is the avalanche voltage well defined?The transistor used in the second schematic is an IT121.

author: wraper
author_rank: Unknown
timestamp: January 23, 2020, 04:31:33 pm
content: Quote from: shakalnokturn on January 23, 2020, 02:46:26 pmIIRC around 6.5V generally.https://en.wikipedia.org/wiki/Zener_diodeQuote

author: David Hess
author_rank: Super Contributor
timestamp: January 24, 2020, 07:24:00 pm
content: Quote from: MasterT on January 23, 2020, 03:57:25 amI think you got that reversed; the base-collection junction is slow and the base-emitter junction is fast. Sometimes the base-collector junction is used because the slow recovery time clamps the RF signal for a short time after the overload is removed like a PIN diode would do. Usually though the base-collector junction is used for higher breakdown voltage.Quote from: james_s on January 23, 2020, 02:46:16 amThat is only very rarely the reason. Usually it is to take advantage of lower leakage than common diodes or the extremely high speed of the base-emitter junction. Even in the past, 12 volt <500ps switching diodes were specialty items because of low demand; most circuit would use a small signal schottky diode instead.TO-225 or TO-220 transistors are sometimes used as thermal sensors instead of axial diodes because the tab makes for convenient mounting.