NPN VbrEBO damages in diode configurations. - Page 1
Forum section: electronics / projects

author: MT
author_rank: Super Contributor
timestamp: February 23, 2025, 12:55:22 am
content: Would this configuration cause damage to the diode configured transistors?VbrEBO, 5V BC847BS, Opamp supply +/- 7,5V, a batch of 353's from early 80ies to mid 90ies from 4 different manufacturers, all ends up clipping at at +6,96V and -6,32V.

author: floobydust
author_rank: Super Contributor
timestamp: February 23, 2025, 02:19:29 am
content: I see the LF353 loses 1.5-3V on output swing, so powering it from 7.5V I would expect it to clip before the zener-connected transistors conduct. It's a bit risky circuit because it depends on the part's limits.Try power it off a higher voltage to see what is really clipping first.IEBOwould be low, a few hundred uA at most in avalanche.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2025, 03:21:21 am
content: One transistor is clamping the reverse voltage across the other, so the base-emitter junctions never reach breakdown. The transistor connected diodes are being used to create a fast low leakage dead zone in the transfer function.Breakdown of the base-emitter junctions would damage the performance of the transistors when used as transistors by lowering hfe, however it will not increase leakage, so would be irrelevant in this circuit. But this does not matter because one transistor protects the other.Incidentally, I prefer the LF353 from National to its contemporary TL072 from Texas Instruments. Its performance is a little bit better in general, and it has better output drive capability. The contemporary LF356 series is even better, but the low power LF355 has not been produced in some time.

author: Ian.M
author_rank: Super Contributor
timestamp: February 23, 2025, 07:20:19 am
content: Also note that a BC847B is rated for 100mA continuous collector current, and it is permissible to operate it saturated at a forced beta of 10, (i.e. with 10mA base current). Pin 6 of the second OPAMP is a virtual earth so to get 10mA through R2 and the forward biased transistor's B-E junction you'd need 100V across R2, and a bit more (one Vbe drop) from the preceding OPAMP output, far in excess of its max. output voltage.David has already pointed out that the transistors cant be degraded by Zenering their B-E junctions as they clamp each other, and I have just shown that they are well within their ratings, so no, this circuit cant damage its transistors.

author: floobydust
author_rank: Super Contributor
timestamp: February 24, 2025, 05:28:17 am
content: I missed that this is a couple diodes really, back to back.BAV199is superior to transistors (E-B) leakage current, although a bit slower trr0.8-3usec.Does the LF353 have the same the phase-inversion and wonky gain as theTL072? It's just that old datasheets don't reveal that.I expect the phase-reversal which is characteristic of most JFET input stages I believe.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2025, 09:57:11 am
content: Quote from: floobydust on February 24, 2025, 05:28:17 amThe BAV199 is tested to 5 nanoamps. A transistor will be tested to 25 or 50 nanoamps. An FD300 diode is tested to 1 nanoamp and a 1N458A diode is tested to 25 nanoamps. All of these parts will have a typical leakage of picoamps, but if you want a guaranteed leakage of picoamps, you either have to qualify them yourself, or use a 2N4117A JFET which has a tested maximum leakage of 1 picoamp.I like using 2N3904s and have never found one that leaked more than a couple picoamps through either junction. The base-collector junction is 40 volts and slow, like a BAV199, and the base-emitter junction is only 5 volts, but very very fast. Many small signal bipolar transistors achieve this, but avoid the gold doped ones.QuoteYes, the LF353 and related parts all suffer from phase reversal.As an aside, I just found another reason to prefer the LF353 series to the TL072 series; the LF353 has greater output drive. Of course one should not be relying on output drive from a JFET part.