BJT with open emitter ?

[shakalnokturn]

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.)

[james_s]

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.

[MasterT]

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.

[wraper]

In first case seem to be used simply as high frequency diode. In second case as bidirectional Zener diode.

[hsn93]

In first case seem to be used simply as high frequency diode. In second case as bidirectional Zener diode.

is it same if they shorted C-B / EB instead of keeping it Not connected?

[shakalnokturn]

In first case seem to be used simply as high frequency diode. In second case as bidirectional Zener diode.

In 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.

[wraper]

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?

IIRC around 6.5V generally.
https://en.wikipedia.org/wiki/Zener_diode

Surface Zeners
The emitter-base junction of a bipolar NPN transistor behaves as a Zener diode, with breakdown voltage at about 6.8 V for common bipolar processes and about 10 V for lightly doped base regions in BiCMOS processes. Older processes with poor control of doping characteristics had the variation of Zener voltage up to ±1 V, newer processes using ion implantation can achieve no more than ±0.25 V. The NPN transistor structure can be employed as a surface Zener diode, with collector and emitter connected together as its cathode and base region as anode. In this approach the base doping profile usually narrows towards the surface, creating a region with intensified electric field where the avalanche breakdown occurs. The hot carriers produced by acceleration in the intense field sometime shoot into the oxide layer above the junction and become trapped there. The accumulation of trapped charges can then cause 'Zener walkout', a corresponding change of the Zener voltage of the junction. The same effect can be achieved by radiation damage.

The emitter-base Zener diodes can handle only smaller currents as the energy is dissipated in the base depletion region which is very small. Higher amount of dissipated energy (higher current for longer time, or a short very high current spike) causes thermal damage to the junction and/or its contacts. Partial damage of the junction can shift its Zener voltage. Total destruction of the Zener junction by overheating it and causing migration of metallization across the junction ("spiking") can be used intentionally as a 'Zener zap' antifuse.[6]

[David Hess]

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.

I 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.

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.

That 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.