3
\$\begingroup\$

I'm thinking about a strange working mode: the CB part is forward biased and trying to control EC current by CB current. EB is reversed. May it work?

Transistor is BJT.

\$\endgroup\$
7
\$\begingroup\$

Yes, BJTs will work in reverse mode (collector and emitter switched).

Most types will have very low gain (5 or 15 rather than 100 or 300) and the breakdown voltage is generally very low, less than 10V.

A few are made to be symmetrical and don't much care which way you connect them.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Are there any practical applications of reverse mode? \$\endgroup\$ – Peter Mortensen Oct 26 at 13:24
  • \$\begingroup\$ @PeterMortensen Yes. See my comment below. Good as a clean, quiet switch to ground if you can tolerate a slight (mV) offset. \$\endgroup\$ – John Doty Oct 26 at 14:22
  • \$\begingroup\$ @PeterMortensen In (relatively) modern times they're sold as mute switches. Before widespread availability of MOSFET-based analog multiplexers they were used as switches in that service. \$\endgroup\$ – Spehro Pefhany Oct 27 at 1:42
3
\$\begingroup\$

The TTL input multiple-emitter transistor works in such a reverse mode when a logical "1" (+5 V) is applied to the input emitter. Then, in contrast to the usual common-emitter configuration, the emitter is connected to Vcc and the collector to ground.

As far as I can remember, this mode has been also used in the past in transistor switches due to the lower collector-emitter saturation voltage VCEsat (at output logical "0").

| improve this answer | |
\$\endgroup\$
2
\$\begingroup\$

I believe such symmetrical bipolars were once used as analog multiplexors.

===================================

Regarding the use of 2N2484 (?) transistors, what was crucial params of that part number?

| improve this answer | |
\$\endgroup\$
  • 2
    \$\begingroup\$ I used upside-down 2N2484s as video baseline clamps for the CCDs on the ASCA satellite (heasarc.gsfc.nasa.gov/docs/asca/asca_sis.html). Charge injection from logic was better (lower) than I could achieve with FET switches. \$\endgroup\$ – John Doty Oct 26 at 0:30
  • 1
    \$\begingroup\$ To get good VCE(sat), inverted, you want high beta in the normal configuration. You also want decent beta in the inverted configuration. You want low parasitic resistances. 2N2484 specs a high beta in the normal configuration. For the rest, I tested VCE(sat), inverted, at low current. I made the prototype boards with MPSA18s, but there's a prejudice against plastic in space. Old 2N2484s from the 1960's weren't very good, but freshly procured devices (~1990) worked well. Gold-doped general purpose devices, 2N2222 and the like, didn't work so well. \$\endgroup\$ – John Doty Oct 26 at 14:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.