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I often find transistors used as shown below. Note that the BJT to the left is being used in a strange configuration, where the base is being pulled permanently high.

What on earth is the transistor supposed to achieve in this odd position?

odd transistor position

On a related note, I've occasionally seen people use a transistor in place of two diodes. However, my own experiments show that this simply does not work. For example, in the diagram below, replacing the input diodes with a single NPN transistor would significantly change the behaviour of the circuit.

diode logic

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  • \$\begingroup\$ With the input at Vcc, Vbe=0 so the transistor is off. With the input at ground Vbe=0.7, and the transistor is on. For the 2nd circuit, the diodes are replaced by a single multi-emitter transistor. \$\endgroup\$
    – Chu
    Commented Feb 9, 2018 at 8:15
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    \$\begingroup\$ Also see this question: electronics.stackexchange.com/questions/135464/… and this one: electronics.stackexchange.com/questions/304642/… \$\endgroup\$ Commented Feb 9, 2018 at 8:16
  • \$\begingroup\$ @Chu but why not just use the transistor in a normal fashion? \$\endgroup\$ Commented Feb 9, 2018 at 17:21
  • \$\begingroup\$ What's normal ? \$\endgroup\$
    – Chu
    Commented Feb 10, 2018 at 0:38
  • \$\begingroup\$ Normal is connecting the emitter to a power rail and using the base as an input. Obviously. You know, the way they're used 99% of the time? \$\endgroup\$ Commented Feb 10, 2018 at 2:11

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That type of BJT configuration is called "common base".

enter image description here

Either base or emitter can act like an input. I suggest you go and read up on the three types of configuration: -

  • Common emitter
  • Common collector
  • Common base
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  • \$\begingroup\$ Thanks. I've looked up "common base" but still don't understand why anyone would use it in preference to a more normal input-on-base configuration. I also don't understand why it even works at all. Surely current can flow between base and collector no matter the state of the emitter? Just as current can flow between the base and emitter even if the collector is disconnected. \$\endgroup\$ Commented Feb 9, 2018 at 17:23
  • \$\begingroup\$ Common base gives no phase shift (or inversion) between input (emitter) and collector making it suitable for some RF oscillators. Differential amplifiers input transsistors interact using common base and common emitter connections so it is used in virtually every single op-amp on the planet. Also note the negative polarities of the supplies in the picture and that the transistor is a PNP type. \$\endgroup\$
    – Andy aka
    Commented Feb 9, 2018 at 17:29
  • \$\begingroup\$ Maybe try this type of picture using an NPN transistor: pcbheaven.com/wikipages/images/trans_theory_1325789466.png. Maybe try this website for help? \$\endgroup\$
    – Andy aka
    Commented Feb 9, 2018 at 17:31
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    \$\begingroup\$ See also this explanation about TTL \$\endgroup\$
    – Andy aka
    Commented Feb 9, 2018 at 17:54
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    \$\begingroup\$ The DTL discrete designs preceded the integrated logic devices. The semiconductor house guys thought "Why change the topology?" and exploited the dual-diodes inside the integrated transistors. \$\endgroup\$ Commented Feb 11, 2018 at 4:09

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