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I'm trying to know What's the usage of this type of biasing BJTs. this is the main schematic. I'm talking about these type of biasing:

figure1

Another question to me is this type of biasing for potentiometer:

figure2

If the desginer would use pot as a variable resistor(not voltage devider) he/she could use such biasing:

figure3

Am I right? then what's the point about this type of biasing?

Edit: Is there any difference between this type of biasing and in this question?

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    \$\begingroup\$ The potentiometer is connected the way it is to prevent it from ever having a resistance greater then the base resistance of the device. Basically, the wiper can fail, and if you don't tie the wiper to one end of the resistive element, the failure of the wiper would result in an open circuit, which could be much less desirable then just the maximum potentiometer resistance. \$\endgroup\$ – Connor Wolf Jul 26 '15 at 17:52
  • \$\begingroup\$ As well as Connor's point about failure, putting the (otherwise unused) portion of the element in parallel with the wiper resistance can somewhat reduce the CRV (contact resistance variation). \$\endgroup\$ – Spehro Pefhany Jul 26 '15 at 19:06
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They're not being biased into active operation. The rheostat is for calibration.

The transistors are connected as low-leakage zener diodes so they will clamp errant input voltages that may occur when the user does something idiotic.

Edit: with regard to your added question, yes there is a difference. The back-to-back series transistors act as back-to-back zeners so they will clamp the voltage at maybe +7V and -7V. The single transistor will clamp at +6/-0.7 or -6/+0.7 depending on the type of transistor and how it is wired.

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  • \$\begingroup\$ Thanks. What do you mean "The rheostat is for calibration"? "clamp errant"? and you didn't say anything about that pot. \$\endgroup\$ – Roh Jul 26 '15 at 17:39
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    \$\begingroup\$ The potentiometer is connected as a rheostat. Same thing. The transistor is effectively not in the circuit for normal operation, only required during episodes of operator error. \$\endgroup\$ – Spehro Pefhany Jul 26 '15 at 17:43
  • \$\begingroup\$ Thanks and would you please answer to my new question that I added as 'edit'. \$\endgroup\$ – Roh Jul 26 '15 at 17:47
  • \$\begingroup\$ If under some condition the gnd potential is higher than the emitter, whixh may happen sometimes, the bjt should enable pretty high current, like normally shottky would. \$\endgroup\$ – Gregory Kornblum Jul 26 '15 at 19:44

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