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This is a post I found on Quora. I don't understand why decreasing Vbe decreases the current. Vbe=Vb-Ve To decrease Vbe you could increase Ve, Vb doesn't necessarily have to change, so neither Ib....

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  • \$\begingroup\$ "Vb doesnt necessarily have to change, so neither Ib" - Wrong conclusion. V[be] is what matters. I[b] depends on V[be], not on V[b] alone. \$\endgroup\$
    – JimmyB
    Nov 12, 2018 at 13:39
  • \$\begingroup\$ Why does increasing Ic increase the voltage drop across the emitter? \$\endgroup\$
    – user203610
    Nov 12, 2018 at 15:13
  • \$\begingroup\$ You are misreading the explanation. It's the transistor that's controlling \$I_C\$, but it's subject to changes in temperature. ). Re-read, carefully. Consider that \$R_E\$ is 'trying' to keep the emitter current constant, and if the emitter current is constant, so is the collector current. \$\endgroup\$
    – TimWescott
    Nov 12, 2018 at 15:44
  • \$\begingroup\$ Michael_a general recommendation: Do not rely too much on Internet contributions.Here you are quoting Quora ...and in other contributions you have found some information in Wikipedia. I know Quora ....a lot of garbage!! Try to find a good textbook - that is the best knowledge source for beginners. Even in this forum, you can find wrong information. \$\endgroup\$
    – LvW
    Nov 12, 2018 at 17:24
  • \$\begingroup\$ Your base voltage is not fixed because the base does not have an important resistor to ground making a voltage divider. Then there is hardly any negative feedback from the emitter resistor. \$\endgroup\$
    – Audioguru
    Mar 31, 2022 at 21:41

3 Answers 3

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To bias a bipolar transistor at 1milliAmp collector (emitter) current, you'll need approximately 0.600 volts Vbe.

For 10X less, at 100 microAmps, expect Vbe to be 0.600 - 0.058, or 0.542 volts.

For 100X less, at 10 microAmps, expect Vbe to be 0.600 - 2*0.058, or 0.600 - 0.116 = 0.484 volts.

For 1,000X less, at 1 microAMps, expect 0.600 -3*0.058 = 0.600 - 0.174 = 0.426 volts across Vbe.

For 10,000X, at 100 nanoAmps, expect 0.600 - 4*0.058

This relation, where each 0.058 volt reduction in Vbe causes a 10:1 reduction in collector (emitter) current, comes from the exponential equation long trusted to describe behavior of a junction.

Vdiode = 2.718...^[(Q * Vdiode)/(K * T * n)]

where "n" is result of the abruptness (or lack of abruptness) of the transition from N doping to P doping in the base-emitter junction (Or the "diode" junction).

Note this equation has a strong factor of Temperature (Kelvin). Classically, at a constant current the Vbe changes by 2 or 2.2 milliVolts per degree C (Kelvin). This behavior is crucial for most voltage references.

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To decrease Vbe you can either lower Vb or increase Ve or both. Decreasing Vbe reduces base current because the base-emitter junction acts as a forward biased diode and lowering the forward voltage across a diode (Vbe) also reduces the current flow through it.

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when someone analyses the temperature, they talk about a small phenomenon and then check that is positive feedback or negative feedback effects.(positive feedback generate instability)

For a simple example,

if you drop a fixed voltage on NTC(a resistor with a negative temperature coefficient) then the temperature is raised( even 0.1 degrees), then a small delta(T) even 0.1 degrees, generate a new R that is smaller than before:

R_new = R_old - NTC_Coef * delta-T

In fixed voltage, decreasing R, make current higher(V_fix/R) and then R*i^2 make the temperature higher than the last step, then makes R smaller -> increase temp -> R is smaller -> increase temp ... .this called positive feedback and not good, and you should avoid instability.

But for your example:

With temperature rise, we know the Beta and Ic will be raised, if Ic is raised even a small value,

V_emitter ~ Re*Ic

will be increased and then

Vcc_fixed = RB * IB + Vbe + Ie * Re ~ constant + Vbe + IC * Re

The equation shows the Vbe should be decreased and this decrease Ib(even pico amp) then Ic decrease,

inc temp -> inc Ic -> inc Beta -> decrease ic -> decrease temp injunction -> ...

This is a stable region for a circuit(all parameters microvolt, 0.1 degree, pico amp, ... )

It is a negative feedback effect, the circuit tries to work on a bias point, even changing the environment temperature.

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