# Stability of BJT circuit using Re resistor

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....

• "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. Nov 12, 2018 at 13:39
• Why does increasing Ic increase the voltage drop across the emitter?
– user203610
Nov 12, 2018 at 15:13
• 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. Nov 12, 2018 at 15:44
• 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.
– LvW
Nov 12, 2018 at 17:24
• 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. Mar 31, 2022 at 21:41

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.

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.

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.

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.