# Electronic – A question about Transistor in forward-active biasing

biasbjttransistors    I have question about transistor biasing.
Between picture 1 and 2, what's difference, why we add the resistor RE in pic 2 ? what's it effect to circuit ?
Similarity for pics 3 and 4, instead of connecting RB to Vcc, why do they connect RB to the collector of BJT ?
And which is the better way to bias a BJT among 4 pics ?
thanks very much 🙂

As Olin said, circuit shown in #1 and #3 are completely open loop. So the bias stability of the circuit is less and it can even lead to thermal runaway. The bias stability can be improved by including a negative feedback mechanism in these circuits. Circuits #2 and #4 does that.

Negative feedback in circuit #2: Assume that the collector current increases. This leads to increase in voltage across \\$R_E\\$. So the voltage at emitter increases. Since \\$V_{BE}\\$ remains almost constant, the voltage at base also increases. This leads to decrease in base current as an effect the collector current also reduces. So there exists a negative feedback to stabilize the operating point.

\$\$I_C\uparrow I_ER_E\uparrow V_E\uparrow V_B\uparrow I_B\downarrow I_C\downarrow \$\$

Negative feedback in circuit #4: Assume that the collector current increases. This leads to increase in voltage across \\$R_C\\$. So the voltage at collector decreases. This leads to decrease in base current as an effect the collector current also reduces. So here also there is a negative feedback to stabilize the operating point.

\$\$I_C\uparrow (I_C+I_B)R_C\uparrow V_C\downarrow I_B\downarrow I_C\downarrow \$\$

PS: The negative feedback affects the ac signal also which will reduce the gain of the amplifier. To avoid that a bypass capacitor is usually connected in parallel to \\$R_E\\$ in circuit #2.