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Emitter Follower

An emitter follower circuit shown in the figure is widely used in AC amplification circuits. The input and output of the emitter follower are the base and the emitter, respectively, while the collector is at AC zero, therefore this circuit is also called common-collector circuit.

emitterfollower.gif

DC operating point


\begin{displaymath}
\left\{ \begin{array}{l}
I_e=(\beta+1) I_b \\
V_{cc}=R_B...
...I_b \\
V_{ce}=V_{cc}-V_e=V_{cc}-R_E I_e \end{array} \right.
\end{displaymath}

Solving the second equation, we get $I_b$:

\begin{displaymath}
I_b=\frac{V_{cc}-V_{be}}{(\beta+1)R_E+R_B}
\end{displaymath}

and $I_e=(\beta+1)I_b$:

\begin{displaymath}
I_e=(\beta+1) I_b=\frac{(\beta+1)(V_{cc}-V_{be})}{(\beta+1)R_E+R_B}
\end{displaymath}


\begin{displaymath}
V_{ce}=V_{cc}-R_EI_e
\end{displaymath}

Example

Assume $R_B=100\,K$, $V_{cc}=10\,V$, $\beta=100$. Find $R_E$ so that the DC operating point is in the middle of the load line.


\begin{displaymath}
V_e=I_e R_E=(\beta+1)I_b R_E=\frac{(\beta+1) R_E (V_{cc}-V_{be})}{(\beta+1)R_E+R_B}
\end{displaymath}

Solving the equation $V_e=V_{cc}/2=5\,V$ for $R_E$, we get $R_E=1.15\,K\Omega$.

\begin{displaymath}
I_b=\frac{V_{cc}-V_{be}}{R_B+(\beta+1)R_E}=0.043\,mA,
\;\;\;\;\;I_e=(\beta+1)I_b=4.34\,mA,\;\;\;\;V_e=5\,V
\end{displaymath}

AC small-signal equivalent circuit

emitterfollower2.gif

Based on this small signal model, we can find the three system parameters: voltage gain, input resistance, and output resistance:

Conclusion:

The emitter follower is a circuit with deep negative feedback, i.e., 100% of its output $v_{out}=v_e$ is fed back to become part of its input $v_{be}$. The fact that this is a negative feedback can be seen by:

\begin{displaymath}v_e \uparrow \Longrightarrow v_{be} \downarrow
<\Longrightarrow i_e \downarrow \Longrightarrow v_e \downarrow \end{displaymath}

Due to this deep negative feedback, it has the following properties:

Although the emitter follower circuit does not amplify the signal voltage, it drastically improves the input and output resistances, compared with the input resistance $r_{in}=r_{be}$ and output resistances $r_{out}=R_C$ of the common-emitter transistor circuit. In fact the emitter follower acts as an impedance transformer with a ratio of $\beta$, i.e., the input resistance is $\beta$ times greater than $R_E\vert\vert R_L$ and the output resistance is $\beta$ times smaller than $R_S+r_{be}$.

Although the emitter follower does not amplify input voltage, due to its high input resistance and its low output resistance, it draws very little current from the source and can drive heavy load (low $R_L$), it is therefore widely used as both the input and output stages for a multi-stage voltage amplification circuit.


next up previous
Next: Multi-stage Amplification Up: ch4 Previous: AC Amplification
Ruye Wang 2014-08-02