Effect of Emitter Resistor next up previous
Next: Experiment: Active Loads Up: Differential Amplifier with Active Previous: Collector Output Resistance (Early

Effect of Emitter Resistor

The circuit in Fig. 6.3[6.3] shows an emitter resistor between Q1 and Q2. Usually we add a potentiometer RE at the emitters of Q1 and Q2 for matching purpose. By adjusting the 1K pot, we can achieve matching between Q1 and Q2.

   Figure: Differential Amplifier


The effect this resistor has on the gain can be seen by realizing that the output configuration has not changed, and looking at the input. Applying KVL at the input loop gives

 
Recall from previous labs that a first order Taylor series gives $\Delta V_{be}=\frac{\Delta I_c}{g_m}$.We then find that

  This gives rise to the following expression for voltage gain

Now, if we account for the collector output resistances, then the the expression for gain becomes:

  Note, that the effect of the emitter resistor in Q2 transforms the collector output resistance of ro2 to $r_{o2}\left(1+\frac{g_mR_E}{2}\right)$. This can be shown with small signal equivalent circuits.


Bassel Al-Annouf
12/7/1998