CE with unbypassed RE; application of CB amplifier

Circuit (voltage divider bias, RE unbypassed)

              +VCC
               |
         +-----+-----+
         |           |
         R1          RC
         |           |         C2
 vs--||--+------B    C----+----||----+---- vo
     C1  |       \  /     |          |
         R2       NPN     (vo node)  RL
         |         |                 |
        GND        E                GND
                   |
                   RE  (no bypass capacitor)
                   |
                  GND

C1 and C2 are coupling capacitors; RE is not bypassed, so it appears in the ac path.

Output impedance

Set vs = 0 and look into the collector. The unbypassed RE (series feedback) raises the resistance looking into the collector:

Zout(collector) = ro [1 + gm (RE || rπ)]  →  ∞  (ro = ∞)
Zo = RC || Zout(collector) ≈ RC

So the output impedance of the stage is approximately RC (RE only raises the collector impedance, which is already very large).

Voltage gain

With the T-model: vb = ie (re + RE) and vo = -ie (RC || RL), so

Av = vo/vb = -(RC || RL)/(re + RE)

(If RE ≫ re: Av ≈ -(RC||RL)/RE, which is set by the resistors alone.) The input impedance is Zin = R1 || R2 || (β+1)(re + RE).

Applications of the common base amplifier

• High-frequency and RF amplifiers (VHF/UHF), because there is no Miller effect and the bandwidth is wide.
• Current buffers, and stages that need a low input impedance (e.g. matching to low-impedance sources such as microphones or antennas).
• The upper stage of a cascode (CE-CB) amplifier.
• Constant current sources and current mirrors.