2.9 Common collector amplifier

The common collector BJT circuit shown in Figure 2.9 has the following hybrid parameters. Input impedance, h _ie, equals 2 10 ³ ohms. Voltage feedback ratio, h _re, equals 4 10 ^?4. Current gain, h _fe, equals 50. Output admittance, h _oe, equals 20 10 ^?6 siemens. Calculate the (a) Z _in, (b) Z _out, (c) A _i, and (d) A _v respectively.

Figure 2.9: Common collector amplifier

Solution:

Since h _oeR _E ? 0.1

1. Z _in = [h _ie + (1 + h _fe)R _E] R ₁ R ₂

   Z _in = [2 10 ³ + (51)5.1 10 ³] (10 10 ³) (90 10 ³)

   Z _in = (27.1 10 ⁴) (9 10 ³) = 8,700 ohms

2. Z _out = h _ie / (1 + h _fe) R _E = (2 10 ³ /51) (5.1 10 ³)

   Z _out = 39 (5.1 10 ³) = 39 ohms

3. A _i = (1 + h _fe) = 51

4. A _v = 1 ? (h _ie / Z _in) = 1 ? (2 10 ³ / 8,700) = 0.77