Analysis of CE Amplifier with an Emitter Resistance

Analysis of CE Amplifier with an Emitter Resistance

• The simple and effective way to obtain voltage gain stabilization is to add an emitter resistance RE to a CE stage as shown in Fig. 6.8.1.

• Fig. 6.8.2 shows an ac equivalent circuit for common emitter circuit with unbypassed emitter resistance as shown in Fig. 6.8.1.

• It is drawn by replacing coupling capacitors and d.c. sources by short circuits.

Ex. 6.8.1 Derive the expression for CE amplifier with bypassed Ry using approximate analysis.

An approximate analysis of the common emitter circuit with Rg can be made using approximate h-parameter equivalent circuit shown in Fig. 6.8.3.

Current gain (A_I):

Input resistance (R₁):

Important Concept

The input resistance due to factor (1+h_fe) R_E may be very much larger than hy Hence an emitter resistance greatly increases the input resistance.

Voltage gain (A_V):

Output resistance (R_O): It is the resistance of an amplifier without considering thesource and load (i.e. V_a = 0 and R_L = ∞). It is defined as a ratio of output voltage V_o to output current with Vs = 0.

R_o = V_o / I_o |_{Vs = 0}

When V_s = 0, the current through the input loop I_b = 0, hence I_c and I_o both are zero. Therefore, R_o = ∞. The output resistance R of the stage, taking the load into account is given as

R_o = R_o ||R_L = ∞ || R_L = R_L  … (4)

Ex. 6.8.2 Fig. 6.8.4 shows a single stage CE amplifier with unbypassed emitter resistance find current gain, input resistance, voltage gain and output resistance. Use typical values of h-parameter.

Sol.: Typical values for h-parameters are h_fe - 50, h_fe = 1.1 K, h_oe = 25 μA/V, h_re = 2.5 × 10^-4. Since h_oe R_L = 25 × 10^-6 (1.1 K || 3.3 K) = 0.02, which is less than 0.1, we use approximate analysis.