Neutralization

Author: Leonard Krugman

The close coupling between the input and the output circuits of the transistor causes the resonant frequency of the coupling circuit to be particularly sensitive to variations in the input and output impedances. In general, the load impedance has a greater effect on the input impedance than the generator impedance has on the output impedance. For this reason, the best procedure to follow in aligning an i-f strip is to start with the last stage and work toward the first.

To avoid the critical tuning problem, the stage may be neutralized. This allows each resonant coupling circuit to be independently adjusted without introducing any detuning on or by the remaining stages. One form of neutralization is illustrated in Fig. 7-6 (A). For reasons of clarity, only the a-c circuit is shown. Neutralization is accomplished by balancing the resistor R_B and the equivalent impedance Z_c of R_c and C against the transistor base resistor r_b and the equivalent impedance of the collector arm z_c composed of r_c and C_c. The balancing conditions are more clearly illustrated in the equivalent circuit of the neutralized circuit, as shown in Fig. 7-6 (B). The circuit is drawn in the form of a conventional bridge. The bridge is balanced when r_b/R_B = z_c/Z_c. Under this condition there is no interaction between the input and output circuits. Then, when the stage is neutralized, the output impedance is independent of R_g and the input impedance is independent of R_L.

Fig. 7-6. (A) Neutralized i-f amplifier. (B) Equivalent circuit of neutralized i-f amplifier.

In practical circuits the neutralization network design can be simplified by omitting the capacitor C if a point-contact transistor is used, or by eliminating R_c if a junction transistor is used. This changes the balance equations to r_b/R_B = r_c/R_c for the point-contact types, and r_b/R_B = C/C_c for the junction types. These simplifications are possible at the intermediate frequencies because feedback is governed primarily by r_c in the point-contact transistor, and by C_c in the junction transistor. The network components are not very critical. Values within a 5% tolerance range are generally satisfactory.

Notice that the lower output terminal is connected to ground through R_B. This makes it important for the value of R_B to be small in order to avoid introducing too much noise through R_B into the output circuit. For satisfactory operation, the value of R_B should not be larger than the base resistance. This fixes the value of C in the range of C_c, and R_c in the range of r_c. The loss in gain due to the neutralizing network will be less than 10% of the total gain in a properly designed circuit.