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NeutralizationAuthor: 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 if strip is to start with the last stage and work toward the first.
Fig. 76. (A) Neutralized if amplifier. (B) Equivalent circuit of neutralized if amplifier. In practical circuits the neutralization network design can be simplified by omitting the capacitor C if a pointcontact 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 pointcontact 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 pointcontact 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.


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