Neutralization

NEUTRALIZATION

Need for Neutralization

In tuned amplifiers, the transistors are used at resonant frequency to amplify the narrow band of high frequencies.

At this frequency, the junction capacitance between base and collector of the transistor becomes dominant. Thus the reactance of the circuit is low.

This reactance provides a positive feedback which causes oscillations in the circuit and makes the system unstable.

To prevent oscillation in tuned amplifiers, the neutralization technique is used.

NEUTRALIZATION METHODS

The stage gain should be reduced to prevent oscillations in the circuit. This can be done by

i. Reducing Q

ii. Stagger tuning

iii. Loose coupling between the stages

The feedback signal can be cancelled by an additional feedback signal that is equal in amplitude and opposite in phase. i.e., a portion of output signal is feedback to the input with same amplitude but opposite phase. This is achieved using neutralization.

Neutralization can be done in three methods.

i. Hazeltine Neutralization

ii. Neytrodyne Neutralization

iii. Rice Neutralization

iv. Neutralization using coil

HAZELTINE NEUTRALIZATION

L.A. Hazeltine introduced this circuit to increase the stability of the amplifiers. Fig. 3.36 show the tuned amplifier with Hazeltine neutralization.

In this circuit a small amount of variable capacitance C_N is connected from the bottom of the coil to the base of the transistor. The output coil in center-tapped and the tap is used as the power supply feedpoint of the amplifier.

This capacitance feeds a signal with equal amplitude and opposite phase, which cancels the interval capacitance C_bc from the top end of the coil.

Drawback

The capacitance value changes with time. The neutralizing capacitor should be adjusted correctly to completely neutralize the internal capacitance.

NEUTRODYNE NEUTRALIZATION

In this circuit, the neutralization capacitor is connected from the lower end of the base coil of next stage (secondary coil) to the base of the transistor.

Fig. 3.37 show the Neutrodyne neutralization circuit.

The capacitor is connected with V_CC and it is not sensitive to changes in the supply voltage. It also provides high stability.

Advantages

i. The capacitor is insensitive to variations in VCC.

ii. High stability.

RICE NEUTRALIZATION

Fig. 3.38 show the Rice Neutralization circuit. It uses a center tapped coil in the base circuit. The signal voltages at the end of the tuned base coil are equal and out of phase.

NEUTRALIZATION USING COIL

Fig. 3.39 show the neutralization of tuned amplifier using coil. In this method, part of the tuned circuit at the base of next stage is used for minimum coupling to other windings.

It is made perpendicular to the coupled windings. The voltage across the inductor will cancel the feedback signal produced by the collector base capacitance C_bc