Neutralization is a technique used to cancel feedback in bipolar junction transistors (BJTs) and field-effect transistors (FETs) that can cause instability over certain frequency ranges. This is done by introducing an additional feedback signal of equal amplitude but opposite phase to cancel the original feedback. In a neutralized transistor circuit, a negative capacitor is typically used to introduce this additional feedback signal. The Hazeltine neutralization method uses a variable capacitor connected from the bottom of an RF amplifier coil to the transistor base to neutralize the feedback from the collector-base capacitance. A modified version called Neutrodyne neutralization connects the capacitor to the secondary coil of the next stage for improved stability and insensitivity to supply voltage variations.

1. Neutralization

2. • Technique used for the elimination of
potential oscillation is called neutralization
• BJT and FET are potentially unstable over
some frequency range due to feedback
parameter(YN ) present in them.
• Feedback can be cancelled by an additional
feedback signal
• That is equal in amplitude and opposite in
sign.

3. Equivalent circuit of a neutralization
transistor

4. • Yre
‘ = Ib / Vce by making Vbe = 0
• Ib = Yre Vce- YN Vce
• Yre
‘= Yre - YN
• For perfect neutralization, Yre
‘ = 0
• Therefore Yre = YN
• desired component used for neutralization is
Negative capacitor

5. Tuned RF amplifier with Hazeltine
Neutralization

6. • Hazeltine technique is used to maintain
Stability
• Undesire effect of collector to base
capacitance is neutralize by introducing a
signal which cancels the signal.
• CN - Variable capacitor
• CN - connected from bottom of the coil
• It introduces a signal to base of transistor
such that it cancels the signal Cbc.

7. Modified Hazeltine -Neutrodyne

8. • Modified version of hazeltine
neutralization called Neutrodyne
neutralization technique.
• CN is connected to the lower end of
secondary coil of next stage.
• Hence it is connected with VCC, It is
insesitive to any variation in supply
voltage
• It provides higher stabilization for the
tuned amplifier

9. Advantages
• Amplify defined frequencies
• Signal to noise at output is good
• Suited for radio transmitter and receiver

10. Disadvantages
• Circuit is bulky because of inductors and
capacitors
• Circuit is Costly
• Design become complex if band frequency
increases
• Not suitable to amplify audio frequency