Neutralizing Amplifier

Posted on Feb 6, 2014

Grid driven tetrodes like 6146, 807, or 4CX250`s have high power gain. High gain systems require very little feedback to become unstable, so they are generally neutralized. The also often require some form of grid loading resistor to reduce or stabilize gain. The following circuit shows a commonly used tetrode grid-driven amplifier with neutralization: L1/C1 is the normal input tuning coil. Being resonant on

Neutralizing Amplifier
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the operating frequency, it inverts phase 180-degrees from end-to-end. C2 is a voltage divider to control the feedback voltage ratio and provide a return path for grid excitation. Cneut is adjusted so its voltage feedback equals the voltage fed through Cgp from plate to control grid inside the tube. Note that this system depends heavily on L1/C1 being resonant at the operating frequency. This proves the tube is only neutralized at the frequency where C1/L1 is set. It does not stabilize the tube on any frequency except where L1/C1 is resonant. While the feedback adjustment setting of Cneut holds true for multiple bands near the initial adjustment frequency, it only actually neutralizes the tube on the band in use at any moment of time! In a 160-10 meter PA, Cneut generally only works properly over two or three bands. It is usually set near 15 meters so it has the most effect where it is needed most. By the time we get down to 40 meters and lower, feedback voltage through Cgp is generally through such a high reactance that the lack of proper balancing is meaningless. Additional stability can be added by loading the grid with a broadband termination resistance. This makes neutralization much less critical, and may at times even eliminate the need to neutralize. This resistor would go from the control grid to ground and ideally be added right at the tube. Unless the resistor is an integral part of the bias system, it must be...

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