Grid Leak versus Plate AM Detection

Grid Leak vs. Plate Detection in Early Radio Circuits:

Grid Leak Detector:

• Function: Demodulates and amplifies amplitude-modulated (AM) signals.

• Operation: Utilizes the non-linearity of the grid current vs. grid voltage characteristic.

• Components: Triode vacuum tube, grid capacitor, grid leak resistor (high resistance).

• Advantages: Simple design, good amplification at small signal levels.

• Disadvantages: Less sensitive than plate detector, distortion at larger signal levels, prone to drift.

Plate Detector:

• Function: Demodulates AM signals.

• Operation: Utilizes the non-linearity of the plate current vs. grid voltage characteristic.

• Components: Triode vacuum tube, grid capacitor, bias resistor, plate load resistor.

• Advantages: More sensitive than grid leak detector, less distortion.

• Disadvantages: More complex design, requires bias adjustment, higher power consumption.

Note:

Regenerative receivers need to show a reduction in gain (regeneration level) with signal amplitude to allow a smooth transition into oscillation. If the gain of the sustaining amplifier increases with signal amplitude (increasing regeneration) the circuit suddenly 'pops' into strong oscillation as the regeneration control level is increased. 

In a tube circuits the factors that control gain versus signal amplitude are:

Tube nonlinearities at the particular biasing point, which often but not always cause an increase in gain with signal amplitude.

Changes in (grid) biasing with signal increase, especially with the grid leak arrangement.

Dynamic changes in loading of the resonant circuit with signal strength, especially with the grid leak arrangement.