Super Regeneration Detector
A super-regenerative receiver is actually an oscillator that is arranged to periodically be allowed to oscillate then stopped (or Quenched as it is called).

The time taken for the oscillations to build up depend upon the signal (or noise) level in the circuit. When there is a signal present at the oscillation frequency the oscillations will start more quickly.

The time taken to start oscillation will affect the average current so by filtering this the modulation can be recovered.

They have have extremely high-gain and be sensitive to microvolts of signal but have rather broad bandwidth as well as radiating at the operating frequency.

Because they can get such high gains from a single active device they were popular when gain was expensive or for very low-cost applications where the limitations were acceptable. The main application until recently was for remote control of garage doors and toys. Garage door openers now use more sophisticated receivers. More stringent FCC requirements also restrict their use.

They were invented in the 1920's by Edwin Armstrong who also invented the Superhet receiver technique (super-sonic-heterodyne).

In the circuit shown Q1 is configured as an oscillator with the frequency set by L1 and C2 with feedback from c3 from collector to emitter.

Since there is a positive feedback loop the antenna can be connected either at the collector or emitter - most circuits I have seen connect it to the tuned circuit (L1/C2).

Oscillations build-up from noise or the signal and cause a rectified version to appear at the base and emitter of the transistor charging up C4 modifying the bias of the transistor. At some point the bias will be such that the oscillations will stop and C4 will discharge through resistors R2 and R3. Eventually the oscillations will start again and the cycle will repeat typically at a few 10's or 100's of kilohertz (the quench frequency).

The average collector current is sensed with R4 causing a voltage that varies with the signal level although it also has a high level of the quench frequency. This is filtered by the resistors R5 and R6 and C7 and C8 leaving the modulation as an output.