The use of feedback in the reflex receiver allowed a single transistor to be used simultaneously as an amplifier for both RF and AF signals. The regenerative circuit however takes the RF output of a transistor detector (which would normally be disregarded) and returns part of it to the input to boost the incoming signal. The transistor detector combines the RF amplifier stage with the diode detector by utilising the base - emitter junction of a suitable transistor.

The gain produced is directly proportional to the amount of feedback, but a limit is reached where the feedback is too great for the transistor to cope with. At this point the transistor ceases to work as a rectifier/amplifier and breaks into oscillation. Thus maximum gain is achieved by designing the feedback circuit so that the point of oscillation is approached as nearly as possible without the transistor actually going into oscillation. The actual gain which may be obtained can be as great as 100:1, thus vastly improving the sensitivity of the receiver. The additional gain however, will be accompanied by some loss of quality or reproduction.

A block diagram of a regenerative receiver is shown in figure 7.

Figure 7

A phase changer is used to ensure that the feedback from the collector to base of the detector transistor is positive. The RF current and its corresponding voltage appearing at the collector of the transistor will be out 180 degrees out of phase with the input to the base. This will tend to cancel rather than boost the RF input which is applied directly to the base. Thus the phase changer is necessary to reverse the phase of the feedback, so that it is applied to the input in the same phase.

A simple single transistor regenerative circuit is shown in figure 8.

Figure 8

The phase changer in this circuit has been implemented by the additional coil L2, which is inductively linked to the aerial coil L1. If the coil is connected to the collector one way around it will tend to produce cancellation with the input; and connected the other way around it will tend to boost the input signal.

The addition of the variable capacitor C4 provides a means of controlling the amount of feedback used. By varying this capacitor, the maximum gain attainable before sending the transistor into oscillation can be found.

The simple circuit shown in figure 8 can be easily expanded by the addition of one or more stages of AF amplification following the detector. Sufficient output power is provided by these circuits to drive a high impedance loudspeaker direct without the need for an output transformer.