Need help with this feedback circuit (bjt amp)
The circuit is a small-signal equivalent diagram for the well known common-collector stage (emitter follower) with "bootstrapping". 
In the circuit, there are two feedback cases:

1.)  It is primarily the resistor RL´ which provides negative feedback for stabilizing the DC operating point and allowing emitter follower operation of  the stage; in this context, the conncted resistors RB1...RB3 play a minor role only (depending on the values if compared with RL). 

2.) With the aim to drastically increase the input resistance ("bootstrap effect") it is the main purpose of RB3 to provide positive feedback to the base node. Because  the stage acts as an emitter follower the signal voltages at the base and emitter node will be nearly identical: 

Therefore, there will be practically no signal current through RB3 (RB3 is virtually increased due to the bootstrap effect). As a result, the signal input resistance of the whole stage will practically not be degraded due to the biasing resistors RB1 and RB2 (because the "bootstrapped" resistor RB3 is between signal input and the common node of RB1 and RB2).  

Summary: Negative feedback (RL) for stabilizing purposes and positive feedback (RB3) for increasing the input resistance.