3.5 Radio Receiver with Increased Sensitivity Audio Amplifier
If you cannot obtain the signal reception that is loud enough with receiver from the previous project, the sensitivity of the LF part of the device must be increased. The simplest way to manage this is adding a transistor pre-amplifier, as shown on pic.3.21. Please note that all the components except C1, C and L are located on the PCB, which allows you to use the input circuit from pics.3.15 and 3.20. The receiver is supplied from the 4.5 V battery but, if you intend to use an adaptor, the supply voltage can be bigger, its maximum value being from 12 V (for the LM386) till 18 V (for the LM386-4).The optimal value for R3 resistor (obtaining the greatest amplification, minimal distortion etc.) depends upon the kind of transistor that is being used. The easiest way to finding it is by experimenting, with a few MOhms’ trimmer resistor, in the way it has already been described in chapter 3.2.