Wednesday, March 16, 2016

One transistor radio signal paths

This circuit provides a good example of the different component functions.
First the antenna, coil, and ground provide a signal to the receiver but that's not all. The coil provides a DC path to prevent static charge building on the antenna and is a high resistance path for high frequency signals blocking them from the receiver.
L2 and C1 are a paralell resonant circuit. They select the frequency that the set will receive and give it a voltage boost.
C2 is a low resistance path for RF signals. The output is being developed across R1. High frequency RF bypasses through C2 charging it quickly and low frequency AF go through R2 producing a voltage drop. The voltage dropped across R1 equal the RF peaks while the voltage across R2 is developed as C3 is charging.
This leaves R3 and C4 which form a filter to allow the DC component to flow back to the battery and the AC component to return to the FET source. R3 must offer enough opposition to the RF component to make it see the C4 as the low return path. Bias on the FET allow a space charge to build in it. As the input is impressed on the gate it modulates the FET channel current and develops the RF across R1 which in turn supplies a charge signal for C3 and develops the AF across R2 which powers the phone.

The question is how many paths are there? The battery is providing a DC which is fairly constant. The RF and AF components are source to drain and back to source bypassing the battery through C4.

No comments:

Post a Comment