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Of course, the reader can use other microphones such as ceramic or crystal high-impedance types simply by removing R1.The signals coming from the microphone are applied to the transistor by C1.This capacitor value must be altered if the reader wants to change the audio frequency response that passes through it. Because this kind of microphone has an internal field effect transistor (FET) that is used to increase the signal amplitude (acting as an amplifier), the sensitivity is very good. As this part of the circuit operates with high-frequency signals, all of the capacitors must be ceramic types.The modulation comes from an electret microphone. So, this resistor must be altered to 82 or 100 ohm.Base biasing is made by resistors R2 and R3, and at the same time C2 decouples this electrode. Values below 47Ģ must not be used, as the current flow through the transistor can rise to high values, heating this component to a dangerous point.If the circuit is powered from a 9 V supply, the signals can reach up to 800 ft, but the value of resistor R4 must be increased to limit the current flow through the transistor to secure values. Resistor R4 limits the current flow through the transistor and therefore the power. Values between 2.2 and 5.6 pF can be used experimentally. Depending on the operational frequency, this capacitor can be replaced with others to achieve better performance. The feedback that keeps the circuit in oscillation is given by C3. The frequency is determined by L1 and CV and can be adjusted to any free point in the FM range between 88 and 108 MHz.ĬV is a common trimmer rated 2 to 20 pF, but this component is not critical, and any trimmer with its highest capacitances in the range between 20 and 50 pF can be used. The transmitter is formed by a simple one-transistor oscillator that produces the high-frequency signal. These characteristics are provided as a reference for the reader who needs to find an equivalent transistor that is suitable for this application. HFE (according suffix) D: from 60 to 120 - E from 100 to 200 - F from 160 to 230 The principal characteristics found in the recommended transistor are: Our project is very interesting from a didactic point of view, as the reader can compare its performance with that of the transmitter shown in Project ART003E Using this transistor, we create a small FM transmitter that can send its signals to receivers placed at distances as great as 300 ft, depending on the power supply voltage, the antenna, and the local topographic conditions (presence of hills, obstacles, etc.). 1 and can produce signals at frequencies up to 150 MHz.įigure 1 – layout of the recommended transistor The recommended transistor has a terminal layout as shown in Fig. In the circuit, we include a PNP Japanese transistor 2SA1177 (or any equivalent) to form a small FM transmitter that resembles the one in Project ART003E. Despite this fact, we can find many PNP transistors that are suitable for use in high-frequency oscillators, reaching frequencies as high as those used in FM transmissions. The NPN transistor has only one P layer to be traversed by the charge carriers, whereas the PNP has two layers. As a result, NPN transistors can operate at higher frequencies than equivalent PNP transistors. The charge carriers in an N semiconductor material can flow at higher speeds than in a P semiconductor. Despite this, the circuit has excellent performance as a substitute for those that use ,NPN transistors. This transmitter uses a PNP (positive-negative-positive) transistor instead of an NPN (negative-positive-negative) type, which is the basis of most of these projects.
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