4 Transistors FM Transmitter

The following diagram is the schematic diagram of 4 transistors FM transmitter circuit designed by Paul K. Sherby.

FM transmitter Circuit with 4 Transistors

Components List:

R1,R2,R8 = 1K
R3 = 100K
R4 = 150K
R5,R7 = 10K
R6 = 220 ohm
R9 = 10 ohm
P1 = 5K trimpot
D1 = 1N4002
Q1,Q2 = 2N3904
Q3,Q4 = 7001, NTE123AP
C1 = 1uF/63V
C2,C3 = 10nF
C4,C5,C9 = 4.7uF/63V
C6,C12,C13,C14 = 1nF
C7,C8,C11 = 5pF
C10 = 220uF/63V
L1 = 3.9uH
L2 = 1uH
L3 = aircoil, 8.5 turns air space, 1/4 inch diameter


FM Transmitter Circuit Notes:
This circuit delivers an FM modulated signal having an output power of about 500mW. The input microphone pre-amplifier is designed close to a pair of 2N3904 transistors (Q1/Q2), and audio gain is restricted by the 5k preset trimmer potensiometer (trimpot).

The oscillator is actually a Colpitts stage, frequency of oscillation governed by the tank circuit built from two 5pF ceramic capacitors along with the L2 inductor. Frequency is about 100Mhz with values shown.

Audio modulation is fed in to the tank circuit via the 5p capacitor, the 10k resistor and 1N4002 controlling the quantity of modulation. The oscillator output is fed in to the 3.9uH inductor (L1) which will have a high impedance at RF frequencies.

The output stage operates as a ‘Class D’ amplifier, no direct bias is applied but the RF signal developed across the 3.9uH inductor is enough to drive this stage. The emitter resistor and 1k base resistor avoid instability and thermal runaway in this stage.

Some substitutes for the 2N3904 are: NTE123AP, 2N4401, BC547 (watch lead orientation), and so on. The 7001 (Q2/Q3) oddly enough also reference towards the NTE123AP. Even though the 2N3904 is really common and effortlessly available.


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