250mW FM Transmitter Amplifier

Published:2012/11/11 20:13:00 Author:muriel | Keyword: 250mW, FM , Transmitter, Amplifier | From:SeekIC

This project is a simple 2-transistor VHF power amplifier, with about 16dB gain, and requires no tuning or alignment procedures. Wideband techniques have been used in the design and the circuit is equipped with a lowpass filter to ensure good output spectral purity. The project has been designed for assembly on a single-sided printed circuit board. The circuit is specifically designed to amplify the output of 7mW to 10mW WBFM transmitters (wide band) to a final level of 250mW to 300mW. The first stage (Q1) operates in Class-A. Although Class-A is the least efficient mode, it does offer more RF gain than other clases of bias, and Q1 is a low-level stage, when compared to the higher power Q2 stage. The output of this stage is around 70mW of RF power. The stage is untuned so that it gives a very broadband characteristic. The transistor is biased by means of R5, R6 and L6, and the residual (standing) DC current is set by R4. The input signal is coupled by C9 to the Base of the transistor. Q2 is operated in Class-AB which leads to greater efficiency, but the RF gain is only about 8dB, but it amplifies the output of Q1 to typically 250mW. Q2 is biased by means of R3, R2 and L4. The input signal from Q1 is coupled to the Base of Q2 via C7. The voltage regulator Q3 (78L08) is used to regulate the supply voltage to Q1 and the bias votages to both Q1 and Q2 so that the output RF power is relatively constant, even with large variations of supply voltage. Q3 also removes supply ripple as well as providing power for an FM transmitter like Kit 3018 wireless microphone with the required DC 8v power. The output of the amplifier is filtered with a low-pass filter to reduce the output spurious and harmonic content. The output filter consists of C3, C4, L1 and L2.



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