Electronic schematics, circuit designs
27MHz Fm Narrow Band transmitter
NBFM or Narrow Band Frequncy Modulation is used in this 27 MHz fm transmitter circuit schematic. This circuit is an application by Motorola MC2833 - VHF transmitter with FM modulation and narrow band in a single chip.
The output power is 10mW (+10 dBm) and the coverage is 10 - 20m.
P1 is used to adjust microphone amplification and P2 to adjust the deviation. Remember that this 27MHz fm transmitter use NBFM with maximum 5KHz deviation, this mean you have to use an nbfm receiver to obtain enough audio level.
The transmitter is easy to adjust: trim C9, C9 and C18 for maximum output power on a artificial 50Ω load. Then listen on a 27MHz radio receiver and adjust P1 and P2 untill you obtain the best modulation.
X1 = 27.005 MHz quartz
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Microphone amplifier with low noise
This low noise microphone amplifier is build with MAT02 produced by PMI. This mic amplifier is very performant and has a very low noise level. The amplification can be 20dB or 23.5dB (10x or 15x ) selected by S1.
The output impedance is 70 Ω at 23.5 dB gain and with R3 = 220KΩ the input impedance is 30kΩ .
Distorsions are 0.045%, the noise level is -65dB at 15mV output voltage.
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40W audio amplifier TDA2030 BD712 BD711
A great 40 W audio mono amplifier build with TDA 2030, BD 712 and BD 711. The output power is 22W on 8 ohm speaker and 40W on 4 ohm. Distorsions are 0.012% at 1khz/8 ohm/11W and 0.032% at 1khz/4 ohm/20W. The IC1 supply curent variates accordingly to the audio input signal so there will be a similar variation on R6, R7, R8 and R9. As long the current is arround 1A, the voltage on the resistors is insufficient to open T1 and T2. This mean that output power smaller than 2W on 4 ohm will be outputed by TDA2030. This is a great audio power amplifier as I know because I’ve build one.
The voltage power supply is between 12V and 40V.
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