Download 1W AM Transmitter circuit and explanation

Amplificatore Audio
12/08/2008 19.06
This is the low frequency power amplifier of which the penumbra article can be composed without
using it hardly
The about 660-mW output can be gotten with the 16-ohm speaker.
This circuit is used for the simple monitor and so on.
The impedance (4-ohm and 8-ohm, and so on), the output power of the speaker and so on are specified by the standard
but are OK rather than strict. Because the sound cracks (warping) when presenting the output above the rating, it uses
naturally within the standard.
When conscious of the timbre and the output power, it is better not to do the use of this circuit.
Micro Power AM Broadcast Transmitter
In this circuit, a 74HC14 hex Schmitt trigger inverter is used as a square wave oscillator to drive a small
signal transistor in a class C amplifier configuration. The oscillator frequency can be either fixed by a crystal
or made adjustable (VFO) with a capacitor/resistor combination. A 100pF capacitor is used in place of the
crystal for VFO operation. Amplitude modulation is accomplished with a second transistor that controls the
DC voltage to the output stage. The modulator stage is biased so that half the supply voltage or 6 volts is
applied to the output stage with no modulation. The output stage is tuned and matched to the antenna
with a standard variable 30-365 pF capacitor. Approximately 20 milliamps of current will flow in the
antenna lead (at frequencies near the top of the band) when the output stage is optimally tuned to the
oscillator frequency. A small ‘grain of wheat’ lamp is used to indicate antenna current and optimum
settings. The 140 uH inductor was made using a 2 inch length of 7/8 inch (OD) PVC pipe wound with 120
turns of #28 copper wire. Best performance is obtained near the high end of the broadcast band (1.6 MHz)
since the antenna length is only a very small fraction of a wavelength. Input power to the amplifier is less
than 100 milliwatts and antenna length is 3 meters or less which complies with FCC rules. Output power is
somewhere in the 40 microwatt range and the signal can be heard approximately 80 feet. Radiated power
output can be approximated by working out the antenna radiation resistance and multiplying by the
antenna current squared. The radiation resistance for a dipole antenna less than 1/4 wavelength is
R = 80*[(pi)^2]*[(Length/wavelength)^2]*(a factor depending on the form of the current
distribution) The factor depending on the current distribution turns out to be [(average current along
the rod)/(feed current)]^2 for short rods, which is 1/4 for a linearly-tapered current distribution
falling to zero at the ends. Even if the rods are capped with plates, this factor cannot be larger than
1. Substituting values for a 9.8 foot dipole at a frequency of 1.6 MHz we get R= 790*.000354*.25 =
.07 Ohms. And the resistance will be only half as much for a monopole or 0.035 Ohms. Radiated
power at 20 milliamps works out to about I^2 * R = 14 microwatts.
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1W AM Transmitter circuit and explanation
This AM transmitter circuit provides a nice, clean output of about 1 Watt (carrier power). Though
designed for the medium wave band (circa 1.5 MHz) it would work equally well on higher
frequencies (6.2 MHz for example) with a few tweaks in component values (see table on left - C15
should be adjusted for maximum output).
The carrier (produced by the 4049) is modulated at low-level by the
MC1496 balanced modulator. There are then a couple of stages of linear amplification to reach the final
output power so no modulation transformer is required. TR2, TR5 and TR6 are BC108 or similar; TR3 is a
2N3053 or 2N4427 or 2N3866 or any low/medium power NPN transistor. The main output transistors, TR4
and TR5 were originally 2SC1162 but BD135 or BD139 or other medium power RF transistors will do equally
well. T1 uses a pre-tuned TOKO KANK3334 coil, the other transformers are wound on the red T50-2 toroids
(the number of turns shown is the ratio, use about 4 to 5 times that number in reality - less at higher
frequencies). The LED lights up if current in the output amplifier goes too high, so it's a kind of 'high SWR'
warning.
500 KHz to 1600 KHz AM Transmitter Circuit
500 KHz to 1600 KHz AM Transmitter Circuit
The circuit is in two half, an audio amplifier and an RF oscillator. The oscillator is built around Q1
and associated components. The tank circuit L1 and VC1 is tunable from about 500 KHz to 1600
KHz. These components can be used from an old MW radio, if available. Q1 needs regenerative
feedback to oscillate and this is achieved by connecting the base and collector of Q1 to opposite
ends of the tank circuit. The 1nF capacitor C7, couples signals from the base to the top of L1, and
C2, 100pF ensures that the oscillation is passed from collector, to the emitter, and via the internal
base emitter resistance of the transistor, back to the base again. Resistor R2 has an important role in
this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal
emitter resistance, re of Q1, and also increases the input impedance so that the modulation signal
will not be shunted. Oscillation frequency is adjusted with VC1.
The Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor and realizing
full gain of this stage. The microphone is an electret condenser mic and the amount of AM
modulation is adjusted with the 4.7k preset resistor P1. An antenna is not needed, but 30cm of wire
may be used at the collector to increase transmitter range.
Thanks To: http://freeelectricalsandtools.blogspot.com/
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Se parliamo di microfono a condensatore ha ragione l'amico, devono essere alimentati, un condensatore
non è un generatore di energia.
Nella stragrande maggioranza dei casi le capsule microfoniche sono del tipo a condensatore, molto spesso
contengono anche il preamplificatore che deve necessariamente essere fatto con un fet, serve una grande
impedenza d'ingresso.
Se la capsula ha due fili uno deve essere alimentato a 5-12 Volt tramite una resistenza da 1K mentre l'altro,
solitamente solidale con la carcassa metallica, è la massa.
Il segnale si ottiene dal terminale alimentato disaccoppiandolo con un condensatore da 4-10 uf con il
positivo rivolto verso il microfono.
Questa è la pagina di un catalogo online di microfoni a condensatore, ci sono sia i modelli a tre fili che quelli
a due fili con relativi schemi di impiego
http://www.sala.com/sala/catalogo/83/83.html
Dipende molto se la capsula è del tipo preamplificato oppure no, nel primo caso in uscita hai già un
segnale di 100-150 mv, nel secondo caso in uscita hai un segnale di pochi mv, tipicamente una
ventina.
L'amplificatore operazionale ti serve comunque, devi connettere l'ingresso positivo a 1/2 della Vcc
tramite un partitore e il guadagno lo regoli a seconda del segnale in uscita della capsula e di quanto
deve essere il segnale in ingresso al trasmettitore audio.
Molto probabilmente la tua capsula è del tipo preamplificato, lo sono quasi tutte, pertanto in uscita
dovresti avere un segnale ampio, se hai un oscilloscopio puoi facilmente verificarlo.
Citazione:
- La resistenza da 1K dipende dalla capacità della capsula vero? Essa influisce sul tempo di
carica/scarica della capacità variabile. E' giusto?
La resistenza serve per limitare la corrente che scorre nel microfono.
Purtroppo il valore di questa resistenza è molto variabile, dipende da come è fatto esattamente
l'amplificatore interno, in svariati modelli di capsule non serve, in altri modelli è molto alta, da 1k a
10k sono i valori tipicamente usati.
Se sulla capsula non c'è segnata una sigla riconducibile ad un data sheet tocca andare per tentativi,
tanto danni non ne puoi fare al limite non funziona oppure distorce molto.
Citazione:
- Il condensatore d'uscita: ha un valore specifico i 10uF. Cioè: Se dovesse servire solo a non far
passare la continua, allora + grande è meglio è.
con 10uf ti passa senza problema tutta la banda HiFi, cioè i classici 20Hz - 20kHz.
simple preamp:
this is a very easy circuit to build - all parts can be found at the local electronics shop - no matter
how worthless it (the local shop,) usually is...
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