AM Receiver Schematic

This is a compact three transistor, regenerative receiver with fixed feedback. It is similar in principle to the ZN414 radio IC which is now no longer available. The design is simple and sensitivity and selectivity of the receiver are good.

AM Receiver Schematic

Notes:
All general purpose transistors should work in this circuit, I used three BC109C transistors in my prototype.The tuned circuit is designed for medium wave. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz. Q1 and Q2 form a compund transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit.

The 120k resistor provides regenerative feedback,between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit. Too much feedback and the circuit will become unstable producing a "howling sound". Insufficient feedback and the receiver becomes "deaf". If the circuit oscillates,then R1s value may be decreased; try 68k. If there is a lack of sensitivity, then try increasing R1 to around 150k. R1 could also be replaced by a fixed resisor say 33k and a preset resistor of 100k. This will give adjustment of sensitivity and selectivity of the receiver.

Transistor Q3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. Audio level varies on the strength of the received station but I had typically 10-40 mV. This will directly drive high impedance headphones or can be fed into a suitable amplifier.

Construction:
All connections should be short, a veroboard or tagstrip layout are suitable. The tuning capacitor has fixed and moving plates. The moving plates should be connected to the "cold" end of the tank circuit, this is the base of Q1, and the fixed plates to the "hot end" of the coil, the juction of R1 and C1. If connections on the capacitor are reversed, then moving your hand near the capacitor will cause unwanted stability and oscillation.

Finally here are some voltagee checks from my breadboard prototype.This should help in determining a working circuit:

All measurements made with a fresh 9volt battery and three BC109C transistors with respect to the battery negative terminal.

Q1 (b) 1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V
Q3 (c) 3.87V

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Use LA1800 Portable AM and FM Radio

This portable AM and FM radio circuit is designed using the LA1800 IC and some other external components. As you can see in this circuit diagram the LA1800 manufactured by Sanyo Semiconductors , require few additional components. The LA1800 am FM portable radio circuit needs to be powered from a 3 volt DC power supply circuit.

You can use an 3 volt battery. This radio receiver circuit has a low current dissipation of 5.6mA for FM band and 3.2mA for AM band .Also the output signal is driven into earphone speakers , but you can use an additional speaker ( in that case you need to connect an additional small power audio amplifier).

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AM Transmitter Double Side Band

AM Transmitter Double Side Band

The ambit of AM transmitter is advised to address (amplitude modulated) DSB (double ancillary band) signals. A articulate AM arresting consists of a carrier and two symetrically spaced ancillary bands. The two ancillary bands accept the aforementioned amplitude and backpack the aforementioned information. In fact, the carrier itself coveys or carries no information. In a 100% articulate AM arresting 2/3 rd of the ability is ashen in the carrier and alone 1/6th of the ability is independent in anniversary ancillary band.

In this transmitter we abolish the carrier and transmitt alone the two ancillary bands. The able achievement of the ambit is three times that of an agnate AM transmitter.

Op Amp IC741 is acclimated actuality as a microphone amplifier to amplify the articulation best up by the condenser microphone. The achievement of the op amp is fed to the bifold counterbalanced modulator (DBM) body about four IN4148 diodes. The accentuation akin can be adapted with the advice of preset VR1.

The carrier is generated application clear oscillator active about BC548 transistor T2. The carrier is added amplified by transistor T1, which additionally acts as a absorber amid carrier oscillator and the counterbalanced modulator. The alive abundance of the transmitter can be afflicted by application crystals of altered frequencies. For multi abundance operation, alternative of altered crystals can be fabricated application a selector switch.

Ths achievement of the DBM contains alone the artefact (of audio and carrier) frequencies. The DBM suppresses both the ascribe signals and produces bifold ancillary bandage suppressed carrier (DSBSC) at its output. However, back the diodes acclimated in the counterbalanced modulator are not absolutely matched, the achievement of the DBM does accommodate some balance carrier. This is accepted as carrier leakage. By adjusting the 100 ohm preset VR2 and trimmer C7 you can absent the carrier leakage.

To accept DSB signals you charge a exhausted abundance oscillator to reinsert the missing carrier. If you don’t accept a exhausted abundance oscillator, or appetite to transmitt alone AM signal, acclimatize preset VR2 to aperture some carrier so that you can accept the signals on any accustomed radio receiver. In AM approach 100% accentuation can be accomplished by adjusting preset VR1 and VR2.

X1 – 8+8 Turns Bifalar 24 SWG On TV Balune Core
X2 – Primary 12 Turns, Secondary 4 Turns. 24 SWG on TV Balun Core (dot indicates start of coil).
X3 – 20 Turns 24 SWG on TV Balun Core

Range of the circuit depends on the type of antenna used. It is very important to use matched antenna to radiate the signals effectively. I used horizontal dipole antenna, which is simple and easy to construct. For 7 MHz, ie 40 meter ham band the length of dipole antenna will be 20 meter. Use 75 Ohms co-axial cable to connect antenna and transmitter. I was able to get 57 report from station 80 kilometer away. You can easily add a Linear RF amplifier using IRF830 to get more power.

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