FM Antenna Booster Circuit

This is a design circuit for a low cost fm antenna booster that can be used to listen to programs from distant FM stations clearly. The antenna fm booster circuit comprises a common-emitter tuned RF preamplifier wired around VHF/UHF transistor 2SC2570 (C2570). This is the figure of the circuit;

Assemble the circuit on a good-quality PCB (preferably, glass-epoxy). Adjust input/output trimmers (VC1/VC2) for maximum gain. Input coil L1 consists of four turns of 20SWG enamelled copper wire (slightly space wound) over 5mm diameter former. It is tapped at the first turn from ground lead side. Coil L2 is similar to L1, but has only three turns. Pin configuration of transistor 2SC2570 is shown in the fm antenna booster schematic.

27MHz Intercom Walkie Talkie Circuit

Intercom walkie talkie are the next logical step in this discussion. They show how a crystal oscillator can be used to transmit voice. Transmitting a voice via a crystal locked oscillator is not easy. This is because the crystal is locking the frequency and it is very difficult to shift it. The only way to do it is to add the audio as an amplitude component so that the amplitude of the oscillator rises and falls with the audio signal but its frequency does not change. This is the figure of the circuit;


Nearly all the components in the 4-transistor circuit are used for both transmitting and receiving. This makes it a very economical design. The frequency-generating stage only needs the crystal to be removed and it becomes a receiver. The operation of this circuit coincides with our discussion on receiver circuits at the beginning of this article where we said the receiver was oscillating all the time, similar to a weak transmitter. A 390R is added to the emitter of the oscillator stage to reduce the activity and turn it into a receiver. The next section of the circuit is called a building block. It consists of three transistors directly coupled to produce an audio amplifier with very high gain. The first transistor is a pre-amplifier and the next two are wired as a super-alpha pair, commonly called a Darlington pair to drive the speaker transformer.

The third block is the speaker. This is a separate item because it is used as a speaker in the receive mode and a dynamic microphone in the transmit mode. A speaker can be used in reverse like this and it is called a dynamic microphone because of the coil and magnet arrangement. When you talk into the cone, the movement of the voice coil in the magnetic field produces a few millivolts output. This can be coupled to a high gain amplifier to get quite good results. When the walkie talkie is in the receive mode, the first transistor is configured as a receiver and the audio is picked off the 4k7 load resistor via a 0.47u electrolytic. It then passes through a volume control and into the three transistor amplifier. The speaker transformer couples the amplifier to the speaker and we hear the result. When the walkie talkie is in the transmit mode, the speaker is placed at the input of the audio amplifier.

Boosting Regulator Current Circuit

This is design of a pass transistor used to increase the current output of a voltage regulator IC. The circuit is design by Andy Collinson. This circuit is control by regulator IC. The IC that using in the circuit is 78xx series. This is the figure of the circuit.


The operation of the circuit is description by a power transistor is used to supply extra current to the load the regulator, maintaining a constant voltage. The currents up to 650mA will flow through the regulator, above this value and the power transistor will start to conduct, supplying the extra current to the load. This should be on an adequate heat sink as it is likely to get rather hot. Suppose you use a 12v regulator, 7812.

AM/FM Antenna Booster Circuit

This is design for antenna booster circuit. This circuit can be used to amplify the weak signal received by the antenna. Antenna for AM/FM is usually not tuned for the optimal dimension of 1/4 wavelength, since we prefer small portable size. This un-tuned antenna has very low gain, so the antenna booster circuit here is very helpful in getting better signal reception. This is the figure of the circuit.


Use around 470uH coil for L1 if you use for AM frequency (700 KHz - 1.5 MHz) and use around 20uH for SW or FM receiver. For short wave performance, using this antenna booster, you’ll get a strong signal as we get from a 20-30 feet antenna, with only a standard 18″ telescopic antenna and this booster circuit. The power supply should be bypassed by a 47nF capacitor to ground, at a point that should be chosen as close as possible to L1.