Thursday, 19 June 2014

1.3W VHF RF Amplifier 2SC1970 88-108 MHz



    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz


    This RF power amplifier is based on the transistor 2SC1970 and 2N4427. The output power is about 1.3W and the input driving power is 30-50mW. It will still get your RF signal quit far and I advice you to use a good 50 ohm resistor as dummy load. To tune this amplifier you can either use a power meter/wattmeter, SWR unit or you can do using a RF field meter.

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

    RF Amplifier Assembly

    Good grounding is very important in a RF system. I use bottom layer as Ground and I connect it with the top with wires to get a good grounding. Make sure you have some cooling at the transistor. In my case I put the 2SC1970 close to the PCB to handle the heat. With good tuning the transistor shouldn't become hot.

    RF Amplifier Printed Circuit Board

    You can download a pdf file which is the black PCB. The PCB is mirrored because the printed side side should be faced down the board during UV exposure. To the right you will find a pic showing the assembly of all components on the same board.This is how the real board should look when you are going to solder the components. It is a board made for surface mounted components, so the copper is on the top layer. I am sure you can still use hole mounted components as well.
    Grey area is copper and each component is draw in different colors all to make it easy to identify for you. The scale of the pdf is 1:1 and the picture at right is magnified with 4 times. Click on the pic to enlarge it.

    Low-Pass Filter

    Some of you might want to add a low-pass filter at the output. I have not added any extra low pass filter in my construction because I don't think it is needed. You can easy find several homepages about low pass filter and how to build them.

    1.3W VHF RF Amplifier 2SC1970 88-108 MHz

Transmitter Circuits Collection


The NOGAnaut QRP Transmitter


The crystal oscillator is the simplest form of transmitter. Normally, oscillators are used to drive buffer amplifiers and power amplifiers, which provide increased output, as well as prevent the output circuit from adversely loading the oscillator.

Most transistors exhibit a characteristic impedance different from the 50-ohm impedance of a well-tuned antenna system. An improper match between the impedance of the transistor and the load (e.g. antenna system) can cause severe power degradation, and worse, can seriously affect the signal, including shifting the oscillator frequency in unpredictable ways.

In the NOGAnaut transmitter, the 2N2222A transistor, which exhibits a characteristic impedance of approximately 200 ohms, is matched to a 50-ohm load via the pi-network filter composed of C1, C2 and L2. The values of these components were chosen to provide a close match between the 200-ohm transistor and a 50-ohm antenna (it is therefore critical that a good 50-ohm antenna system be used with this transmitter). It so happens that these values also form the familiar half-wave harmonic filter, thus satisfying FCC spurious emissions requirements.

NOGAnaut Schematic

Figure 1. NOGAnaut 80M Transmitter Schematic.

Capacitor C5 provides the necessary feedback to begin oscillation. You may find that you can operate your NOGAnaut without this capacitor--stray capacitance in the circuit provides a certain amout of feedback without C5. However, it was found during development of this circuit that the oscillator can have troubles starting at times, therefore it is recommended that you leave C5 in the circuit.

The 0.01 uF capactor, C3, serves as a DC-blocking capactor. At 3.6864 MHz, this capacitor is essentially a dead-short to RF, but blocks the DC current from flowing into the load.

This is a familiar Colpitts oscillator, operated in "common-base mode." The usual base-bypass capacitor is replaced by the capacitance of the crystal. With a 15V supply, this transmitter has been measured to deliver as much as 134 milliwatts into a perfectly matched 50-ohm load ("your mileage may vary"). With a 9V supply, about 20-50 milliwatts should be expected.

The transmitter is keyed by interrupting the positive supply voltage. You can modify this to be grounded keying, if necessary (just interrupt the negative supply voltage instead of the positive voltage). This may be necessary if you use a keyer that expects grounded or negative keying.

For a very good description of crystal oscillators, check out Solid State Design for the Radio Amateur by Wes Hayward, W7ZOI, and Doug DeMaw, W1FB. This is one of the most popular amateur radio books ever written and is packed full of practical information about how solid state circuits behave. It is published by the ARRL, and can be purchased directly from them, as well as from many electronics retailers.

Further information about pi-network filters can be found in The ARRL Electronics Data Book, by Doug DeMaw, W1FB, also published by the ARRL. This book contains most of the nuts and bolts of basic circuit design, and is a must for any ham shack.