10 Watt CB Linear amplifier

Posted on Apr 16, 2012

It is quite easy to get a watt or more with very simple equipment, but to get more than 5 watts becomes a little more difficult. This article describes a 10 watt linear amplifier that is capable of delivering over 15 watts into 50 ohms and uses cheap plastic transistors that are used in CB equipment. If you have difficulty in finding 2SC2078 then lift the lid of your CB set to find a suitable alternative. The bias generator transistor, TR4, is marked TIP31 in the circuit diagram, but here you can use just about anything that will fit. You could even use another 2SC2078, if you had money to burn, but more practical components would be TIP41, TIP3055, MJE3055. All that matters is that it will pass up to 1 Ampere and have the correct base details in a TO220 case.

10 Watt CB Linear amplifier
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The amplifier has a wide bandwidth, from 1.8 MHz through to over 30 MHz. The drive level required is only about 2 - 5 mW under 14 MHz, rising to 10 mW at 30 MHz. You can therefore make a good QRP CW rig with nothing more than this PA and a simple crystal oscillator. I can achieve 12 watts out of mine using a 10-turn loop around my Grid Dip Oscillator! I can get over 15 watts from my Marconi signal generator, but above about 12 watts it is being over-driven an may not be very nice to look at on the spectrum analyser. The circuit was designed to be as clean as possible. The circuit was originally designed to accompany my phasing-type SSB exciter, but it can be used to amplify almost any HF signal from 2mW in the HF band. Note that there is needed a Low-Pass filter between the amplifier and antenna. This is a requirement for ALL transmitters. L1 matches a 50-Ohm input to the 10-Ohm input impedance of TR1. The output of TR1 is coupled via T1 to TR2 and TR3 bases. T1 also transforms the impedance to the very low input impedance of these two transistors, so the secondary winding must be quite thick wire. TR2 and TR3 amplify the signal even further, up to about 12-Watts. They are in a push-pull configuration so T1 secondary and T2 primary must both be symetrical. T2 increases the output impedance to 50-Ohms. TR1, TR2 and TR3 all have a 330R and 10n between the collector and base terminals. This is needed for stability and...

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