This project is a highly useful RF amplifier that can be used in a variety of ways. It can be used as a preamplifier for receivers operating in the 3- to 30-MHz shortwave band. It can also be used as a postamplifier following filters, mixers, and other devices that have an attenuation factor. It is common, for example, to find that, mixers and crystal filters have a signal loss of 5 to 8 dB (this is called insertion loss). An amplifier following these devices will overcome that loss. The amplifier can also be used to boost the output level of signal generator and oscillator circuits. In this service, it can be used either alone, in its own shielded container, or as part of another circuit containing an oscillator circuit.
The circuit is shown in Fig. 7-16A. This circuit was originated by Hayward and used extensively by Doug DeMaw in various projects. The transistor (Q0 is a 2N5179 broadband EF transistor. It can be replaced by the NTE-316 or ECG-316 devices, if the original is not available to you. The NTE and ECG devices are intended for service and maintenance replacement applications, so they are often sold by the local electronic parts distributors.
This amplifier has two important characteristics: the degenerative feedback in the emitter circuit and the feedback from collector to base. Degenerative, or negative, feedback is used in amplifiers to reduce distortion (i.e., make it more linear) and to stabilize the amplifier. One of the negative feedback mechanisms of this amplifier is seen in the emitter. The emitter resistance consists of two resistors, R5 is 10 A and R& is 100 il. In most amplifier circuits, the emitter resistor is bypassed by a capacitor to set the emitter of the transistor at ground potential for RF signals, white keeping it at the dc level set by the resistance. In normal situations, the reactance of the capacitor should be not more than one-tenth the resistance of the emitter resistor. The 10-ft portion of the total resistance is left unbypassed, forming a small amount of negative feedback.
The collector to base feedback is accomplished by two means. First, a resistor-capacitor network (Ri/RdCo) is used; second, a 1:1 broadband RF transformer (Tj) is used. This transformer can be homemade. Wind 15 bifilar turns of no. 26 enameled wire on a toroidal core, such as the T-50-2 (RED) or T-50-6 (YEL); smaller cores can also be used.
The circuit can be built on perforated wireboard that has a grid of holes on 0.100-in centers. Alternatively, you can use the printed circuit board pattern shown in Fig. 7-16B. In this version of the project, the PC board is designed for use with a
7-16 Feedback NPN transistor preampliGer.
Mini-Circuits 1:1 broadband EF transformer Alternatively, use a homebrew transformer made on a small toroidal core. Use the size 37 core, with no 36 enameled wire. As in the previous case, make the two windings bifilar.
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