R-F Amplifiers

Author: Leonard Krugman

Transistor r-f amplifier circuits, like their counterpart vacuum-tube circuit types, are most often used for improving the gain, over-all signal-to-noise ratio, or selectivity characteristic of a multistage circuit.

Figure 7-7 illustrates a typical transistor r-f amplifier circuit. The design is basically the same as that of an i-f amplifier. The chief problem is the selection of a transistor having a sufficiently high a cutoff. The power gain of a r-f amplifier is inversely proportional to the frequency. In a typical case a transistor having a maximum gain of 40 db at 10 mc will have a maximum gain of 20 db at 40 mc. There are two critical parameters, the emitter bias and the base resistance. The base resistance is determined by the physical construction of the transistor and, therefore, low base-resistance transistors, designed specially for high frequency applications, should be used. The importance of emitter bias was considered in the analysis of oscillator circuits. The bias should be selected to be far enough away from the unstable region of the characteristics to avoid oscillation, and yet not so far away that the gain is very low. Special care must be taken to avoid introducing stray capacitance into the emitter input circuit. These reactances tend to lower the input emitter impedance, and thereby decrease circuit stability.

Fig. 7-7. Typical transistor r-f amplifier.