I.F. Class A Amplifiers

Author: J.B. Hoag

There is a type of radio-frequency amplifier which is designed to amplify only one frequency region. This means that, once its circuits have been properly adjusted, they need not be changed again. As used in certain broadcast receivers, this frequency region is centered at 455,000 cycles per second (455 kHz), a value which is the intermediate between audio and broadcast frequencies; hence the name intermediate-frequency amplifiers. There are two main types of i.f. amplifiers, one for phone reception, where the fixed band of frequencies has a definite width, over which the amplification is as nearly uniform as possible, and the other for the reception of dots and dashes where the band width is made as narrow as possible. These cases are illustrated in Fig. 27 B.

Fig. 27 B. I. F. band widths

I.F. amplifiers usually contain one or two stages. Figure 27 C shows the circuit of a one-stage i.f. amplifier of the type which covers a band of frequencies.

Fig. 27 C. A one-stage i.f. amplifier. T = variable-μ pentode. Typical values; C1, C3, C4, C5 = 0.1 μfd at 455 kHz (= 0.01 μfd at 1600 kHz or higher); R1 = 300 ohms; R2 = 0.1 megohm; R = 2000 ohms

It is to be noted that the circuit differs from that of a tuneable r.f. amplifier only in that there are additional condensers CC across the primaries of the transformers, and that a decoupling resistor R has been added to prevent stray feedback. The tuning units in the shields (dotted lines) use air-core or powdered-iron-core universal-wound coils, the latter offering greater selectivity and gain. The adjustment of these tuned circuits is made by varying the inductance of the iron-core coils or by changing the capacitances across their primaries and secondaries. High stability is important in order to keep all resonant circuits tuned to the same frequency. Any frequency " drift" will reduce the selectivity and gain. More complicated LC circuits are sometimes used to sharpen the sides of the frequency response curve (Fig. 27 B(a)) and to make its top flatter.

In other i.f. amplifiers, it is desirable to sharpen the tuning as much as possible, as in Fig. 27 B(b), in order to amplify only one, rather than a band of frequencies. Crystal filters offer the best method of obtaining this high selectivity. It is to be remembered that a quartz crystal acts like a series circuit of very high Q. A crystal of proper thickness to resonate at the desired i.f. is inserted in the coupling circuits between two tubes, as in Fig. 27 D.

Fig. 27 D. A crystal-filter circuit. T1 = center-tapped i.f. input transformer with high-L, primary closely coupled to secondary. C1 and Cs = 100 pF. C2 = 10 to 15 pF.

It will be noticed that the coupling circuit forms a bridge, with the upper and lower halves of L₂ on one side of ground and with the crystal, X, and condenser, C₂, balancing each other. Adjustment is made so that C₂ is equal to the capacitance of the holder of the crystal. This balance prevents undesired frequencies from reaching the output circuit. Condenser C₁ is used to control the selectivity of the circuits, i.e., the sharpness of the response curve, (b) in Fig. 27 B. The selectivity is a minimum when the circuit L₂C₁ is tuned to the crystal frequency, and increases as C₁ is changed from this condition.