The IF amplifier provides most of the gain in a superhet receiver, and it also supplies the narrowest bandwidth in the signal chain. Thus, the IF amplifier is chiefly responsible for both the sensitivity and selectivity nf the radio receiver. In fancy radio receivers, the IF amplifier can be quite complex, as witnessed by the designs used in modern shortwave and ham radio receivers in the kilobuck range. This one is a con siderably simpler design, but is nonetheless effective. For those who need a more complex design, I recommend various editions of Bill OrrTs The Radio Handbook or The ARRL Radio Amateur's Handbook.
Figure 5-21 shows the basic circuit for an IF amplifier building block. For receivers requiring moderate gain, for example, an AM receiver intended primarily for local reception, then only one stage needs to be used. In shortwave receivers, or wherever superior sensitivity is sought, it is permissible to cascade two or three such stages to boost amplification. The gain of this stage is on the order of 50 dB, and some projects might need as much as 80 to 110 dB.
5-21 Integrated circuit IF amplifier.
The basis for the IF amplifier building block is the MC-1350P gain block chip (this chip is also available as NTE replacement part number NTE-746).
The input circuit is tuned by either a single- or double-tuned transformer of the same sort used at the output of the NE-602 stage covered previously. The secondaty Impedance of this transformer should be on the order of 1000 ft in order to properly match the input impedance of the MC-1350P. The output of the MC-1350P is also tuned by a similar IF transformer.
The gain in the M01350P device.is controlled by the voltage applied to pin 5. This voltage must be varied between +3 and +9 Vdc in order to set the overall gain of the stage. The gain-control voltage supplied to pin 5 can be derived from a manual gain control (i.e., potentiometer voltage divider), automatic gain control (AGC) circuit, or a combination of both.
Figure 5-21 shows a sample manual gain control circuit based on a 10-kfl potentiometer and a 5.1-kfi fixed resistor. If a fixed gain is desired, then a fixed-ratio voltage-divider chain can be used instead. Also, it is not strictly necessary to use the same gain control for all stages in the combined IF amplifier. You can, for example, make one stage a variable-gain circuit and the other(s) fixed-gain.
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