I-F Amplifiers

Author: Leonard Krugman

In general, the upper frequency limit of the junction transistor is considerably lower than the limits of the point-contact type. On the other hand, the junction type has a lower noise factor, and better stability in some applications. These factors frequently make it advantageous to use the junction transistor in some high frequency applications even if an additional stage or two may be required.

Fig. 7-4. Typical transistor i-f amplifier.

Figure 7-4 illustrates one stable form of i-f amplifier stage using a WE 1752 N-P-N transistor. The operating frequency is 455 kc, and the gain is 18 db.

Due to the natural regenerative feedback path through the collector junction capacitance and the base resistance, and the close coupling between the input and output circuits, the circuit, when connected in tandem, is likely to oscillate unless the stage is carefully tuned. The alignment procedure is easiest if the last stage is tuned first. For an input resistance R_g = 500 ohms, the output resistance r₀ averages 12,500 ohms, and C_c is about 15 pF.

The cascading of transistor i-f is more complicated than that of vacuum tubes. The main contributing factors are the effect of the output load on the input impedance, and the effect of the generator impedance on the output impedance. These factors show up largely in the design of interstage coupling networks.