Examples and Analysis of Application Circuits of High Frequency MOS Field Effect Transistors

The first use of high-frequency MOS field effect transistors, and currently the most used are the RF stage and the first mixing stage of FM receivers; with the appearance of cascaded transistors, they have begun to be used in the RF stage of VHF TVs. , It plays a role in removing all kinds of clutter and improving the characteristics of cross-talk modulation. Figure 2.45 is an example of using MOS field effect transistors to form a frequency modulation pre-circuit in the RF stage and the mixing stage. Since automatic gain control circuits are widely used in the radio frequency stage of VHF TVs, some people think that bipolar circuits are sufficient for the mixing stage.

Applied to the intermediate frequency stage, the bias circuit of the first intermediate frequency stage of the TV can be simplified, the intermediate frequency transformer is easy to obtain a high Q value, and the frequency change of the automatic gain control is small. High frequency MOS field effect transistor application circuit. Despite these characteristics (except for Figure 2.46), the intermediate frequency stage has a trend towards integrated circuits. This kind of circuit has no advantages in terms of cost.

The application examples of the double-grid type are: the product detector with beat frequency added to the second grid 29); as shown in Figure 2.47, the control signal-subcarrier is added to the first grid, and chroma is added to the second grid. Color demodulator 30)~32) for signal phase discrimination.

In the application of frequency doubling, in addition to the same method of use as the vacuum tube, there is also the use of square law characteristics to reversely excite two MOS field effect transistors, so that the leakage current flows into the common load, thus canceling the broadband of the fundamental component. Examples of frequency multipliers.

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