دارات المازج (mixer) .. كتاب التلفزيون الملون والعادي

E- 7 The mixer circuits (MIXER):

It is one of the voter circuits in addition to the RF amplification circuit. The oscillator (Fig. Intermediate Frequency (IF). The mixer may be an old vacuum valve, a transistor, or a UHF diode. These are non-linear elements that work according to the super heroin method. The resultant of the mixer is the sum of the two signals and the difference between them, and the desired signal IF whose frequency is equal to the frequency of the oscillator - the frequency of the RF signal. The circuit then tunes to the smaller frequency, ie the difference between the two frequencies.
Transistor mixer circuit: There are several ways to connect the transistor, including the common base method, and it has several ways to inject the local oscillator signal - the common base method is acceptable because its work is stable.

The common emitter method is also used, in which the RF and mixer signals can be injected into the base or the emitter, and sometimes a signal is injected through the base and the other signal through the emitter, and the super heterodyne collision transistor is used.

Figure (5-20) is also used with the CASCODE method, which is a transistor connected by the common emitter method with another connected by the common base method. With a direct connection between the emitter and the collector of the two transistors, it achieves high gain in amplification, good isolation between the input and the output, and little noise. Figure (5-20) shows a Cascode mixer circuit, where the oscillator signal and the RF signal are applied to
The base of the transistor is a common emitter (Q2) and the intermediate frequency (IF) is taken from the collector (Q1). When the voter is placed on the (UHF) range, the IF signal resulting from the UHF voter is fed to the transistor (Q2) via the diode isolation DI. Thus, the cascade amplifier provides an additional amplification stage for the intermediate frequency IF for UHF stations. A UHF voter may not contain an RF amplification stage.

The base 01 is grounded by the capacitor -C1, and the base bias is secured for the transistor Q1 and Q2 through the voltage divider 1 2 R3, and the emitter stability is secured by R4 and C2 - and it is noted that there is no need for neutralization in this circuit due to the low entrance impedance in the grounding stage of the base.

The use of a MOSFET transistor in the mixer: The use of a two-gate Mosfet transistor is ideal in the synthesizer amplifier circuit - one of the two gates is used for the RF signal input. The second gate of the oscillator signal - the two input signals can pass through one gate, and in this case the second gate is used as a barrier to isolate the entrance and the exit in the mixer stage.

The use of the diode in the UHF mixer: We notice in the simple diagram Figure (5 - (21) the UHF mixer diode. The RF signal of the UHF field is applied to the diode of the mixer CR1 from file L28 - and the oscillator signal is connected to the same diode via L31. Since the diode is an element It is non-linear, in which the super-heterodene conflict occurs and produces the two-frequency difference, which is the intermediate frequency IF - and the binary used in this circuit is what is called the Schottky barrier, which has great advantages in the UHF mixer circuit.
Circuit of using transistors to amplify the RF signal: The transistor is used to amplify the RF signal. Its connection can be in the way of the grounded (common) radiator or the grounded (common) base - it is similar to the fuse circuit that is in the common cathode way and its gain is greater and needs to be neutralized, while in the common base method similar to the common network it does not need to be neutralized (Nutralizing)

co-radiator method:

In this typical circuit, Figure (5) - (22), the collector signal voltage differs in its page 180 from the base voltage signal. The capacitive resistance between the collector and the base acts as a negative feedback - that is, the incoming collector voltage signal. By means of the capacitive resistance between the base and the collector, it cancels part of the voltage of the resulting signal, and leads to weakening the gain a lot. In order to avoid this, the base must be fed a signal of the same page difference as the signal, to cancel the negative feedback. Therefore, it is fed from the secondary of the transformer by means of a neutralizing capacitor.