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Capacitors - Amplifier - Schematic diagrams of electronic circuits .. Electronics

Capacitors

A capacitor consists of two parallel and close metal plates that do not touch each other. The capacitor has the ability to store an electrical charge (45).

If a certain voltage is applied between the two plates S - S, the number of electrons on one of them increases, while their number on the other plate decreases.

In this case, an electrostatic voltage is generated on the insulator separating them, and the capacitor in this case becomes "charged" and its capacity depends on its "capacitance" or its thirst to store the charge. The capacitance of the capacitor is measured in units of farads. However, if the two plates S - S are connected together, the capacitor discharges its charge immediately. The capacitor's ability to retain its charge, i.e. its capacity, can be increased by reducing the distance between the two plates without contact between them, or by increasing the surface area of ​​the two plates that make it up. The insulator between the plates is called the “electric insulator” and is made of mica and ceramic. The plates are made of aluminum foil wrapped in cylindrical coils to increase their surface area and insulated from each other by paper (46). Capacitors can also be made by depositing a conductive metal surface on both sides of the insulator.

As for electrolytic capacitors, they are made so that the direction of the voltage applied to them is correct. This type of capacitors has capacitances

45 - Fixed capacitor.
46 - Some capacitors used in electronic devices.
47 - Variable capacitor.
48 - Two single-column capacitors and a trimmer capacitor.

Fixed, do not change, and therefore they are called "fixed capacitors". We often need to change the capacitance of the capacitor during operation, and in this case it is preferable to use "variable capacitors". The variable capacitor contains a fixed plate and another movable plate fixed on an axis or rotating leg (47) that can be turned to change the overlapping area of ​​the movable plates and thus change the capacitance of the capacitor. It is worth noting that the capacitance of the capacitor is zero when the two plates become. In the case of non-overlapping, that is, when their overlapping area becomes zero.

If we want to tune two circuits at the same time, as is usually the case in receivers, we use group capacitors, i.e. those that contain two or more groups (48). These capacitors can be manufactured using several fixed plates, the total capacitance of each branch of which can be increased until we obtain the required value for the total capacitance. Sometimes it is necessary to adjust the capacitance of the capacitor when starting to put the circuit in its correct operating conditions. In this case, a trimming capacitor or "refiner" is used, which can be adjusted using a screwdriver to bring the moving plates closer to the fixed plates.

Amplifier

The circuit shown in Figure (49) constitutes the first stage designated for amplifying audio frequencies in the receiver or in the wide-band audio amplifier. It works to amplify the voltage of the audio signals that it obtains from the phonograph pickup or from the detector in the receiver.

In this circuit, capacitor S A isolates the DC circuits from the base of the transistor, which is fed by the two resistors M2 and M3. The resistor M3 is responsible for feeding the emitter B with a relatively small negative holding voltage, the value of which increases when the emitter current intensity increases. This helps to stabilize the operating conditions in it. As for capacitor S2, it connects the emitter to the ground at audio frequencies. In addition, the current coming out of the collector M passes through the resistor with which the amplified signals appear before being taken to the next stage via capacitor S. Figure (100) shows the actual structure of the components drawn in Figure (49). Figure (50b) gives a more consistent and specific shape for the same circuit.

49 - Single-stage transistor amplifier ".
50 - Practical assembly of the circuit in Figure 49.