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Some points of the circuit - Dynamic operating conditions of the transistor - Valves with a heated cathode .. Electronics

Some points of the circuit

Transistors are connected in circuits in ways some of which are shown in Figure (113). Figure (A) shows the connection of the transistor in the circuit in a way that makes the input signal between the emitter and the base, and the output signal between the collector and the base. As for the resistance M, it is the load resistance through which we obtain the required power, and it can be replaced by a tuning coil or any other device. In this type of connection, the base is shared between the input and the output, so it is called the common base and grounded arrangement.

Figure (B) shows the common-emitter-grounded transistor connection system, where the input signal is inserted between the base and the emitter, and we get the output signal between the collector and the emitter. In the same way, Figure (C) shows the common-emitter-grounded transistor connection system, where the input signal is inserted between the base and the emitter. • In all connection methods (A, B, C), transistors of the type SM were used. As for the method shown in Figure (D), an MS transistor was used, in which the signal is inserted between the base and the emitter, and we get the output signal through the collector and emitter circuits. This circuit is called the common-emitter-grounded transistor. Some of these connection methods provide us with high power gain, and some of them work efficiently at high frequencies. Others are characterized by a high input impedance and a small output impedance, or vice versa. The impedance is the value of the "resistance" at the frequency used (operating frequency). Figure (114) shows the use of the transistor as a current generator in which MA plays the role of the input resistance, and M2 plays the role of the output resistance. In this circuit, the maximum possible power can be obtained when the input and output circuits match both M1 and M2.

Dynamic operating conditions of the transistor

Figure (115) shows how to use a common-emitter transistor, and how to obtain the base bias using the resistors M1 and M2. As for the output signal, it is obtained via the resistor M3. The isolation capacitors S A and S2 are useful in isolating the input and output stages from the influence of direct current. As for the resistor M, it gives the emitter a negative bias to ensure stable operating conditions.

It is worth noting that the base current Tq in this case as well as the collector current Tmj have a stable value, and it is also noted that the collector current is equal to the sum of both the emitter current and the base current. The base current may be in the opposite direction to the emitter current and the signal to be amplified enters through the capacitor S1, so the value of the base current changes according to the intensity of the signal current as in Figure (116 - Tq). Any slight increase in the base current leads to a large increase in the emitter and collector currents, which results in a fluctuation in the value of the collector current that is exactly similar to the fluctuation of the signal to be amplified. A moving coil meter is usually used to indicate the average value of the currents passing through (115), and an oscilloscope may be used to determine the dynamic operating conditions of the circuit.

113 - Methods of using the transistor.
114 - Impedance matching.
115 - Static operating conditions of the transistor.
116 - Dynamic operation of the transistor during amplification.