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Audio equipment, radio, television, tape recorders... Electronics

Amplifiers

Amplifying signals by audio amplifiers means strengthening them. Amplifiers operate in the frequency range between 10 oscillations per second and 15,000 oscillations per second. Musical frequencies lie between 50 oscillations per second and 2,000 oscillations per second, but their "harmonics" are regenerated to improve musical performance. The electrical output from a gramophone receiver, when listening to a record, may exceed a few millivolts. Therefore, we need an amplifier that amplifies and strengthens the small output signal so that the loudspeaker can be supplied with the power it needs (117). Figure (118) represents an audio amplifier that uses a valve. It is worth noting that the negative bias of its control network is three-way, equal to 2 volts, and can be obtained by means of the resistor M1. If a current of one milliamp passes between the cathode and the anode, it will pass through the anode circuit through the resistor M2, which gives a voltage on both ends equal to 100 volts according to Ohm's law (V = V × M). If we assume that a positive signal with a peak value of one volt (V) has reached the control network (1119), and the negative bias of the network is -1 volt, then the anode current intensity increases to become 1.5 milliamps flowing in the resistor M2, in addition to the voltage drop changing to become 150 volts.

Thus, the voltage applied to the elevator = 150 100 = 50 volts. However, if the signal connected to the network is negative and its peak value is one volt, the negative bias of the network increases to become one volt, and thus the intensity of the elevator current decreases to become 0.5 milliamps, and the voltage drop in the resistor M2 becomes 50 volts, meaning that the elevator voltage is 200 - 50 = 150 volts. In short, when a signal with a voltage ranging between +1 volt and -1 volt is connected to the network, we get a highly amplified signal in the elevator circuit ranging between 150 volts and 50 volts. The capacitor S1 (120) is a coupling capacitor (horn capacitor) that works to block the passage of any current at the same time. It has a small reactance at continuous frequencies, but it is useful in the same way in feeding the next stage with the amplified signal.

118 - Audio amplifier with a triode.
119 - Signal amplification by a triode.
120 - The coupling capacitors prevent the passage of direct current to the next stages.

We usually need the control network to have a negative voltage, even if it is a small value. Therefore, we find in Figure (121) that to obtain a negative possession for the network, the cathode must be connected to the bias resistor M3 to give a voltage drop of 2 volts, and a current of one milliamp passes through it. Using Ohm's law G / T = M. The value of M3 becomes = 2000 ohms to achieve the required operating conditions . As for the capacitor S3 connected across the resistor M3, its task is to connect the cathode to the ground at the operating frequency. The pentode is usually used instead of the triode to obtain a certain percentage of the signal with sufficient accuracy and at the desired value. In this case, it is used as a loudness regulator. As for MS, it is the resistor feeding the blocking network S?, while Ss is the blocking network capacitor that passes the signals with audio frequencies.

121 - The bias voltage arises from the passage of current in M3.

122- A pentode to control the loudness.

It is worth noting that the blocking network voltage is equal to the cathode voltage itself, and the amplified signal is obtained through the load resistance M2. The valve and all other components, resistors and capacitors attached to it are usually called the "stage", and it is said that the "stage gain" or the resulting amplification is less than the amplification factor of the valve itself. The internal resistance of the valve is (123A), and the resistance M2 is chosen to match this resistance, and in this case it is called the "limit" resistance or the optimal load. Figure (123B) shows the amplification circuit itself, which includes a five-way valve in addition to a transformer and a loudspeaker. The ratio of the number of turns in the transformer is chosen in a way that makes it provide the optimal load that we choose. The impedance of the loudspeaker ranges between 2 and 15 ohms. The ratio of the number of turns in the transformer ranges between 5:1 and 60:01

123 - Load resistance in the valve riser.

124 - Three-stage amplifier.

Successive stages

We may usually need more than one stage of amplification in transmitting or receiving devices, etc. We may use two or more stages (124), the first of which is the primary amplification stage A (122), and the next amplification stage B is represented by the amplification circuit shown in Figure (121). As for the amplification stage C, the output is used to operate the loudspeaker.