محولات الربط - التغذية المرتدة السالبة - المضخمات السمعية .. الإلكترونيات

Link Transformers - Negative Feedback -
Audio Amplifiers .. Electronics

Link Transformers

Figure (125) shows a link transformer whose primary coils are connected to the audio frequency signal from the previous stage. As for the secondary coils of the transformer, they feed the control network of the valve of the next stage with the required signal.

This system shown is not widely used these days. If one end of the secondary coil is negative in one half of the signal wave, the other end is positive and the polarity of each end is reversed in the other half of the wave.

Figure (126) shows a link transformer with a secondary coil containing interconnection points. Valve S1 receives a negative signal in one half of the wave, but valve S? The positive signal is obtained, then the polarity is reversed during the other half-wave, and the intensity of the current passing through the anode circuit in valve S increases, while the intensity of the anode current in valve S2 decreases. The output of the two valves, which are half-power, is taken from the output of valves S, S and are collected together on the primary coil of the output transformer, while the secondary coil of the transformer is connected to the loudspeaker. This output method is usually called "push-pull", and in this method the value of the negative bias of the network of either valve can be increased. When its polarity is reversed in the other half-cycle and becomes positive, it allows the current to pass through only one cycle. This amplification method is called "model B" amplification and is characterized by a large output power while saving the battery power consumed.

125 - Audio link transformer.
126 - Push-pull circuit.

It is possible to use resistors and capacitors instead of the previous connection transformer. In Figure (127), we find that the first valve receives the audio frequency signal via the network SH A. If the anode current increases, the drop increases across each of the two resistors MA and M2. Accordingly, point S becomes negative and point S becomes positive. As for the signals, they pass through the capacitors SA and S2. The resistors M3 and M4 play the role of the network resistors, while the resistor M5 ensures the negative bias of the cathode. It is possible to take part of the output signal and return it to become the opposite of the original signal, since the properties of some components, such as capacitors, change according to the change in frequency. If we assume that the amplifier gives most of its power when amplifying high tones, i.e. it has the ability to amplify high frequencies, and if the negative feedback process is carried out by opposing part of the external signals to the input signals, then this negative feedback reduces the strength of the high-frequency signals. While it affects low frequency signals to a lesser extent.

The overall result is amplification fidelity, i.e. better "reliability" in it. The feedback in Figure (128) is done through the resistance that it transmits to the valve cathode.

127 - Push-pull circuit in a normal amplifier.

128 - Negative feedback.