If for example, the amplifier is driven into clipping, large spikes are generated at point A as the NFB attempts to compensate for the flattened peaks at the output. In practice this effect is quite easy to detect with an oscilloscope. During this period, the collector of Q14 rises to the full supply voltage (connection B), energizing LED1 via the current limiting resistor R28. This stored voltage holds Q14 in conduction for an extended period, while C20 is discharged by R27 and the transistor's base current. When sufficient energy arrives to bias Q13 ON momentarily, its collector falls - charging C20. The signal from point A is AC coupled to Q13 by C19 and R25 (which raises the circuit's input impedance). The circuit basically detects transient AC signals, extends the pulse length and illuminates the LED for that period. Now if the signal at the collector of Q6 represents the corrected output (that is, less the distortion components), and Q5 and Q6 represent a true differential amplifier, we can expect the signal difference to appear at the collector of Q5 - or in effect, the distortion components which are cancelled at the output - thanks to the overall NFB. This occurs since the other half of the differential pair Q6 generates the full output signal swing - if we consider the MOSFETs as electrically transparent for the moment. The signal at the collector of Q5 (point A) represents the amplifier's overall error or correction voltage. In practice, the circuit will energize LED1 if the amp's distortion rises above about 0.05% - a lower figure than many amplifiers can ever hope to achieve! While its operation is very straightforward, the circuit utilises a complex effect which occurs within the amplifier during overload conditions. we have also included a novel overload detector circuit based around transistors Q13 and Q14. An externally hosted image should be here but it was not working when we last tested it.
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