In the mid seventies was a concept developed in Europe about the advantages of low global feedback amplifiers. It is said that feedback amplifiers have problems because the signal fed back "takes" a while back at the entrance and this causes loss of quality and ¨ ° to sound natural. You said that amortiguaiento factor (damping factor) improved low-feedback amplifiers.
Today the discussion continues, but restricted to the amount of feedback.
The reality is that you can get excellent results with a synergy of local and global feedback feedback. To extend the latter
explore a bit the concept of delay between input and output. " Electromagnetic energy travels in a conductor at a speed of 230,000 km / s. This means say that a cable of 23 cm long (distance comparable to the size of an amplifier) \u200b\u200bis traversed by an electromagnetic wave in 1 nanosecond, which is the period of a 1Ghz signal. A 20,000 Hz signal has a wavelength within a conductor of 11.5 km or 11,500 meters. Therefore the concept of "delay" is not applicable to an audio amplifier, but try running a 11,500 m long cable into your amplifier.
When feedback or an article you read damping factor "delay" so read is "phase", and the phase is inherent in the reactors' own input devices drivers and output and transfer characteristics of the fed back locally and globally.
seriously can not be assessed if the feedback is high or low if you consider what kind of devices and topologies are involved in the design. Let
History:
problems with the amplifiers of the early 70's are two: the design criteria and semiconductor devices that were available.
be considered an audio amplifier and an operational amplifier that is high gain and a dominant pole, generally in the input stage, which by the amp stable to close the loop in any condition. Moreover, we see in many designs of an operational period, type UA741, the input of the circuit to take advantage of the differential pair and the dominant pole.
know that a dominant pole at low frequency, usually 10Hz, brings two problems: one consisting of two events of similar nature, the slew-rate and the reduction of intermodulation, and the other, the damping factor degradation by falling the loop gain at medium frequencies.
should be taken into account for the correct understanding that these dynamic processes, such as slew-rate and cutting intermodulation are independent phenomena for feedback, the show, the feedback is cut (see *)
To solve these problems changed the design criteria. Today is seeking the power band width, ie maximum excursion of tension, is greater than the small-signal, ie when it falls midrange 3db. To achieve this criterion
need two things: first input differential pair with discrete components, resistors degenerate emitter, or appropriate local realimenación bias current (see *) and second drivers and output devices very fast ie MOSFETs, to take the pole of the input stage, which is no longer dominant, up to about 15000 Hz
With these two design criteria, the feedback amplifier multiple discrete components and transistors to the output MOSFETs is the best option as the audio amplifier to date.
* A complete analysis can be seen in Gray & Meyer "Analysis and Design of Analog Integrated Circuits" John Wiley and Sons. P. 1977. 541 and following
Subject: ESS.
Student: Pedro Jose Contreras Urbina
Source: http://www.vn-amps.com.ar/zero.htm
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