What is wrong with negative feedback?

Kirkus, my technique for measuring propagation delay is simple: compare the input to output while using a squarewave source. Observe the difference in time between the rising input waveform and the rising output waveform. That's the delay time. I have yet to see an amplifier where I could not see that on the 'scope.

Since negative feedback only exists if the open-loop (feedback-free) gain is above unity, and since the open-loop response falls off at 6dB/octave . . . the input/output phase response must be 90 degrees or less. So if we're going to talk about "transit time", how would you define that?

It really seems to me that something is glossed over here. In this model phase and time become the same, and is inadequate to explain the behavior of an amplifier that has wide (+200KHz) open loop bandwidth. In such amplifiers the model below falls apart:

Since we know that comparing the phase at the input the output will give us 90 degrees, the "transit time" at 100KHz will be 2500 nanoseconds. At 200KHz, it will be 1250 nanoseconds. At 20KHz, it will be 25000 nanoseconds. So it seems that talking about "transit time", or "propegation delay"[sic], or "delayed feedback", or whatever . . . is a wholly inadequate way of understanding what's going on. Rather, classical Control Theory uses phase relationships to analyze feedback.

Propagation Delay does not alter with frequency anywhere near the audio band, and at those frequencies the delay time is easily measurable. In fact, we can see that at low frequencies feedback works pretty well, but as frequency increases, the feedback is progressively inadequate due to the fixed propagation delay of the circuit having a larger effect as the waveform time decreases. This introduces a time-domain distortion- ringing and odd-ordered harmonic enhancement. It is this phenomena that requires networks in many amplifier designs to prevent negative feedback from becoming positive feedback due to the phase at very high frequencies that are out-of-band but can cause the amp to go into oscillation if not addressed. The model you are proposing relies on propagation time being mutable, which it certainly is not. I'm with Spectron on this one. Sounds to me like control theory is being misapplied here.

The model you are proposing relies on propagation time being mutable, which it certainly is not.

Atmasphere, forgive me if I'm being a snot . . . but I think you need to brush up on some basic electrical theory. Pole/zero networks do indeed have different delays based on frequency. If you don't believe me, try constructing a simple R-C lowpass network with, say, a .47uF capacitor and a 750 ohm resistor. Compare the "Propegation Delay" between input to output, using SINEWAVES, at 10KHz and 20KHz. For the former, you will find it to be about 24uS, for the latter about 12uS. For both, the phase shift is about 90 degrees. Or you can do it in SPICE in just a few minutes.

Again, some basics here. A real-world amplifier circuit contains mechanisms that produce both frequency-dependent and frequency-independent delays. In a typical well-designed Miller-compensated amplifier, the goal is to choose the compensation capacitor so that the frequency-independent delay is completely swamped by the frequency-dependent delay of a first-order slope, yielding a phase margin of 90 degrees at all frequencies above unity gain.

Here's the conceptual error with your square-wave timing test. If we assume that it's indeed a perfect square-wave on input, and the circuit in question doesn't have infinate bandwidth . . . then the output square-wave will have a longer rise time and more rounded leading edge than the input. So we set up our scope, and use the markers to decide where to measure on the x-axis. For the input side, it's easy to locate the marker because the rise-time is infinately short. But on the output, it's comparatively slopey and rounded . . . so when you look at the output and place the marker, the exact placement across the slope determines for which frequency you're measuring the delay. If you just place the marker where it "looks about right", then you're simply meauring the delay of "kinda one of those frequencies" . . . one of an infinate number contained in the perfect squarewave on the input.

But really the time-honored method is to use X/Y mode on your scope to compare the phase as you vary the frequency of a sinewave. You can then CALCULATE the precise delay for any frequency, based on phase. And no, there won't be just one number.

If a person cares to look in any book covering filter theory, they'll find gain/phase graphs that illustrate propagation or group delay. For lowpass filters, which is what most amplifers are classified as, low frequencies have little or even no delay while higher frequencies have more, such as the nominal 45 degree phase lag at the -3db point. A phase lag corresponds to a delay.

Kirkus you know who Atmasphere is right?

Negative feedback falls into the same category as damping factor both which alot of people dont understand including myself to a point,my counterpoint amp has a damping factor of i think 70 while the great or my bias is NOT GREAT digital amps go on and on about the high amounts of damping factor they trump on their stats.,My Counterpoint has plenty of bass ,it just has to be on the recording in the first place.