FEEDBACK On Feedback

Negative feedback is a technique where the output of a circuit is compared with its input, and a small difference often occurs. This difference becomes just a bit of correction that modifies the instantaneous voltage that is about to journey through the amplifier. This new voltage is shaped just right so that when it goes through the non-linearities of the amplifier it comes out just like the input signal, perfect. When all is done properly, there is no penalty to pay for this. The input signal comes in on one terminal (the base of a transistor or the grid of a tube), the feedback is often applied to the terminal beneath it (the emitter or the cathode), and the "corrected signal" appears at the collector or plate. The feedback provides the "correcting information" in perfect time. The amount of correction is generally just a few percent. It is exactly equal to the open-loop distortion of the amplifier before the feedback loop was closed.

Some look at feedback and think that since the output is being compared to the input, only differences or errors generate any action by the circuit with negative feedback. This would be incorrect as this makes it sound like the detector is idle waiting for an error to occur. It is not, in fact, it is passing the main signal all the time. In the example from the above paragraph, the input simply passes through the comparator unchanged if no change is needed. It really does leave it alone, I promise.

Now is too much feedback a bad thing? Not necessarily. A cathode or emitter follower has 100% feedback. It does this because the output terminal simply follows the input terminal. Kind of like a dog on a very short leash. If the leash gets too long, as it might going through a whole amplifier, things can get out of hand. That is where feedback gets a bad name. It can lead to good test bench results and poor field results as any apparatus will that is tuned for one condition. A designer can easily get great performance into an 8 ohm resistor only to have

miserable spurious oscillations in the field when the load looks nothing like an 8 ohm resistor. It is indeed challenging to make a feedback amplifier stable in the real world. However, it can be and has been done.

AFTER THE FACT

It is often said that feedback is bad because it is after the fact. Well, how much? Modern audio amplifiers generally have no phase shift in the audio band, i.e. the feedback is constant at all frequencies and virtually no phase shift is present. Amplifiers from the 70's often had a 90 degree phase difference purposely created to provide stability. This resulted in ever decreasing feedback across the audio band. The classic example is the Phase Linear 400 where the feedback started decreasing at 60 Hz. To end up with any feedback at 20 kHz they had to start with about 60 dB at the low frequencies. What is interesting is that an amplifier with constant

feedback performs just about as well as an amplifier with decreasing feedback if both are done properly. You would think something obvious would happen when we sweep through the frequency where the feedback factor starts to fall and the phase goes to 90 degrees. Well, nothing much happens, the feedback just does less correcting, the distortion rises and the damping factor falls.

Why does feedback have such a bad reputation? I believe it is because many times it has been poorly applied and results in misbehaving amplifiers in the field. Many designs have marginal stability. That does not mean they cannot be stable; they just are not. Amplifiers without feedback are inherently stable and free from these problems, but not necessarily better. They often have poor damping factors and more distortion than their feedback brothers. In my amplifiers, I apply as little feedback as possible because stability is very important to me. More feedback always decreases stability. If the distortion is down to a few tenths of a percent I

would rather have it than an amplifier that would oscillate under some conditions. I could also increase the feedback and improve the specs but at the cost of circuit and transformer complexity that results in unnecessary cost to you. Unfortunately, some designers have just increased the feedback without proper attention to the new requirements for the amplifier inside the loop. That is where unstable amplifiers come from.