Negative Feedback(이하 NFB)는 흔하게 잘못이해되는 주제이다. 전역 NFB는 회로의 후반부에서 전반부로 소량의 신호가 되돌려지는 것(feeding back)으로 일반적으로 출력 트랜스포머에서 phase inverter로 되돌려진다.
What does negative feedback accomplish?
전역 NFB의 용도는 다음과 같다.
– 주파스 응답(frequency response)을 평탄화하고 확장함
– 피드백 루프로 둘려싸여진 스테이지에서 발생하는 디스토션을 감소시킴
– 앰프의 유효 출력 임피던스를 감소시켜, 댐핑 팩터를 증가시팀
이 모든 것들은 톤에 동일하게 영향을 미치는 요소들이다.
평탄화되고 확장된 주파수 응답은 출력단의 “humps”를 제거하고 저고역 주파수의 종단의 확장을 통해 톤 특성에 명백한 변화를 발생시킨다.
디스토션의 감소는 앰프를 더 깨끗하고, hi-fi하게 만들며, clipping 발생점을 높이고, 출력단 노이즈를 감소시킨다.
Perhaps the main difference for the “feel” in a negative feedback amplifier,
as opposed to a non-negative feedback amplifier, is the increased damping
factor produced by the negative feedback loop. The decreased effective
output impedance causes the amp to react less to the speakers. A speaker
impedance curve is far from flat; it rises very high at the resonant frequency,
then falls to the nominal impedance around 1kHz, and again rises as the
frequency increases. This changing “reactive” load causes the amp output
level to change with frequency and changes in speaker impedance (a dynamic
thing that changes as the speakers are driven harder). Global negative
feedback generally reduces this greatly. This can be good or bad, depending
upon what you are looking for. Negative feedback makes the amp sound “tighter”,
particularly in the low end, where the speaker resonant hump has the most
effect on amplifier output. This is better suited for pristine clean playing
or a tight distorted tone, while a non-negative feedback amp has a “looser”
feel, better suited to a bluesy, dynamic style of playing.
The other disadvantage of a negative feedback amplifier is that the
transition from clean to distorted is much more abrupt, because the negative
feedback tends to keep the amp distortion to a minimum until the output
stage clips, at which point there is no “excess gain” available to keep
the feedback loop operating properly. At this point, the feedback loop
is broken, and the amp transitions to the full non-feedback forward gain,
which means that the clipping occurs very abruptly. The non-negative feedback
amp transitions much more smoothly into distortion, making it better for
players who like to use their volume control to change from a clean to
a distorted tone.
How much feedback to use?
The amount of voltage fed back determines the amount of gain
reduction and the amount of distortion reduction, as well as the effective
output impedance. The more voltage fed back, the less distortion,
the lower the effective output impedance, the higher the damping factor,
and the lower the gain of the stages enclosed by the feedback loop.
Typically, in a guitar amp, somewhere around 6-10dB of feedback is used.
If you have 6dB of feedback, for instance, and it takes 2V at the phase
inverter input to achieve output clipping, if you removed the feedback,
it would only take 1V at the phase inverter input to achieve output clipping.
In other words, there is a voltage gain reduction of 6dB, or a factor of
two, in the stages enclosed by the feedback loop. This is achieved
by feeding back a certain percentage of the output voltage to an earlier
point in the circuit, the phase inverter. The more voltage fed back,
the more the voltage gain reduction, as mentioned previously.
The series feedback resistor, in conjunction with the resistor to ground,
determines the amount of voltage being fed back. If you want to feed back
more voltage, you make the series resistor smaller, or the shunt resistor
larger, or you use a higher impedance tap on the output transformer.
The actual resistor values used in the feedback attenuator aren’t that
important, as their ratio determines the amount of feedback. The shunt
resistor value is usually fixed by the phase inverter design requirements,
and the series resistor is then sized according to the desired amount of
feedback, given the voltage available at the output. Note that Marshall
typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms.
You can get the same attenuation from a 10K/500ohm pair as you would from
a 100K/5K pair. In addition, if you were using a 100K/5K attenuator
running from the 16 ohm tap, you would get roughly the same amount of feedback
if you used a 47K/5K attenuator running from the 4 ohm tap. Note
that the tap voltages are not linear with respect to the impedance, it
varies linearly with the square root of the impedance, that is, the voltage
on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the
voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It
helps if you think of the equation for power: P = V^2/R. If you have
100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have
100W into 8 ohms, the voltage is V = sqrt(100*8) = 28.28V RMS. If
you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS.
Frequency response shaping via feedback
Closely related to the subject of negative feedback is the
use of frequency-dependent elements in the feedback loop to shape the overall
response of the amplifier. Most guitar amps have a “presence” control,
which boosts the high frequencies. It accomplishes this not by actually
boosting the highs in the forward path of the output circuit, rather by
cutting the amount of high frequencies being fed back. This effectively
reduces the amount of negative feedback at those higher frequencies, which
results in a boosting of the highs at the output. Some guitar amplifiers
have a “resonance” control, which does a similar thing, by cutting the
amount of low frequencies present in the feedback loop, thereby boosting
the low frequencies in the output. The amount of boost is equal to
the amount of negative feedback. If the amp has 6dB of feedback, there
can be at most a 6dB presence or resonance boost. This means that
if you reduce the amount of feedback for more gain, you will also reduce
the effectiveness of the presence and resonance controls, likewise, if
you increase the amount of feedback, you will increase the effectiveness
of these controls.
There is a danger in using too much negative feedback, however, as the
amplifier can become unstable and oscillate, particularly with reactive
loads. In addition, the more stages the feedback is applied around,
the more likely the chance for oscillations, as there are more phase shifts
within the forward path, due to coupling capacitors and other circuit capacitances.
Other types of feedback
Some amplifiers, most notably the Marshall Valvestate transistor
amps, use negative current feedback in lieu of negative voltage feedback,
or a combination of the two. Global negative current feedback has
a similar effect on distortion reduction, but instead of decreasing the
effective output impedance and increasing the damping factor, it actually
increases the effective output impedance and decreases the damping factor.
This makes the amplifier’s output voltage vary with variations in speaker
impedance. Since a speaker’s impedance varies radically with frequency,
a current feedback amplifier will tend to feel more “tubey” than a voltage
feedback amplifier, because of this speaker/amplifier interaction.
In addition to global negative feedback, amplifiers usually have some
form of local negative feedback, but sometimes this is not as apparent.
A cathode follower is an example of an amplifier stage with 100% negative
feedback. This is what gives it the high input impedance and low
output impedance, and the near-unity maximum gain. Some amplifiers
will use a single-stage inverting amplifier circuit with local feedback
from the plate to the grid via a large resistor and coupling cap.
The gain of these inverting stages is set by the value of the feedback
resistor in proportion to the value of the input resistor.
Copyright © 1999, Randall
Aiken. May not be reproduced in any form without written approval
from Aiken Amplification.