What Is Loop Gain?

M.J. Casey

Loop gain is the term applied to the gain in a parameter when a portion of the output of a device in the circuit or system is returned as input to the device. There are negative and positive loop gain systems. This feedback mechanism is important as a control technique in chemical process control, manufacturing lines, electronic circuits, and biological systems.

Feedback can be created when a microphone picks up its own audio signal being projected from a speaker.
Feedback can be created when a microphone picks up its own audio signal being projected from a speaker.

The screech of a speaker when a microphone is pointed at it illustrates an out-of-control loop gain. The microphone picks up the sound from the speaker and sends it back to the amplifier, which magnifies the sound and sends it out the speaker again. As the cycle continues, the signal is greater than the operating input range of the speaker, and the annoying sound issues.

In negative feedback, information is moving backward. Input streams arrive at a processing step, the process or change occurs, and the output parameter is measured and compared to the setpoint, or desired value. An upstream adjustment is made, and the loop repeats. This is a closed loop gain feedback system.

Positive feedback, also called feed-forward, occurs when input stream parameters are measured prior to the process, and the controller is adjusted before the process occurs. Both systems have merit and typically are combined using a feedback controller to feed information to the feed-forward controller. The feed-forward controller processes this data with the input stream parameter data to make the adjustment.

The heating of a liquid stream to a set temperature by passing the liquid through a heat exchanger demonstrates this combined control system. The temperature and flow rate of the incoming liquid is sent to the feed-forward controller. The downstream temperature is sent to the feedback controller, which compares it to the setpoint. The feed-forward controller modifies the change required by the feedback controller with the current information it is receiving. By use of this combined closed loop gain control system, swings in the temperature of the heated liquid are avoided, and the remaining process proceeds more smoothly.

The combined feed-forward/feedback control system could be called a buffer because it works to keep a constant output value. An op amp is an electronic device that does the same thing. It accepts two inputs and has a single output that goes either high or low, depending on the difference of the inputs. The design of certain op amps causes one of the input values to be inverted or change sign, and then the output is sent back as one of the inputs. The goal is to balance both inputs to the same value.

Open loop gain refers to a control mechanism that does not use any process information to control the system behavior. An op amp may be characterized as an open loop gain device, but it is put into a larger feedback control system, which is closed. It serves as the buffering device in the circuit.

Manufacturing uses closed loop gain feedback systems, although often via manual processes. The basis of statistical process control is another expression of a feedback system. Biological systems use gain loop controls, from the control of cellular respiration to the balance of entire ecosystems.

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