Question

What is the difference between open-loop and closed-loop gain?

Answer 1

The terms open-loop gain and closed-loop gain are used to describe the gain behavior of amplifiers, and they are especially important in control systems and electronics. Here's a simple breakdown:

1. Open-Loop Gain:

1. Definition: This is the gain of an amplifier when there is no feedback loop. In other words, it's the gain measured from the input to the output without any signal being fed back from the output to the input.
   

1. Characteristics:
   

  - Open-loop gain is typically very high (sometimes in the thousands or even more).
  - It can be unstable, meaning small variations in the input can cause large changes in the output.
  - The amplifier works without any corrections for distortion or changes in gain.

1. Example: If you have an op-amp (operational amplifier) with an open-loop gain of 100,000, and you apply a small input signal (like 1 mV), the output will be 100,000 times that signal (100 V). But without feedback, the system might become unstable.
   

2. Closed-Loop Gain:

1. Definition: This is the gain of an amplifier when there is feedback from the output back to the input. Feedback is used to control and stabilize the gain.
   

1. Characteristics:
   

  - Closed-loop gain is usually lower than open-loop gain because the feedback controls the system and reduces the total gain.
  - The feedback helps stabilize the system and make the gain more predictable, linear, and less sensitive to changes in temperature or other external factors.
  - In many cases, you can design the closed-loop gain to be exactly what you want, even if the open-loop gain is very high.

1. Example: If the same op-amp has a closed-loop gain of 10, it will always amplify the input signal by a factor of 10, even if the open-loop gain is much higher. The feedback keeps the output stable.
   

Key Differences:

1. Stability: Open-loop gain can be unstable, while closed-loop gain is stable and predictable.
   

1. Gain Control: Open-loop gain is uncontrolled and very high, whereas closed-loop gain is controlled and usually lower.
   

1. Feedback: Open-loop gain doesn't use feedback, while closed-loop gain relies on feedback to stabilize and control the amplification.
   

In summary:

1. Open-loop gain is the amplification without feedback (unstable and high).
   

1. Closed-loop gain is the controlled amplification with feedback (stable and typically lower).