page_title: Negative-feedback amplifier
section_title: Bandwidth extension
hierarchical_section_title: Negative-feedback amplifier / Classical feedback / Bandwidth extension
caption_attribution_description: Bandwidth comparison with and without feedback
context_page_description: A Negative-feedback amplifier is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal. The applied negative feedback can improve its performance and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, many amplifiers and control systems use negative feedback.
  An idealized negative-feedback amplifier as shown in the diagram is a system of three elements:
  an amplifier with gain AOL,
  a feedback network β, which senses the output signal and possibly transforms it in some way,
  a summing circuit that acts as a subtractor, which combines the input and the transformed output.
context_section_description: Feedback can be used to extend the bandwidth of an amplifier at the cost of lowering the amplifier gain. Figure 2 shows such a comparison. The figure is understood as follows. Without feedback the so-called open-loop gain in this example has a single-time-constant frequency response given by
  where fC is the cutoff or corner frequency of the amplifier: in this example fC = 10⁴ Hz, and the gain at zero frequency A₀ = 10⁵ V/V. The figure shows that the gain is flat out to the corner frequency and then drops. When feedback is present, the so-called closed-loop gain, as shown in the formula of the previous section, becomes
  The last expression shows that the feedback amplifier still has a single-time-constant behavior, but the corner frequency is now increased by the improvement factor (1 + β A₀), and the gain at zero frequency has dropped by exactly the same factor. This behavior is called the gain–bandwidth tradeoff. In Figure 2, (1 + β A₀) = 10³, so AFB(0) = 10⁵ / 10³ = 100 V/V, and fC increases to 10⁴ × 10³ = 10⁷ Hz.
caption: Figure 2: Gain vs. frequency for a single-pole amplifier with and without feedback; corner frequencies are labeled
exif: {"Image Tag 0x0301": "20000/9091", "Image Tag 0x0303": "0"}