page_title: Negative-feedback amplifier
section_title: Input and output resistances
hierarchical_section_title: Negative-feedback amplifier / Two-port analysis of feedback / Input and output resistances
caption_attribution_description: Schematic for feedback amp input resistance
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 is used to better match signal sources to their loads. For example, a direct connection of a voltage source to a resistive load may result in signal loss due to voltage division, but interjecting a negative feedback amplifier can increase the apparent load seen by the source, and reduce the apparent driver impedance seen by the load, avoiding signal attenuation by voltage division. This advantage is not restricted to voltage amplifiers, but analogous improvements in matching can be arranged for current amplifiers, transconductance amplifiers and transresistance amplifiers.
  To explain these effects of feedback upon impedances, first a digression on how two-port theory approaches resistance determination, and then its application to the amplifier at hand.
caption: Figure 6: Circuit set-up for finding feedback amplifier input resistance
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