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Schematic of an AGC used in the analog telephone network; the feedback from output level to gain is effected via a Vactrol resistive opto-isolator.. Automatic gain control (AGC) is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, the purpose of which is to maintain a suitable signal amplitude at its output, despite variation of the signal amplitude at the input.
The Barkhausen stability criterion gives the necessary condition for oscillation; the loop gain around the feedback loop, which is equal to the amplifier gain multiplied by the transfer function of the inadvertent feedback path, must be equal to one, and the phase shift around the loop must be zero or a multiple of 360° (2π radians).
The delay locked loop is a variable delay line whose delay is locked to the duration of the period of a reference clock. Depending on the signal processing element in the loop (a flat amplifier or an integrator), the DLL loop can be of 0th order type 0 or of 1st order type 1.
The input signal is applied to the amplifier with open-loop gain A and amplified. The output of the amplifier is applied to a feedback network with gain β, and subtracted from the input to the amplifier. The loop gain is calculated by imagining the feedback loop is broken at some point, and calculating the net gain if a signal is applied.
The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery (the battery is the active device).
Low-density parity-check (LDPC) codes are a class of highly efficient linear block codes made from many single parity check (SPC) codes. They can provide performance very close to the channel capacity (the theoretical maximum) using an iterated soft-decision decoding approach, at linear time complexity in terms of their block length.
[8] [9] That is, the amplifier uses current feedback. It frequently is ambiguous just what type of feedback is involved in an amplifier, and the asymptotic gain approach has the advantage/disadvantage that it works whether or not you understand the circuit. Figure 6 indicates the output node, but does not indicate the choice of output variable.
A schematic of a simple superhet broadcast FM receiver. Note that there is no AGC loop, but simply uses a high-gain IF amplifier which is intentionally driven into saturation (or limiting). For single conversion superheterodyne AM receivers designed for medium wave (AM broadcast) the IF is commonly 455 kHz.