This avoids distortion due to excessive amplification of the amplifier's input offset voltage. The dc "noise gain" is reduced to unity by capacitively coupling the feedback with a zero established by R1 and C1, so that the dc level of the output is equal to the bias voltage. The average level of the ac-coupled input is biased to V s/2 by the R A-R B divider pair, and the in-band gain is G = 1 + R2/R1. The signal is capacitively coupled in and out. Illustrating the problem, the circuit of Figure 1, which has several design weaknesses, is an ac-coupled non-inverting amplifier. Though apparently simple, there are problems with it. The most effective way to achieve this is with a regulator, as in Figure 6 however, a popular method involves tapping the supply voltage with a pair of resistors. Since it is usually desirable for large output values to limit symmetrically, the bias is usually established at the midpoint of the rated amplifier output range, or (for convenience) at one-half the supply voltage. With a single supply, such a node must be created artificially, by introducing additional circuitry to provide some form of biasing, to maintain signal common at an appropriate mid-supply voltage. The two supply voltages are usually equal and opposite (and often tracking), but that's not an absolute necessity. The principal advantage of dual supplies is that their common connection provides a stable, low-impedance zero-reference. The fundamental issue is that, if the signal is to swing both positive and negative with respect to "common", this zero-signal reference voltage must be at a fixed level between the supply rails. Single-supply op-amp applications have inherent problems that are not usually encountered in dual-supply circuits.
If this is not carefully thought through and executed, instability and other problems may be encountered. In processing analog signals, a common feature of single-supply operation is the need for additional components in each stage for appropriate signal-biasing. Even line-powered equipment, such as computers, may have only a single-polarity built-in supply, furnishing +5 V or +12 V dc for the system. For example, battery power, in automotive and marine equipment, provides only a single polarity.
#BIAS AMP 2 NOT WORKING PDF#
Avoiding Op-Amp Instability Problems In Single-Supply ApplicationsĬharles Kitchin Download PDF Single or Dual Supply?Īlthough it is advantageous to implement op-amp circuits with balanced dual supplies, there are many practical applications where, for energy conservation or other reasons, single-supply operation is necessary or desirable.
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