Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER subject of Graph 12. C is chosen so that at low frequencies the effect of C and i?2 is negligible, but at 17,500 c/s the reactance of C is small and the two resistances are effectively in parallel. The gain is much reduced. At some intermediate frequency the complex load draws a leading current and thus introduces some phase shift, but this is permissible because the shift in the three coupling circuits is not
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 NEGATIVE VOLTAGE FEEDBACK AND THE STABILIZED GAIN AMPLIFIER subject of Graph 12. C is chosen so that at low frequencies the effect of C and i?2 is negligible, but at 17,500 c/s the reactance of C is small and the two resistances are effectively in parallel. The gain is much reduced. At some intermediate frequency the complex load draws a leading current and thus introduces some phase shift, but this is permissible because the shift in the three coupling circuits is not yet approaching 60 degrees. Without going into the matter too deeply it may be said that the effect of correcting circuits such as these is to reduce the amplifier pass-band. There- fore to apply successfully feedback to an existing amplifier it is helpful if the amplifier has a useful gain over a band which is wider than is finally required. This is the reason for the dictum that efforts to salvage a bad ampHfier with negative feedback may be disappointing. If the amplifier is coupled there will be another frequency—a very low one—at which oscillations can take place, due to phase advance in the coupling capacitance. To deal with this we connect across the coupling capacitance a correcting network comprising R^ and C^ {Figure ) producing the high- Reactance of C2 must be negligible /?2 h ;^i Figure pass circuit described in Graph 12. At the oscillation frequency R^ is com- parable with the reactance of C, and C^ must be sufficiently large for its own reactance to be small compared with the resistance of R^. The effect is to reduce the phase shift without increasing the loop gain. An alternative approach to the stability problem is to make B deliberately frequency con- scious, so that B is substantially real within the pass-band but complex with- out it and of such a nature as to introduce compensating phase shifts and/or reduce the loop gain at the freque
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