A small signal voltage amplifier is shown in Fig. 1.3. In Fig. 1.3(a) the operational amplifier has no external components connected to it and the signal is fed in as shown. The operational amplifier has an extremely high gain under these circumstances and this leads to saturation within the amplifier.
As saturation implies working in the non-linear section of the characteristics, harmonics are produced and a ringing pattern may appear inside the chip. As a result of this, a square wave output is produced for a sinusoidal input. The amplifier has ceased to amplify and we say it has become unstable.
There are many reasons why an amplifier may become unstable, such as temperature changes or power supply variations, but in this case the problem is the very high gain of the operational amplifier.
Figure 1.3(b) shows how this may be overcome by introducing a feedback network between the output and the input. When feedback is applied to an amplifier the overall gain can be reduced and controlled so that the operational amplifier can function as a linear amplifier.
Note also that the signal fedback has a phase angle, due to the inverting input, which is in opposition to the input signal (Vi).
Negative feedback can therefore be defined as the process whereby a part of the output voltage of an amplifier is fed to the input with a phase angle that opposes the input signal. Negative feedback is used in amplifier circuits in order to give stability and reduced gain.
Bandwidth is generally increased, noise reduced and input and output resistances altered. These are all desirable parameters for an amplifier, but if the feedback is overdone then the amplifier becomes unstable and will produce a ringing effect.
In order to understand stability, instability and its causes must be considered. From the above discussion, as long as the feedback is negative the amplifier is stable, but when the signal feedback is in phase with the input signal then positive feedback exists.
Hence positive feedback occurs when the total phase shift through the operational amplifier (opamp) and the feedback network is 360° (0°). The feedback signal is now in phase with the input signal (Vi) and oscillations take place.
No comments:
Post a Comment