22-01-2013, 02:29 PM
Feedback Amplifiers
Feedback.ppt (Size: 3 MB / Downloads: 233)
Introduction
It’s impossible to think of electronic circuits without some forms of feedback.
Negative feedback
Desensitize the gain
Reduce nonlinear distortion
Reduce the effect of noise
Control the input and output impedance
Extend the bandwidth of the amplifier
The basic idea of negative feedback is to trade off gain for other desirable properties.
Positive feedback will cause the amplifier oscillation.
The Four Basic Feedback Topologies
Voltage amplifier---series-shunt feedback
voltage mixing and voltage sampling
Current amplifier---shunt-series feedback
Current mixing and current sampling
Transconducatnce amplifier---series-series feedback
Voltage mixing and current sampling
Transresistance amplifier---shunt-shunt feedback
Current mixing and voltage sampling
The Practical Situation
The load effect of the feedback network on the basic amplifier is represented by the components h11 and h22.
The loading effect is found by looking into the appropriate port of the feedback network while the port is open-circuit or short-circuit so as to destroy the feedback.
If the connection is a shunt one, short-circuit the port.
If the connection is a series one, open-circuit the port.
Determine the β.
Summary
Ri and Ro are the input and output resistances, respectively, of the A circuit.
Rif and Rof are the input and output resistances, respectively, of the feedback amplifier, including Rs and RL.
The actual input and output resistances exclude Rs and RL.
The Practical Situation
The load effect of the feedback network on the basic amplifier is represented by the components Z11 and Z22.
Z11 is the impedance looking into port 1 of the feedback network with port 2 open-circuited.
Z22 is the impedance looking into port 2 of the feedback network with port 1 open-circuited.
Determine the β.