Sunday, January 20, 2008

INCREASING THE FREQUENCY OF AN AMPLIFIER

VIDEO FREQUENCY AMPLIFIERS

To increase the frequency bandwidth of an amplifier, an inductor is introduced into the circuit.  As we know, increase in frequency causes more EMF to develop across an inductor which enables a greater load to develop across the inductor.  The inductor acts as a store of the energy.

The bandwith of an amplifer is limited in a circuit because of the internal capacitance of the circuit and the capacitance of the amplifer (transistor).  

In the first case, the capacitance usually shunts the signal to ground.  As frequency increases, the capacitance acts like a short and most of the signal is shunted to ground.
In the second case, the capacitance of the transistor causes the base and the emitter to increase at the same time causing the transistor to not amplifer the signal as much.

There are two types of methods to increase the frequency response of a transistor amplifer.  One is my using Shunt Peaking, whereby an inductor is placed on the output (collector) of the amplifer in parallel to the output signal path (but in series to the load (signal) usually a resistor), and the other is by Series peaking, by placing a coil on the output stage, but in series with the output signal.

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