NETWORK ANALYSIS

 

 

You know that the circuit consists of a source, a load and an interface between them. The source and the load, as a rule, are one port networks. Interface is represented as a two port network. A systems approach is used where we regard to two port network (for example, the amplifier) as a 'black box' having an input and an output. We do not need to know what is inside the box as it is represented by a pair of simultaneous equations which show how the box behaves to the outside world. The advantage of this method is that, having once been given the equations, we need only a rudimentary knowledge of electronics in order to predict the behavior of the box. The treatment given is perfectly general; it could be applied to almost any system that can be represented as a two port network.

Finally, we should note one assumption about the 'black box' approach that we

are about to adopt. This is that the system in the box is assumed to be linear.

There are two important properties of a linear system:

1. That the principle of superposition holds. By this we mean that if an

input signal Vi1 to our system gives an output Vo1 and an input signal Vi2

gives an output Vo2, then an input of (Vi1 + Vi2) will give an output

(Vo1 + Vo2).

2. That frequency conservation is maintained. In other words there are no

frequencies present in the output that were not present in the input.

Please note the following:

(i) Much use is made of the 'ratio method' for determining the current in each

of a pair of parallel resistors:

(ii) Use is also made of the 'method of units' to determine the nature of the

variables in an equation. For example, consider the equation

If P is power in watts, then each term on the right hand side must also

represent power in watts. For this to be so, k1 must represent resistance

and k2 conductance.

(iii) We assume that all voltages and currents are r.m.s. values. As is the convention, these are represented by 'V' and 'I' respectively.