TABLE OF CONTENTS An ideal (iron-core) transformer can be used as an impedance level changing device. We recall from basic circuit theory that to achieve maximum power transfer, we must adjust the resistance of the load to make it equal to the resistance of the voltage source. But this is not always possible. A power amplifier for example, has an internal resistance of several thousand ohms. On the other hand, a speaker which is to be connected to the output of a power amplifier has a fixed resistance of just a few ohms. In this case, we can achieve maximum power transfer by inserting an iron-core transformer between the output of the power amplifier and the input of the speaker as shown in Figure 9.31 where N 2
Let us suppose that in Figure 9.31 the amplifier internal impedance is 80000 ? and the impedance of the speaker is only 8 ?. We can find the appropriate turns ratio N 2/N 1=a using (9.97), that is,
or
or
that is, the number of turns in the primary must be 100 times the number of the turns in the secondary.
