3.9 Power Relations in the Basic Transistor Amplifier

In our subsequent discussion we will denote time-varying quantities with lower case letters and lower case subscripts. We will represent average (DC) values with upper case letters and upper case subscripts. We will use lower case letters with upper case subscripts for the sum of the instantaneous and average values.

Let us consider the circuit of Figure 3.49. The collector current and the collector-to-emitter voltage can be expressed as

and

where I _C and V _CE are the average values i _c and v _ce are time-varying components whose average value is zero.

Figure 3.49: Circuit for the derivation of power relations

The power drawn from the collector supply at each instant is

Since V _CC is constant and i _c has zero average value, the average value of the term V _CCi _c is zero. ^[*] Therefore, the average power drawn from the collector supply is

and if there is negligible or no distortion, the current I _C and power P _CC are both independent of the signal amplitude.

The power absorbed by the load at each instant is

The term 2R _LOADI _Ci _c is zero since 2R _LOADI _C is constant and the average of i _c is zero; hence the average power absorbed by the load is

The power absorbed by the transistor at each instant is

Therefore, the average power...