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Power GainAuthor: Leonard Krugman Before determining the power gain included in transistor circuits, some definitions must be considered. Figure 316 illustrates a signal generator E_{g} with an internal resistance R_{g} feeding into a load R_{L}. The total power delivered by the generator ; the power dissipated in the load . Since then . Fig. 316. Simplified transistor equivalent circuit for analysis of power gain. By using conventional calculus methods for determining conditions for maximum power, it is found that the load power is maximum when R_{g} = R_{L}. Under this condition the power available from the generator The operating gain, G, of a network is defined as the ratio of the power dissipated in the load to the power available from the generator. For the general transistor circuit of Fig. 39
The power dissipated in the load
The operating gain
The available gain, AG, of a network is defined as tne ratio of the power dissipated in the load to the power available from the generator when the load is matched to the output resistance. When R_{L} = r_{o} = , then . Substituting in equation 346, the available gain
The maximum available gain, MAG, of a network is defined as the ratio of the power dissipated in the load to the power available from the generator when the generator internal resistance is matched to the input transistor resistance, and when the load resistance is matched to the transistor output resistance. In order to solve for the maximum power gain in terms of the opencircuit parameters, the imagematched input and output resistances, previously determined, are substituted in the operating gain equation 346. Then, the maximum available gain,
where
and
Substituting: equations 337 and 341 in equation 350, for r_{1} and r_{2},
from which derives
For the typical pointcontact transistor, when r_{11} = 250 ohms, r_{12} =100 ohms, r_{21} = 24,000 ohms, r_{22} = 12,000 ohms, and when assuming R_{g} = 50 ohms and R_{L} = 8,000 ohms, the operating gain G, becomes Notice that the stability factor, , is = 0.8. If the stability factor is greater than one, the numerical value of the quantity must be negative, which indicates an unstable condition. For the typical junction transistor in which r_{11} = 550 ohms, r_{12} = 500 ohms, r_{21} = 1,900,000 ohms, and r_{22} = 2,000,000 ohms; when assuming R_{g} = 100 ohms and R_{L} = 1,000,000 ohms, the operating gain From equation (355)


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