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Voltage Gain VG

Author: Leonard Krugman

Looking again at Fig. 3-9, it is seen that the voltage gain transistor_basics_03-69.gif. Since E2 = i2RL and Eg = i1 (Rg + r1),


Eq. (3-22)

Since transistor_basics_03-71.gif, if equation 3-8 is substituted for transistor_basics_03-72.gif in equation 3-22 and if equation 3-13 is substituted for r1 in equation 3-22 the voltage gain becomes:


Eq. (3-23)


Eq. (3-24)*

Notice that the voltage gain is maximum when RL is infinite and Rg is zero. Under these conditions the maximum


Eq. (3-25)*

For the typical point-contact transistor, the maximum transistor_basics_03-78.gif= 96. Assuming typical values of RL = 25,000 ohms, and Rg = 200 ohms transistor_basics_03-79.gif

For the typical junction transistor, the maximum transistor_basics_03-80.gif= 3,450.Assuming typical values RL = 1 megohm, and Rg = 200 ohms transistor_basics_03-81.gif

A comparison of the maximum and operating gains of the typical point-contact and junction transistors indicates that the junction is capable of furnishing much larger voltage gains. This explains why the junction transistor is invariably used in audio amplifier circuits.

The power gain (PG) of the transistor can be calculated from the product of the current gain and the voltage gain or found directly from the ratio of output power to input power.

PG = α(VG)

The theoretical maximum power gain is the maximum current gain and the maximum voltage gain. However, the condition for maximum current gain is RL = 0, and the condition for maximum voltage gain is RL = infinity. Since these conditions are in opposition, the problem of finding the maximum power gain involves matching the input and output resistances of the transistor. The maximum power gain is obtained when the internal resistance of the signal generator is equal to the input resistance of the transistor, and the load resistance is equal to the output resistance of the transistor, that is Rg = r{ and Rl = ro- When these conditions are simultaneously satisfied, the transistor is image impedance matched.

Last Update: 2010-11-17