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Current Gain in the Grounded Emitter Connection

Author: Leonard Krugman

The current gain in terms of the general four-terminal parameters was defined by equation 3-8 as:

transistor_basics_04-8.gif Eq. (4-5)

In terms of the transistor parameters in the grounded emitter connection now being considered, the current gain is

transistor_basics_04-9.gif Eq. (44-6)*

In the case of the grounded-emitter point-contact transistor, r21 and r22 are both negative. The value of the load resistor, RL, determines whether the current gain is positive or negative. If RL is less than the absolute value of -r22, α is positive; if RL is greater than the absolute value of -r22, α is negative. A negative value of current gain indicates simply that the input current is inverted in phase. This is normal in the grounded emitter connection. Theoretically, an infinite current gain is attained when RL = -r22. The current gain of a typical point-contact transistor with a load RL = 15,000 ohms is


(Equation 3-8A for maximum current gain, transistor_basics_04-11.gif , does not apply in this connection, since it is found that the point-contact transistor is unstable when RL is less than -r22.) Notice that the current gain becomes very large for values of RL slightly larger than -r22. For example, if

RL = 12,500 ohms, transistor_basics_04-12.gif

The current gain in the junction transistor is always negative in the grounded emitter connection, since r22 is always positive. The current gain for the typical grounded-emitter junction transistor with a load

RL = 100,000 ohms, is transistor_basics_04-13.gif

The maximum current gain, as in the case of the grounded base connection, is:


Last Update: 2010-11-17