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:

Eq. (4-5)

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

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, R_L, determines whether the current gain is positive or negative. If R_L is less than the absolute value of -r₂₂, α is positive; if R_L 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 R_L = -r₂₂. The current gain of a typical point-contact transistor with a load R_L = 15,000 ohms is

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

R_L = 12,500 ohms,

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

R_L = 100,000 ohms, is

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