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Bias Selection

Author: Leonard Krugman

It can be shown mathematically that the condition for locating the operating point in the center of the negative-resistance region is: Ee (2αRc + RE) = EcRE. This relationship indicates that the extent of the negative-resistance range depends upon the bias batteries and the values of RE and Rc. The emitter-to-collector current gain α is, of course, fixed for a given transistor. For the characteristic in Fig. 6-10 (B), then, all the parameters are specified with the exception of Ee and RE. Notice, however, that these quantities are related to the value of d-c emitter current bias Ie that is required to establish a d-c operating point in the center of the negative resistance region. This condition is:

Ee = REIe.

The two conditional equations can be combined to evaluate RE in terms of known quantities:

transistor_basics_06-24.gif Eq. (6-2)

Since RE also equals transistor_basics_06-25.gif , equation 6-2 limits the value of the emitter bias battery to less than that of Ec. The limiting value of Ee = Ec is reached when Rc = 0.

As a numerical example, if the values associated with Fig. 6-10 (B) are used so that the bias current at the center of the negative resistance region is Ie = 0.75 ma, then



Ee = IeRE = .75 × 10-3 × 31.5 × 103 = 23.6 volts.

The negative resistance of the oscillator is equal to the slope of the

characteristics in that region. Then


This value defines the maximum limit of the impedance of the L-C series emitter circuit at resonance.

Last Update: 2010-11-17