The analysis of base biased circuits depends on the current gain which can vary in a wide range. In an amplifier we need circuits whose Q-points are immune to changes in current gain. The solution for this problem is the emitter biased circuit:
Find the Q-point:
Given : VBB = 5V, VBE = 0.7V, VCE = 15V
RE = 2.2KΩ, RC = 1KΩ
VE = VBB - VBE = 5V - 0.7V = 4.3V
An emitter biased circuit is immune to changes in current gain. Analysing summary:
HO: What is the collector voltage?
VE = VBB - VBE = 2V - 0.7V = 1.3V
HO: What is the collector-emitter voltage?
VE = VBB - VBE = 1.1V
Effect of Small Changes
For example, tolerances of resistors (+/- 10%) are small changes.
See Fig. 5-4 on the next page.
Before we can analyse the effects of small changes we have to find out which values are dependent or independent.
independent values: VBB, VCC, βdc, RE, RC
Suppose the independent values will increase one after another. What will be the effect on the dependent values:
U = up
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