Page 45 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

However if bdc increases, the collector current increase. This inturn increases the voltage at the emitter. This
increased emitter voltage decreases the voltage across the base-emitter junction and therefore, the base current
reduces. This reduced base current results in less collector current, which partially offsets the increase in Ic due
to increase bdc.
Emitter bias is also referred to as emitter feedback bias. This is because an output quantity, i.e., the collector
current, produces a change in an input quantity i.e., the base current. The term feedback means a portion of
the output is given back to the input. In emitter bias, the emitter resistor is the feedback element because it is
common to both the output and input circuits.
3 VOLTAGE-DIVIDER bias: Collector to base bias: Fig 4 shows a typical voltage-divider bias. This type of
biasing is also called the universal bias because, this is the most widely used type of biasing in linear circuits.

Fig 3 Fig 4

This type of biasing is known as voltage divider bias because of the voltage divider formed by resistors R1 and
R2. The voltage drop across R2 should be such that it forward biases the emitter diode.
Emitter current in voltage divider bias : Assume that the base lead is open as shown in Fig 5b. Looking back at
the unloaded voltage divider,

NOTE: VTH is known as the Thevenin’s voltage. Refer reference books for Thevinin’s theorem.
Now assume that, the base lead is connected back to the voltage divider as in Fig 5a. then, voltage VTH drives
the base of the transistor. In other words, the circuit simplifies to Fig 5a and the transistor acts like the controlled
current source.
Because the emitter is boot-strapped to the base,

Fig 5 Fig 6

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CITS : Power - Electrician & Wireman - Lesson 60-69