Page 16 - Semester-IV-Electronics

DC Level Shifter

 In direct coupled amplifiers, the input current at the base of each

amplifier stage comes from the collector of previous amplifier.

 As a result, the dc-voltage of each next stage amplifier increases with

respect to the ground voltage.

 Hence, to bring the dc level at the base of the amplifier of each stage at

the level at which with respect to the ground voltage, the output

voltage = 0 for the zero input voltage, a dc-level shifter is used.

 The basic structure of dc-level shifter is shown in Figure 3.

 Transistor Q acts as emitter follower in which the input signal is
1
applied between the base and ground terminal.

 The output voltage is obtained between the emitter of Q and ground
3
terminal. This also acts as emitter follower.

 The transistor Q is a constant current device at the base of which
2
input current is given from a constant reference voltage source.

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