Level Translator

There are two good reasons for using a level shifter in an IC(op-amp).As we want an op-amp to operate down to dc,no coupling capacitor is used.Because of direct coupling,the dc level rises from stage to stage.The increase in dc level tend to shift the operating point of next stage.This,in turn,limits the output voltage swing and even distort the output signal.It,therefore becomes essential to shift the quiescent voltage before it is applied to the next stage.Another requirement to be satisfied is that the output should have voltage level of 0V for input signal.

he simplest type of a level shifter is shown in fig.1 given below.This is basically an emitter follower.Hence the level shifter also acts as buffer to isolate the high gain stages from output stage.The amount of shift obtained,

V_O - V_i = -V_BE ≈-0.7V

If this shift is insufficient,the output can be taken at a junction of two resistors R₁ and R₂ (fig.2).the voltage in shift is now increased by the drop across R₁ .How ever this arrangement has the disadvantage that signal voltage also get attenuated by R₂ /(R₁ +R₂) .This can be avoided if R₂ is replaced by ‘I’(fig.3).The shift is now

V_O - V_i = -(V_BE +IR₁)

There is no ac attenuation due to high resistance of the current source.

Another voltage source commonly used in μA741 op-amp is shown below.It can be seen that if base current is negligible compare to the current in R₃ and R₄ ,then the circuit behaves as a multiplier as.

V=V_BE (R₃ + R₄)/ R₄ = V_BE[1+( R₃/ R₄)]

The voltage source can also be used to replace R₁

 

Written by John on September 26th, 2008 with 2 comments.
Read more articles on Linear Integrated Circuits and op-amp and Stages of op-amp.

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