On Sun, 16 Jun 1996 23:12:08 GMT, aleung@engr.ukans.edu (Artur Leung)

>     I would like to receive some help about dc-biasing the single
>supply op-amp.
>     The one I am using is the LM324N from National Semiconductor and
>my primary supply voltage will be +12V.  Assume that there is an input
>signal of 1V peak sine wave.  In the data sheet National Semi. states
>that the input voltage cannot go lower than -0.3VDC.  If I want to use
>the LM324 in active band-pass filters, non-inverting amp., and
>inverting amp, what should I do to dc-bias the input voltage source?
>The information given by National Semi.'s data sheet is limited.  It
>only suggests using a reference voltage which is half of the supply
>voltage, but I don't know where to put the reference voltage.
>     Thank you very much for the help in advance.

  Have you tried using a capacitor-coupled input?

                      -o- V+
                       > R1
                       |       |`.
                       |       |  `.
        Signal   Cin   o-------|+   `.
          In   o-||----o       |      >---o-----  To other stages
                       |     ,-|-   ,'    |       (DC biased)
                       <     | |  ,'      |
                       > R2  | |,'        |
                       <     |            |
                       |     o---/\/\-----'
                       |     |
                       |     <
                       |     >
                       |     <
                       |     |
                      -o-----o- Gnd

  In this case (a non-inverting input stage), R1 and R2 determine the
DC bias point for the rest of the circuit, and the AC portion of the
input signal is coupled (added on) to the DC component.

  Cin needs to be large enough to allow the lowest signal frequency to
pass -- Cin forms an RC high-pass filter with R1 and R2, with the 3dB
cutoff frequency at 1/(2 pi Cin (R1 // R2)).

  The output of this stage will already be DC-biased, and other stages
downstream can be dc-coupled from this one.


Date: Mon, 17 Jun 1996 15:31:50 GMT

Original Subject: Re: [Q] HELP on Single-Supple Op-Amp Biasing......

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