Since the output (impedance)of an opamp is "virtually" zero, I don't know
what the 22k is for either. Maybe it was initially used in conjunction with
the U02 capacitor for the filtering circuit. Not needed in this
configuration. The 741 is really a general purpose opamp and not really
designed for voltages below about 6 volts. Especially in the peak region as
you are describing. In the low level linear region at the 2.5 volt idle, it
will perform satisfactorily. The internal offset bias will determine which
side of the waveform will clip first. One side usually clips first. You
could offset the input if you really wanted to, to symmetrically clip.

Microchip and a host of others have better suited rail-rail opamps for
operation at voltages as low as 2.5 rail.
Rick

michael brown wrote:

> I'm playing with first stage of this ultrasonic receiver circuit:
> http://www.atsemi.com/article/Howto.htm
>
> The gain of the 741 is set to 1000 (way above what I can actually get at
> 25kHz), I'm getting close to a voltage gain of 100 as I'd expect at that
> frequency.  I'm using 5V instead of the 9V indicated in the schematic
> for Vdd of the 741.  The output pin idles at 2.5V as I'd expect, but
> when the output reaches about 2.5V p-p, it starts clipping on the bottom
> first, quickly followed by clipping at the top.  I checked everything
> with the scope, and the output is centered on the 2.5V dc bias (pin 6).
> I know that many op-amps can't go rail to rail, but surely shouldn't I
> be able to get closer to the rails than about 1.25V?
>
> I just noticed that I didn't include the 22k resistor to ground on the
> output of the 741.  What is it's purpose?  Is it to reduce the amount of
> feedback, prevent oscillation, set the output impedance?  BTW, I found
> it interesting that the circuit doesn't produce anything out unless you
> have some kind of negative feedback resistor.  I expected the op-amp to
> run at max gain without the negative feedback.
>
> Thanks for any info
>
> michael brown

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