Ian Bell wrote:
> There are several issues to be addressed here.  First even with a 330K
> collector load you have a very low collector current and the
> transistor gain depends significantly on collector current.

OK, that's a key thing for me to know.

> Second you need to think about the load you are driving.  The pic
> inputs may be relatively high impedance but they also have
> capacitance and this will slow the edges if the transistors output
> impedance (determined in part by the collector resistor) is too high.
> Both these factors suggest you should reduce the collector resistor
> further.  I would suggest reducing it to about 33K.

I'll try that now........that seemed to help a little, but so does
increasing the base pullup.

> Third, the emitter resistor reduces the available gain so I would
> normally suggest you remove it.  However, it does increase the (small
> signal) input impedance of the transistor so you may not be able to
> get rid of it entirely. Whether you can depends on the source
> impedance of the 200mV signal.  Do you know the value of this?  With

I would say the source impedance is around 600 Ohms, but that is based
on available material from the net and could be quite wrong.  Plus I
feed that thru a .1uF cap and 4K7 resistor (resistor was 47K, but this
value gives more gain).  Omitting the emitter resistor seems to have
very little effect on things aside from the collector voltage (with no
signal applied) dropping a bit closer to 0 (190mV).

> a 33K resistor load the collector current will be about 0.15mA so the
> transistor emitter resistance will be about 165 ohms. if we assume a
> transister beta of 100 then the effective input resistance is about
> 16K ohms.
>
> Fourth, you need a rail to rail swing so we are no longer in the
> realms of small signal analysis so the usual formulae for gain do not
> apply.

> Fifth, feeding the base via a 3.3M resistor will not drive the
> transistor into saturation, which you probably do want to do to get
> close to 0V.  So you need to reduce the collector resistor a lot,
> probably down to 100K as this is still well above the input impedance.

This doesn't seem to be exactly true in my scenario.  The collector
voltage idles around 190mV indicating saturation with the 3M3 resistor.
A lower value base pullup resistor (1M) seems to have a negative effect
on the gain.  In fact, a 6M7 resistor actually boosted the signal a bit
more.

>
> Try these values and let me know how you get on.

There you have it.  ;-)  Thanks for the information.

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