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off the top of my head- use a second 2n3904 and set up the pair in a
darlington config?
D
At 11:43 PM 10/6/03 -0500, you wrote:

>I have a transistor (2n3904) that is used as a buffer/amp/conditioner in
>my software based 1200 baud modem receiver.  AFAICT, the input is in the
>200mV range and I need a 5V swing on the output.  Right now, the
>transistor is connected as follows.  The base is fed the DC blocked
>audio signal.  The base is pulled up to Vcc with a 3M3 resistor and has
>a 5V zener to ground to protect it from the ringing signal voltage.  The
>collector is tied to Vcc with a 1M resistor.  The emitter is routed to
>ground thru a 2K resistor.  As I understand this, the transistor is held
>in saturation by the self biasing action.  The collector "idles" at
>about 200mV and the base at about .5 - .6V The problem is that this only
>gives me a 2V swing at the collector with a 200mV drive applied to the
>base.
>
>After some tinkering, I changed the collector resistor to 330K and the
>emitter resistor to 1K and this gives me 4V peak at the collector, but I
>don't really know why.  Ideally, I want to have as much gain as possible
>to the point of massively clipping the signal in order to shape the sine
>wave into a pseudo square wave for driving the PIC pin digitally.  I
>don't need any real current from the transistor, just some more voltage
>gain and faster switching.
>
>Can someone recommend something using the same 2n3904 that will give me
>considerably more gain and consequently a steeper rising edge to the
>PIC.  I'm using a 16F628, but don't have enough pins available to use
>the internal comparator to do the job.  I'd also like to accomplish this
>with just discrete components and no extra ICs.
>
>michael brown
>
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