> 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.

5.0/0.2 = Gain of 25.

Max gain possible is about 38.4 x Vcc ~= 200 so it can be done.
(People will query that gain calculation ! :-) )

The **** FIRST **** thing to try is to place an electrolytic capacitor from
emitter to ground - say 10 uF should do. More wont hurt.

> 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

This is not a legitimate way to bias silicon transistors.
It will work after a fashion sometimes maybe.
It's legit for Germanium.
(People will query ... :-) ).

A silicon transistor should have the bias point set explicitly. In this case
with two R's - one each to supply and ground from base.

> and has
> a 5V zener to ground to protect it from the ringing signal voltage.  The

probably not needed but OK

> collector is tied to Vcc with a 1M resistor.  The emitter is routed to
> ground thru a 2K resistor.

Gain with emitter bypassed with cap as above would be about
    Rc/26 * IcmA
    Here Icmax = 5/1m = 5  uA = 0/005 mA
    So gain = 1m/26* 0.005 = 200 = OK

Try the emitter bypass capacitor !!


> As I understand this, the transistor is held
> in saturation by the self biasing action.

if it germanium :-)

>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.

You can get a truly lovely result with 2 transistors in a long tailed pair
(Google ...) but letssee if we can make the transistor worlk OK (although it
may be already with that cap.)

Set Vc at 1/2 supply quiescent.
Aim at say 1 mA Ic to start
Rc = (Vcc-Vcquiesc)/Ic = (5-2.5)/.001 = 2500 ohms.

We want a smll emitter resistor to allow base to float away from ground
clamped position.
Say Re = 100r gives 100/2500 * 2.5 = 0.1 volt dc at 1mA so OK.
Place 10 uF+ across Re

Ve = 0.1v so Vb = 0.6 + 0.1 = 0.7v or so.
V Rb-+ = 4.3v
V Rb-gnd = 0.7v
ratio of Rs about 6:1
10K:1.5K
12K:2k2 etc using R12s

10:1.5K
Vb = 1.5/11.5*5 = 0.65 too low

12k:2k2
Vb = 2.2/14.2 x 5 = 0.767v = OK

Try
R C-V+ = 2500r
R B-V+ = 12K
R B-gnd = 2k2
R E-gnd = 100r
C E-gnd = 10 uF
C coupling in = whatever (10 or 100 uF?)

Required Ib = Ic/beta = 1 mA/100 say = 10 uA
I in 12K Rb-V+ is about 4.2/12k = 300 uA so plenty for base.

Howzat ?


        RM






Try Rb-Vcc = 10k
Rb-gnd = 2k7
Vb = 2.7/12.7 * 5 =








>
> michael brown
>
> --
> http://www.piclist.com#nomail Going offline? Don't AutoReply us!
> email listserv@mitvma.mit.edu with SET PICList DIGEST in the body
>

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body