BJT voltage amplifier question, transistor basics

[panfilero]

I have a small voltage.... millivolts, like 20mV, that I need to
amplify to around 1V, how would I do this? I was thinking about using
a BJT, and feeding this voltage into the Base, but then I remembered
something about the Base Emitter voltage having to be over 0.7 V in
order for the BJT to be on, so I'm not sure how to turn on the BJT to
have it amplify my small signal.

thanks
Joshua

 

[panfilero]

Is this a very low frequency 20 mV (essentially DC) or some
definite AC frequency?

it's DC, it's actually to feed into an Analog to Digital converter
input on a microcontroller, I need to amplify my signal before feeding
it in there.

Amplification is simpler for AC signals, because you can
capacitively couple successive gain stages, blocking the DC
bias voltages between stages. DC amplification requires not
only canceling the bias voltages, but compensating for
temperature caused changes in those bias voltages.

Oh, sounds complicated, I thought it would be simpler

Operational amplifiers (opamps) were invented to do those
last two things, and if you can get them, they would
definitely make life a lot easier for something like this.

Here is a basic tutorial on opamp applications. It shows
how to program them for all sorts of uses, including gain
blocks. At DC or low frequency AC, one would have little
trouble amplifying 20 mV to 1 V (gain of 50).http://www.national.com/an/AN/AN-20.pdf

thanks, I'll look into that option, so then would using a BJT not be a
good solution for this?

 

[MassiveProng]

Why is your PC clock ALWAYS fucking wrong?

Are you just on the other side of a date line?

Or do you have some lame Usenet retard desire to see your post
appear at the top of a new header pull?

 

[Jamie]

By the time you got it to work well, you would have just about
reinvented the opamp. Much easier to buy one and program it with a few
resistors. After you have read about them a bit, come back with the
power supply voltages available in your system, an we can help you
select an opamp that will work well in that situation. By the time you
get this working, you will wonder how you lived with opamps until now.

..............................................Lived with out? )
just poking!

 

[Charles Schuler]

Why is your PC clock ALWAYS fucking wrong?

Are you just on the other side of a date line?

Or do you have some lame Usenet retard desire to see your post
appear at the top of a new header pull?

PLONK

 

[MassiveProng]

snip

FIX YOUR FUCKING PC CLOCK, BOY!

 

[John Popelish]

I have a small voltage.... millivolts, like 20mV, that I need to
amplify to around 1V, how would I do this? I was thinking about using
a BJT, and feeding this voltage into the Base, but then I remembered
something about the Base Emitter voltage having to be over 0.7 V in
order for the BJT to be on, so I'm not sure how to turn on the BJT to
have it amplify my small signal.

Is this a very low frequency 20 mV (essentially DC) or some
definite AC frequency?

Amplification is simpler for AC signals, because you can
capacitively couple successive gain stages, blocking the DC
bias voltages between stages. DC amplification requires not
only canceling the bias voltages, but compensating for
temperature caused changes in those bias voltages.

Operational amplifiers (opamps) were invented to do those
last two things, and if you can get them, they would
definitely make life a lot easier for something like this.

Here is a basic tutorial on opamp applications. It shows
how to program them for all sorts of uses, including gain
blocks. At DC or low frequency AC, one would have little
trouble amplifying 20 mV to 1 V (gain of 50).
http://www.national.com/an/AN/AN-20.pdf

 

[John Popelish]

it's DC, it's actually to feed into an Analog to Digital converter
input on a microcontroller, I need to amplify my signal before feeding
it in there.


Oh, sounds complicated, I thought it would be simpler


thanks, I'll look into that option, so then would using a BJT not be a
good solution for this?

By the time you got it to work well, you would have just
about reinvented the opamp. Much easier to buy one and
program it with a few resistors. After you have read about
them a bit, come back with the power supply voltages
available in your system, an we can help you select an opamp
that will work well in that situation. By the time you get
this working, you will wonder how you lived with opamps
until now.

 

[John Popelish]

John Popelish wrote: (snip)

.............................................Lived with out? )
just poking!

Yes. Thanks.

 

[panfilero]

snip

FIX YOUR FUCKING PC CLOCK, BOY!

what are you talking about clock? I'm asking how to amplify a voltage
signal, has nothing to do with my PC clock, did you even read my post?

PLONK!


Joshua

PS - I think I'm going to go with the OpAmp solution, the main part
that confused me about the BJT was that if I want to amplify a real
small signal.... how do I get the BJT to even turn on if it need the
Base Emitter Voltage to be 0.7 V...... no problem like this with the
op-amp. Or maybe some kind of open collector high impedance type of
thing.

 

[John Popelish]

PS - I think I'm going to go with the OpAmp solution, the main part
that confused me about the BJT was that if I want to amplify a real
small signal.... how do I get the BJT to even turn on if it need the
Base Emitter Voltage to be 0.7 V...... no problem like this with the
op-amp. Or maybe some kind of open collector high impedance type of
thing.

You have to arrange for the transistor to see the sum of
your signal and a much larger bias voltage that changes as
temperature changes the voltage it takes to keep the
transistor conducting a fixed amount of collector current.
In an opamp, this is done by using a pair of input
transistors configured as a differential pair, biased in a
way that keeps the sum of their collector currents fixed,
while canceling each other's bias voltage, but combine the
effect if the input signal on the two of them. The output
signal is the difference between their two collector
currents. It is essentially impossible to duplicate the DC
accuracy of this arrangement with a string of single
transistor gain stages.

If you want to actually get inside an opamp to understand
how this 'magic' works, see:

http://www.national.com/an/AN/AN-A.pdf

So what supply voltage is available to power your opamp, and
what is the range of input signal voltage and the desired
range of output voltage the opamp should produce with this
range of input voltage?

 

[Rich Grise]

Yes. Thanks.

Kind of like Women (or Men, if you happen to be a Woman):

Can't live with'em, can't hunt'em down and kill'em. ;-)

[ NOTE: THIS IS A JOKE!!!!! -----^ ]

Cheers!
Rich

 

[Rich Grise]

[troll crap snipped]
PS - I think I'm going to go with the OpAmp solution, the main part
that confused me about the BJT was that if I want to amplify a real
small signal.... how do I get the BJT to even turn on if it need the
Base Emitter Voltage to be 0.7 V...... no problem like this with the
op-amp. Or maybe some kind of open collector high impedance type of
thing.

Well, once you get into looking at opamp input circuits, you'll come
to realize that the way they did this in the old days was to use a
"differential pair" - that's two transistors with their emitters
connected together and to a common resistor to the negative supply.

Then, just ground the input of the other transistor, and that provides
a reference. The output is the differential voltage between the two
points where the two collectors of the transistors connect to their
load resistors:

+V +V
| |
[R] [R]
| |
+--------------------- -
| | Differential output
| +---------- + (might need level-shifting)
| |
| |
|/ c c \|
in -----| b b |-----+
|> e e <| |
| | |
+-----+----+ 0 V ref.
|
[R]
|
-V

But, to get around zero at the input, you need a bipolar supply. (both
positive and negative relative to the 0V reference, usually called ground.)

As John has said, this is pretty much the way opamps are designed, but
with more buffering & stuff. And the R values can be important, depending
on the application.

Hope This Helps!
Rich