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Electronics => Beginners => Topic started by: texane on January 17, 2010, 08:36:50 am

Title: [ comparator with npn transistors ]
Post by: texane on January 17, 2010, 08:36:50 am

Hi,

I am working on a small project for
which I need to compare a voltage
to 2.5v. The compared voltage is
called Vin. If Vin is more than 2.5,
I need a Vout to 5V, otherwise to 0V.
I do not want to use a comparator,
but transistors for learning purposes.

I was thinking about using the a
voltage divider so that Vin is scaled
from [0, 5 volts] to [0, 1.4] thus my
comparing voltage become 0.7v (ie.
was 2.5v in [0,5] becomes 0.7v in
[0,1.4]) which I plug to an opamp
which is linked to the transistor Base
pin.

< 0.7v wont switch the transistor ON,
otherwise it will. I can replace the opamp
with another transistor.

Is the basic idea good? Any comment
welcome! I have read relying on the
BaseEmitter voltage drop is not a good
thing...

Thanks for helping,

Title: Re: [ comparator with npn transistors ]
Post by: Andrew on January 17, 2010, 12:12:49 pm

You want to use an OpAmp, but not a comperator?

I'll tell you a secret. A comperator is an OpAmp, just specially optimised for working as a comperator. So I don't see the learning experience here - replacing a comperator (which is an OpAmp) with an OpAmp, albeit one not specially suited for comparing.

Title: Re: [ comparator with npn transistors ]
Post by: texane on January 17, 2010, 02:32:43 pm

hi,

forget about the opamp, and replace it
with a transistor providing the same
voltage (ie. Ve = Vb - 0.6) but with a
gain in current. My question was about
relying upon the drop of 0.6v (0.7v) at
Vbe.

Thanks,

f.

Title: Re: [ comparator with npn transistors ]
Post by: Zero999 on January 17, 2010, 05:53:11 pm

You'll need several transistors, not just one.

Google for 'differential pair' and learn the basics of differential amplifiers which is what goes into op-amps and comparators.

Look at the datasheet for a comparator and op-amp IC and base your design on it.

Accept, that your circuit will never be as good as an IC because none of the transistors are matched.

Title: Re: [ comparator with npn transistors ]
Post by: texane on January 17, 2010, 06:03:59 pm

ok, thanks for pointing me to the right direction.

Thanks,