How to use a differential amplifier ?

[jalbers@bsu.edu]

After some reading, I understand that a differential amplifier
amplifies the voltage difference between the two inputs. Further
reading hints that differential amplifiers are good for rejecting
noise in the input signal but don't give any concrete practical
examples with real numbers that are elementary enough for me to
understand.

About the only thing that makes any sense to me is that the (input
signal) and a (copy of the input signal only inverted) are both sent
to the inputs of the differential amplifier. Any noise in the
transmission lines would effect each input equally and not be
amplified. Is this even close? If not, then how does a differential
amplifier tend to reject noise?

Any help would be greatly appreciated. Thanks

 

[Rich Webb]

On Thu, 9 Oct 2008 12:39:02 -0700 (PDT), "jalbers@bsu.edu"
<jalbers@bsu.edu> wrote:

After some reading, I understand that a differential amplifier
amplifies the voltage difference between the two inputs. Further
reading hints that differential amplifiers are good for rejecting
noise in the input signal but don't give any concrete practical
examples with real numbers that are elementary enough for me to
understand.

About the only thing that makes any sense to me is that the (input
signal) and a (copy of the input signal only inverted) are both sent
to the inputs of the differential amplifier. Any noise in the
transmission lines would effect each input equally and not be
amplified. Is this even close?

Yup, pretty much. There's a brief discussion at
http://en.wikipedia.org/wiki/Common-mode_rejection_ratio

--
Rich Webb Norfolk, VA

 

[John Fields]

On Thu, 9 Oct 2008 12:39:02 -0700 (PDT), "jalbers@bsu.edu"
<jalbers@bsu.edu> wrote:

After some reading, I understand that a differential amplifier
amplifies the voltage difference between the two inputs. Further
reading hints that differential amplifiers are good for rejecting
noise in the input signal but don't give any concrete practical
examples with real numbers that are elementary enough for me to
understand.

About the only thing that makes any sense to me is that the (input
signal) and a (copy of the input signal only inverted) are both sent
to the inputs of the differential amplifier. Any noise in the
transmission lines would effect each input equally and not be
amplified. Is this even close? If not, then how does a differential
amplifier tend to reject noise?

---

OK, from National's AN31,:

http://www.national.com/an/AN/AN-31.pdfhttp://www.national.com/an/AN/AN-31.pdf

here's a differential (difference) amplifier: (View in Courier)


R2
+--[10K]--+
R1 | |
V1-----[10K]-+-|+\ |
| >----+-->Vout
V2-----[10K]-+-|-/
R3 |
[10K]R4
|
GND

In general:

/ R1 + R2 \ / R4 * V2 R2 * V1 \
Vout = | --------- | | --------- - --------- |
\ R3 + R4 / \ R1 R1 /


For R1 = R3 and R2 = R4:

R2
Vout = ---- (V2 - V1)
R1

Let's say that V1 is 1V and V2 is 3V. Then the output voltage will be:


R2 10kR
Vout = ---- (V2 - V1) = ------ * (3V - 1V) = 2V
R1 10kR

Now let's add a little 'noise' to both of the inputs in the form of an
identical DC signal added to both inputs.

Actually, let's add a whopping big signal, 5V, to each of the inputs.

Then we'll have:


R2 10kR
Vout = ---- (V2 - V1) = ------ * (3V + 5V) - (1V + 5V) = 2V
R1 10kR

Same as before, even with a huge common mode signal in there as well.
"Common mode" because it appears in common at the input ends of R1 and
R3.

Amazing, huh?

JF

 

[Jon Slaughter]

<jalbers@bsu.edu> wrote in message
news:c6ad7317-303f-4091-bdae-16d25297fbf6@a18g2000pra.googlegroups.com...

After some reading, I understand that a differential amplifier
amplifies the voltage difference between the two inputs. Further
reading hints that differential amplifiers are good for rejecting
noise in the input signal but don't give any concrete practical
examples with real numbers that are elementary enough for me to
understand.

About the only thing that makes any sense to me is that the (input
signal) and a (copy of the input signal only inverted) are both sent
to the inputs of the differential amplifier. Any noise in the
transmission lines would effect each input equally and not be
amplified. Is this even close? If not, then how does a differential
amplifier tend to reject noise?

Any help would be greatly appreciated. Thanks

Yes, that is mainly how they are used(in fact I do not know any other way
but I'm not an expert).

You input f and -f on on the inputs and you get f - (-f) = 2f on the
output(obviously when configured as an amplifier).

Now if you end up with some noise on the two inputs, say e1 and e2, then you
have

f + e1 - (-f + e2) = 2f + (e1 - e2)

If you can make e1 and e2, which in general are small anyways, have a high
correlation, then e1 - e2 ~= 0 and you get 2f. if e1 and e2 do not
correlate, or even say that e2 = -e1 then obviously it does no good... but
such things don't really happen in practice such as using twisted pair or
even just straight(twisted pair makes the correlation between e1 and e2 even
greater).