Connect with us

Diff amp - one or two current sources?

Discussion in 'Electronic Design' started by Mats Karlsson, Aug 16, 2003.

Scroll to continue with content
  1. I have occationally seen differential amp stages (long tailed pairs) using
    two separate current sources rather than a single one, common to both
    transistors.

    What is the advantage of this - if any?

    To me it appears to be counterproductive and a waste of parts but I guess
    there may be a reason for it.

    Note - I am not talking about current mirrors as collector loads.


    Mats
     
  2. Ban

    Ban Guest

    If you use a single resistor from emitter to emitter, you do not have the
    voltage drop of the current through it, thus you gain a higher input voltage
    range or you can use a smaller negative voltage for the current sources.
    since you parallel the current sources, the additional part is only one
    transistor. it will also allow you to adjust the gain of that stage with a
    pot. If you use a trimmer for balancing you can do that by connecting the
    wiper to -Vee and use the two ends as emitter resistors for the current
    sources. This point is low impedance and doesn't induce noise. Sometimes you
    do not even need the transistors and use just a resistor as the current
    source.
    In this case there will be only advantages.
    But you are right, both arrangements are of equal effect.

    ciao Ban
     
  3. I'm going to quibble here a bit on the phrasing of this sentence. The
    fact that it is a low impedance re point, does not mean that it is a low
    noise pick up point. That is, low impedance does not imply low noise
    pickup. The classic example is the summing junction of an op amp. Sure
    the impedance -> zero, but Av -> inf. Noise will be injected into this
    node with a relative gain of Re/Roffset. If the trim resisters are low,
    the wiper wants to go through a large resister.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  4. Sorry I don't follow you - single resistor from emitter to emitter??
    I guess my questions wasn't to clear - sorry.

    To clarify - my two examples were:

    - Text book version: Q1 and Q2 emitters tied together and fed by a common
    current source.

    - Less common version: Q1 emitter fed from current source 'a' and Q2 emitter
    fed from current source 'b'.

    In the latter example, there is no point of contact between the two emitters
    or their respective current sources
    except that both ultimately ties in to -V.

    Mats
     
  5. Norm Dresner

    Norm Dresner Guest

    Doesn't it decrease crosstalk?

    Norm
     
  6. Ban

    Ban Guest

    Well, the latter case is not a differential amplifier at all in fact it
    won't amplify anything since the collector current is constant. :-((

    Are you sure you are talking about long-tailed pairs?

    Have another look into your book and draw an ASCII- schematic (easy with
    this tool):

    +----------+o Ucc
    | |
    .-. .-.
    | |R | |R
    | | | |
    '-' '-'
    | |
    +----------)----------o out-
    | |
    | +----------o out+
    | |
    |/ \|
    in+ -| long-tailed |- in-
    |> pair <|
    | |
    +----+-----+
    |
    |
    (v) current source
    |
    o
    Uee
    created by Andy´s ASCII-Circuit v1.23.080803 Beta www.tech-chat.de


    ciao Ban
     
  7. Ban

    Ban Guest

    Maybe you saw something like this?
    +----------+o Ucc
    | |
    .-. .-.
    | |R | |R
    | | | |
    '-' '-'
    | |
    +----------)----------o out-
    | |
    | +----------o out+
    | |
    |/ \|
    +-|cascoded pair |-+fixed potential
    | |> <| |
    | | | |
    +---)----------)---+
    | |
    |/ \|
    in+ -| |- in-
    |> <|
    | |
    +----+-----+
    |
    |
    (v) current source
    |
    o

    or the resistors were replaced by a current mirror?

    ciao Ban
     
  8. Well that was my first thought - this could NOT be a differantial amp as Q1
    and Q2 does not share anything apart from the supply rails!

    This is of course true as you point out!


    Thanks!

    Mats
     
  9. Jim Thompson

    Jim Thompson Guest

    Please post a schematic so we're sure what you are saying.

    It could be something as simple as needing a good 2:1 ratio.

    ...Jim Thompson
    --
    | James E.Thompson, P.E. | mens |
    | Analog Innovations, Inc. | et |
    | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
    | Phoenix, Arizona Voice:(480)460-2350 | |
    | [email protected]_innovations.com Fax:(480)460-2142 | Brass Rat |
    | http://www.analog-innovations.com | 1962 |

    For proper E-mail replies SWAP "-" and "_"

    I love to cook with wine. Sometimes I even put it in the food.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-