Transistor base as a diode

Discussion in 'Electronic Components' started by Gazza, Nov 29, 2006.

  1. Gazza

    Gazza Guest

    Hello,

    I have seen several schematics where a transistor base has been used as
    a diode where the base is also short circuited to the collector (NPN).
    Please can anybody tell me why this is done and what the benefits are?

    Many thanks in advance.

    Gary
    Gazza, Nov 29, 2006
    #1
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  2. Gazza

    Bill S. Guest

    In article <>,
    says...
    > I have seen several schematics where a transistor base has been used as
    > a diode where the base is also short circuited to the collector (NPN).
    > Please can anybody tell me why this is done and what the benefits are?


    The most common case that comes to mind is a current mirror circuit,
    where you need an input diode with essentially the same I/V curve as
    your output transistor.
    Bill S., Nov 29, 2006
    #2
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  3. Gazza

    ian field Guest

    "Bill S." <> wrote in message
    news:...
    > In article <>,
    > says...
    >> I have seen several schematics where a transistor base has been used as
    >> a diode where the base is also short circuited to the collector (NPN).
    >> Please can anybody tell me why this is done and what the benefits are?

    >
    > The most common case that comes to mind is a current mirror circuit,
    > where you need an input diode with essentially the same I/V curve as
    > your output transistor.


    The way I read that, is as the OP mentioning a transistor with its CB
    shorted and the BE used as a diode - as opposed to in a configuration such
    as a CM. One application that springs to mind is that it yields a slightly
    lower Vf than a regular diode, the collector will begin to draw current at a
    VBE of around 0.5V. This is also very dependent on temperature, I recall
    that the old Germanium AC128 was often used in this way as a temperature
    compensator in the convergence panels of old delta-gun CTVs. Also most Si
    transistors exhibit a zener voltage in reverse - usually around 5V or so.
    ian field, Nov 30, 2006
    #3
  4. Gazza

    Joerg Guest

    ian field wrote:

    > "Bill S." <> wrote in message
    > news:...
    >
    >>In article <>,
    >> says...
    >>
    >>>I have seen several schematics where a transistor base has been used as
    >>>a diode where the base is also short circuited to the collector (NPN).
    >>>Please can anybody tell me why this is done and what the benefits are?

    >>
    >>The most common case that comes to mind is a current mirror circuit,
    >>where you need an input diode with essentially the same I/V curve as
    >>your output transistor.

    >
    >
    > The way I read that, is as the OP mentioning a transistor with its CB
    > shorted and the BE used as a diode - as opposed to in a configuration such
    > as a CM. One application that springs to mind is that it yields a slightly
    > lower Vf than a regular diode, the collector will begin to draw current at a
    > VBE of around 0.5V. This is also very dependent on temperature, I recall
    > that the old Germanium AC128 was often used in this way as a temperature
    > compensator in the convergence panels of old delta-gun CTVs. Also most Si
    > transistors exhibit a zener voltage in reverse - usually around 5V or so.
    >


    Sometimes the reason is much more mundane: Logistics. If the board
    doesn't need many diodes but already has transistors then using a
    transistor instead can reduce the number of BOM line items by one. Saves
    inventory costs, rigging costs etc.

    --
    Regards, Joerg

    http://www.analogconsultants.com
    Joerg, Dec 1, 2006
    #4
  5. Gazza

    Chris Jones Guest

    ian field wrote:

    >
    > "Bill S." <> wrote in message
    > news:...
    >> In article <>,
    >> says...
    >>> I have seen several schematics where a transistor base has been used as
    >>> a diode where the base is also short circuited to the collector (NPN).
    >>> Please can anybody tell me why this is done and what the benefits are?

    >>
    >> The most common case that comes to mind is a current mirror circuit,
    >> where you need an input diode with essentially the same I/V curve as
    >> your output transistor.

    >
    > The way I read that, is as the OP mentioning a transistor with its CB
    > shorted and the BE used as a diode - as opposed to in a configuration such
    > as a CM.

    That is the same configuration, for the input side of the current mirror.

    > One application that springs to mind is that it yields a slightly
    > lower Vf than a regular diode, the collector will begin to draw current at
    > a VBE of around 0.5V. This is also very dependent on temperature, I recall
    > that the old Germanium AC128 was often used in this way as a temperature
    > compensator in the convergence panels of old delta-gun CTVs. Also most Si
    > transistors exhibit a zener voltage in reverse - usually around 5V or so.


    But if you ever make them conduct in reverse like this (as a Zener) then
    those devices should be treated as suspect parts from then on. It has been
    alleged to do funny things to the noise performance etc. Certainly on some
    IC processes it is frowned upon to let that junction break down, even at
    low current levels.

    Chris
    Chris Jones, Dec 2, 2006
    #5
  6. Gazza

    Guest

    Gazza wrote:
    > Hello,
    >
    > I have seen several schematics where a transistor base has been used as
    > a diode where the base is also short circuited to the collector (NPN).


    The logarithm characteristics of a perfect diode are shared by this
    kind
    of connected transistor; so-called 'ideal' diode behavior is used in
    logarithm
    converters, and few 'switch' diodes (which are optimized for low
    capacitance) or
    rectifier diodes (which are optimized for high currents) can compete
    with
    a good low-noise transistor.
    , Dec 4, 2006
    #6
  7. Gazza wrote:
    > I have seen several schematics where a transistor base has been used as
    > a diode where the base is also short circuited to the collector (NPN).
    > Please can anybody tell me why this is done and what the benefits are?


    This is sometimes called a "super diode"; it has a maximum current
    rating equal to the maximum collector current of the transistor, and
    tends to have a nice low forward voltage drop at moderate currents
    compared with average small signal diodes, but the main advantage is
    that the diode characteristics match the characteristics of other
    transistors in the circuit so you get (for example) good current mirrors
    , especially if both are part of the same IC, so good thermal tracking
    and in some situations low distortion or little error over a wide
    operating current range.

    Mark A
    Mark Aitchison, Feb 10, 2007
    #7
  8. Gazza

    Eeyore Guest

    Mark Aitchison wrote:

    > Gazza wrote:
    > > I have seen several schematics where a transistor base has been used as
    > > a diode where the base is also short circuited to the collector (NPN).
    > > Please can anybody tell me why this is done and what the benefits are?

    >
    > This is sometimes called a "super diode"; it has a maximum current
    > rating equal to the maximum collector current of the transistor, and
    > tends to have a nice low forward voltage drop at moderate currents
    > compared with average small signal diodes, but the main advantage is
    > that the diode characteristics match the characteristics of other
    > transistors in the circuit so you get (for example) good current mirrors
    > , especially if both are part of the same IC, so good thermal tracking
    > and in some situations low distortion or little error over a wide
    > operating current range.


    It avoids much of the IR voltage drop of using the b-e diode alone since most of
    the current flows c-e.

    Graham
    Eeyore, Feb 10, 2007
    #8
  9. Gazza

    Jamie Guest

    Mark Aitchison wrote:

    > Gazza wrote:
    >
    >> I have seen several schematics where a transistor base has been used as
    >> a diode where the base is also short circuited to the collector (NPN).
    >> Please can anybody tell me why this is done and what the benefits are?

    >
    >
    > This is sometimes called a "super diode"; it has a maximum current
    > rating equal to the maximum collector current of the transistor, and
    > tends to have a nice low forward voltage drop at moderate currents
    > compared with average small signal diodes, but the main advantage is
    > that the diode characteristics match the characteristics of other
    > transistors in the circuit so you get (for example) good current mirrors
    > , especially if both are part of the same IC, so good thermal tracking
    > and in some situations low distortion or little error over a wide
    > operating current range.
    >
    > Mark A

    They make pretty good Zeners also if you need one around that voltage
    region that is heat sinkable! :)



    --
    "I'm never wrong, once i thought i was, but was mistaken"
    Real Programmers Do things like this.
    http://webpages.charter.net/jamie_5
    Jamie, Feb 10, 2007
    #9
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