> From:         Mike Singer[SMTP:m_singer@POLUOSTROV.NET]
> Sent:         Wednesday, January 07, 2004 8:40 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: How to measure light? Phototransistor?

>> John N. Power wrote:
>> Second, this amplifier is not meant for precision measurement of
>> low currents. I suggested it only for use in a collision detector.
>>The fact that there is a 5% maximum change in mirror gain at the low
>> end of the scale does not mean that that rate is maintained over the
>> entire range.

> This could mean much worse situation. By the way, 20uA or 200uA
> aren't lower end of the range, 0.00002 ua is. 6 mA input current will
> cause tremendous temperature drift (unspecified).

      The following is from National Semiconductor application note AN-72,
      "The LM3900 - A new current-differencing quad of +/- input amplifiers.
      (September 1972)",
      page 3:
      " The open-loop gain changes only slightly over the complete power
      supply voltage range and is essentially independent of temperature
      changes."

>>           I agree that a 20 times change in gain would be bad. I
>> don't believe that it is that bad. The change in mirror gain is only
>> given for the range 20uA to 200uA; that indicates that the effect is
>> typical of the low current end of the scale.

> That indicates nothing for < 20uA or > 200uA. One must suspect
> worst case.

      On page 5 of the same application note, this appears:
      "Only the (-) input must be supplied with a DC biasing current, Ib.
      The (+) input couples only to the (-) input and then to extract from
      this (-) input terminal the same current (A, the mirror gain, is
      approximately equal to 1) which is entered (by the external
      circuitry) into the (+) input terminal."

      Clearly, there is no expectation that the mirror gain changes
      significantly. There are numerous examples of triangle, sawtooth
      and ramp generators in this apnote. A change in mirror gain
      with input current would cause a supposedly linear output to be
      curved. The applications given in this note would not work if this
      happened. I would breadboard a circuit myself and check this,
      but I can't do it right now. I trust National to know something
      about these matters.

      In any case, the amplifier does have limitations. The most pronounced
      is its lack of speed. It is suitable only up to audio frequencies. It
      is not intended for precision work, and that was not the original
      application. The chip is second sourced by Motorola, has been
      around for a respectable amount of time, as you can see from the
      date on the apnote, and is still available. This would not happen
      if it had serious problems with linearity.

      John Power

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