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Re: Interesting LT/Analog op amp

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On 02/10/18 19:37, 698839253X6D445TD@nospam.org wrote:
> bitrex wrote
>> The LT1364/1365 dual/quad, 70MHz GBW and 1000V/uS slew rate:
>>
>> <http://www.analog.com/media/en/technical-documentation/data-sheets/13645fa.pdf>
>>
>> The simplified internal schematic is quite different than most
>> traditional three-gain-stage V -> I -> V amps, looks like a cascade of
>> "diamond"-configuration differential current amplifiers.
>
> Dunno what you mean by 'diamond shape'
> input is complementary emitter followers, nothing differential there, via R1 to next stage.
>
> The opposite phases are generated by driving the complementary transistor emitters with R1.
> Some current sources follow..
> IN+ drives in fact .. complementary emitter followers... sets the working point.
> Output is complementary emitter followers,
>
> Temp tracking should be really good, else it distorts like ..
>

The term comes from the OPA660 'diamond transistor' which, I believe,
was the first to use this architecture in an IC. The complementary
emitter followers combined with the current mirrors behave much like
a single transistor, with a high impedance 'base' at +IN, a high 
impedance 'collector' at the C-Rc node and a low impedance 'emitter'
at the right side of R1, with the added twist that currents can be
flowing either way. It's a nice architecture really, although the
OPA660 never got very popular.

The LT1364 adds another emitter follower at -IN, turning it into
something resembling a long-tailed pair, but owing to the symmetry,
the 'tail' of the pair is no longer needed. A pity R1 is so high,
because that sets the transconductance of the central stage to a
very modest 2mS at most.

Jeroen Belleman

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Interesting LT/Analog op amp bitrex <user@example.net> - 2018-10-02 12:22 -0400
  Re: Interesting LT/Analog op amp <698839253X6D445TD@nospam.org> - 2018-10-02 17:37 +0000
    Re: Interesting LT/Analog op amp Jeroen Belleman <jeroen@nospam.please> - 2018-10-02 21:39 +0200

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