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Old 1st October 2001, 10:12 PM   #11
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Join Date: Jul 2001
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Xavier

I've understood we are talking about *double* differential input stage, with one npn differential pair, and one pnp differential pair, base connected together....

Regards, P.Lacombe.

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Old 2nd October 2001, 07:27 PM   #12
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Yeah I think that is correct. One differential towards the plus power supply and current source towards negative power supply and the opposite.
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Old 2nd October 2001, 07:56 PM   #13
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Then I'm right u'll are talking about Mirror-image differential Input stage( One differential stage complementary to other and their bases tied together).As I have said before this provides excellent PSSR and slew rate.

PSSR:
An amplifier with single differential input stage can be made hum free at ideal( without input signal) but its difficult to know whether it will remain hum free at all loads, dual/mirror-image differential stage deals with it seemlessly.

Slew-rate:
In an amplifier slew-rate is defined by how Cdom sources and sinks current while input stage sinks and sources. Sourcing and sinking of currents through Cdom( Capacitor at dominant pole) does not follow the same positive and negative levels because the current passes through two different routes. This leads to asymmetry in the output. Since dual/mirror-image differential stage is complementry to each other the output is exactly symmetrical.

Other advantages:
Complementry actions cancels transistor parameter variations.
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Old 2nd October 2001, 08:49 PM   #14
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For me this is the best kind of input circuit for an amplifier. It has excellent performance and it solves a lot of problems.
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Old 2nd October 2001, 08:53 PM   #15
Michel is offline Michel  Canada
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john t
tube designers do not shy away from dual diff. stages....they have nochoice as there are no complementary tubes.
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