For the record, I only spoke to Bob Widlar once about audio. (I may have been interviewed by him for a job at Fairchild, in 1963, but I didn't get the job).
This was at an ISSCC conference, in PA, USA in 1974: Bob was on a panel of analog designers. I got up 'knee's quaking' (really) and asked that with the new knowledge of TIM put forth by Matti Otala, where do linear IC's stand with this? He said, that I SHOULD MAKE MY OWN AUDIO OP AMPS, because his designs and those of the others, as well, were NOT designed for audio.
Walt Jung was in the audience, and he came up to me afterward, we went to dinner that night, and have remained friends ever since.
This was at an ISSCC conference, in PA, USA in 1974: Bob was on a panel of analog designers. I got up 'knee's quaking' (really) and asked that with the new knowledge of TIM put forth by Matti Otala, where do linear IC's stand with this? He said, that I SHOULD MAKE MY OWN AUDIO OP AMPS, because his designs and those of the others, as well, were NOT designed for audio.
Walt Jung was in the audience, and he came up to me afterward, we went to dinner that night, and have remained friends ever since.
john curl said:You first.Or let's let the rest of the class respond first.
Just home from a party and quite tired, but let me risk a guess at it.
There is nothing that sets a well defined Q current for the Rush cascodes in the VAS, so there is a potential risk for thermal runaway in them. If Tj increases, Ic will increase for a given Vbe,causing Tj to rise even more and... well you get it. Althogh the global NFB will maintain a near zero output voltage, I cannot see how it could prevent such themal problems in the VAS.
Of course proper cooling may be sufficient to make this a non-issue in practice.
Or a resistor between the emitters in the Rush cascodes could also prevent this problem, at the expense of lower gain.
It's the third time during this week we see Rush's name in this forum. Each time, some of us have pointed the fact that a circuit having a common base transistor does not imply that it is necessarily a cascode.
I even wonder if some circuits called "folded cascodes", where the biasing voltage of the input device is referenced to ground and the biasing voltage of the common base device is referenced to one of the power supply rails, can really be considered as a cascode.
There already were some previous threads about the Rush's circuit : it was said that the curious and interseting feature of the circuit is that it is a non-inverting (between the input base and the collector output ) amplifier circuit.
I even wonder if some circuits called "folded cascodes", where the biasing voltage of the input device is referenced to ground and the biasing voltage of the common base device is referenced to one of the power supply rails, can really be considered as a cascode.
There already were some previous threads about the Rush's circuit : it was said that the curious and interseting feature of the circuit is that it is a non-inverting (between the input base and the collector output ) amplifier circuit.
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