[time-nuts] Switching transistors, current sources, nonidealties and noise

David davidwhess at gmail.com
Sun Jun 19 18:24:15 EDT 2016

>Why do people use general purpose transistors in these places, even
>though RF transistors definitly improve switching behaviour?

Commercial designs do use RF transistors but only old ones are

The Tektronix 7A11 uses 2 GHz PNPs and 1 GHz NPNs but its design is
unusual since it can integrate positive or negative time and while the
transistors use emitter switching, they are not configured as
differential pairs.

The Tektronix 2440 uses a 1.2 GHz NPN differential pair for the fast
ramp switching and a 2N3906 differential pair for the slow ramp

>Having put the circuit through Spice, I see that the current through
>the tail fluctates violently during the time when the current switches
>from one transistor of the pair to the other. The reason for this seems
>to be that the f_t of the current source transistor is too low to compensate.
>Trying to replace the current source with an RF transitor like BFU520
>that has an f_t of 10GHz helps to dampen these fluctuations by a factor of 2,
>but they are still there.

Of interest in these designs is that they do not use separate
transistor current sources where fast switching is involved; the
differential pairs do double duty and the tail current is set by a
resistor to a separately decoupled bias supply.

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