Bob wrote:
There were (and maybe still are) SOT-89 versions of the 2N3804 and 3906. They will handle more power than most of the other versions. That gives you better Vce on the string.
Bruce wrote:
The PZT3904 and PZT3906 are still available.
With most of these old circuits reducing the LF noise contribution to the emitter/collector current by the biasing circuit by utilising lower noise power supplies and/or using improved biasing methods can improve the clse in PN significantly.
The lower transistors on the totem pole operate in current mode and typically have only 3-5v from C to E, so their dissipation is very low even if the standing current is high. I used TO-92 and SOT-23 transistors there. The top transistor, which must develop the output voltage, dissipates more than a TO-92 or SOT-23 should be asked to handle, even with a good heatsink, so I typically used TO-5 (TO-39) or SOT-89 transistors (with heatsinks) there.
It is definitely true that the published NIST designs did not minimize circuit noise. I used *much* larger base bypass capacitors, and improved capacitance multipliers. I also used separate capacitance multipliers for the base divider string and the collector supply. I played with ultra-low-noise DC regulators on each base and for the collector supply, which gave modest improvements in both noise and distortion -- but the added complexity was not justified by the gains, IMO. I did find that doing without the interstage resistors (from the collector of one transistor to the emitter of the next higher transistor) reduced both noise (modestly) and distortion (slightly). In the very rare cases when a circuit oscillated (I experimented with *many* combinations of transistors), I used ferrite beads in place of the resistors or on one or more of the base leads. Choosing transistors with very low base spreading resistance was another key to lowering noise.
Best regards, Charles _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
