Norton CB transformer feedback amps are quiet but they have low reverse isolation.
I've used that bias scheme with 2N5943's in the circuit using a 3 winding transformer with one winding in the collector one in the emitter circuit (driven via a series resistor) and one driving the load. Series shunt feedback circuits can be quiet but have poor reverse isolation. However if a transformer driving the load is used between the collector and the feedback circuit collector node the reverse isolation is improved significantly and the amplifier is a bit quieter than an amplifier with the same gain just using a collector load transformer and an emitter series resistor. Bruce > On 19 August 2018 at 20:48 Charles Steinmetz <[email protected]> wrote: > > > Rick wrote: > > > The bias > > circuit resembles the ones used for applications where > > the transistor's emitter is connected directly to ground > > This is often called a "wrap-around" bias circuit. > > > since there is this 68 ohm resistor, > > I don't see why it isn't sufficient to simply > > connect a fixed bias of about 1V to the base. You > > could even temperature compensate the voltage > > > I'm not > > saying the circuit won't work, just suggesting it > > is needlessly complicated. > > That is more or less correct, although see below re: noise. > > The wrap-around circuit creates an approximately temperature-compensated > current with an LED, the bias transistor, and the bias transistor's > emitter resistor. This current runs through a 1k resistor to ground to > generate an approximately temperature-compensated voltage that is used > to set the amplifier transistor's base voltage. > > That said, the amplifier transistors run quite hot, particularly the > last one, which operates at about 30mA -- nowhere near the much lower > temperature of the LEDs and bias transistors -- so the temperature > compensation does not really do much to stabilize the amplifier current > (note that the same would be true of your posited ~1v voltage source, > even if it were temperature-compensated). The wrap-around circuit *is* > relatively quiet, so your voltage source would likely need to be at > least as complicated to match its bias noise. > > > Can I make a high power version of this by > > simply changing to 2N3566/2N5109/2N5943, etc. > > transistors? > > In principle, yes. However, note that the maximum DC current rating for > the specified transformers is 30mA. For more power, you would almost > certainly want to pass more quiescent current through the amplifiers. > You would need a different transformer at least for the third stage, > since that stage already draws about 30mA. > > > Is the transformer feedback a poor man's Norton > > amplifier scheme? > > It is a so-called "noiseless feedback" circuit, yes. > > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
