[time-nuts] TNS-BUF update
bruce.griffiths at xtra.co.nz
Sun Aug 19 05:19:33 EDT 2018
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.
> On 19 August 2018 at 20:48 Charles Steinmetz <csteinmetz at yandex.com> 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.
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