[time-nuts] TNS-BUF update
bruce.griffiths at xtra.co.nz
Sun Aug 19 04:22:28 EDT 2018
I devised the bias circuit for the TNS-BUF.
It exploits the fact that for a RED LED at least the difference between the LED forward voltage and the transistor Vbe is ~ 1V and has a fairly low tempco and has low noise (at least for RED LEDs).
(Most of the LTSpice LED models do not correctly predict LED forward voltage drop tempco.)
Most classical schemes for biasing BJTs use a resistive voltage divider which inevitably couples power supply noise into the BJT collector current.
John Miles changed the bias circuit of some classical series shunt amp buffers to one similar to this and the buffer flicker phase noise was significantly reduced.
In principle an LED could be used to directly set the dc bias at the base of the amplifier transistors, however inductors may be required to shunt part of the emitter series resistance at dc to allow the desired dc collector current to be established. A pair of series connected LEDs buffered by an npn emitter follower would allow the bias voltage to be shared by all stages and allow the inductor to be replaced by a capacitor bypassing part of the emitter to ground resistance required to establish the desired collector current whilst achieving the required resistance from RF to ground for RF.
Classical bias schemes are usually much noisier especially at low frequencies. Even regulators like the LT3042 are quite noisy at frequencies below the the pole of the reference circuit low pass filter.
One requirement was operation from a standard not necessarily ultra low noise power supply.
Sensing and controlling the collector current has the advantage of simplifying the emitter to ground network to a single series resistor.
This emitter series resistor also suppresses parasitic oscillations somewhat above 100MHz possibly due to feedback transformer parasitics.
> On 19 August 2018 at 15:31 "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
> I was reverse engineering the circuit so I could verify
> that the high power transistors are used in the amplifier,
> and the low power ones are bias controllers. The bias
> circuit resembles the ones used for applications where
> the transistor's emitter is connected directly to ground
> to reduce parasitic inductance at microwave frequencies.
> But this is 10 MHz and in any event, there is a 68 ohm
> unbypassed emitter resistor, which I assume is there to
> reduce flicker noise, which is indeed very low.
> Anyway, 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
> with a temperature sensing diode to cancel
> out drift of the base-emitter voltage. I'm not
> saying the circuit won't work, I just suggesting it
> is needlessly complicated.
> Can anyone clarify this?
> Can I make a high power version of this by
> simply changing to 2N3566/2N5109/2N5943, etc.
> Is the transformer feedback a poor man's Norton
> amplifier scheme?
> Rick N6RK
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