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Old 12th January 2009, 06:10 PM   #1
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Default Novel current source: the Tail Lengthener

Contemplating regulators one day, I thought of the general form of some typical circuits. Of course you are all aware of the series/parallel and voltage/current identities which occur in basic electronics analysis.

The typical forms are:
Pass voltage regulator: these start with a not-very-good voltage source (most often a somewhat resistive power supply) and have a dynamic (active) resistance (such as a transistor) in series with the load. To regulate voltage, the error amplifier reads the voltage in parallel with the load. Input, output and ground make this a three terminal regulator.
Shunt voltage regulator: a not-very-good constant current source (most often a series resistor from a voltage source) drives the load with excess current. A dynamic resistance is in parallel with the load, shunting excess supply current as needed. This regulator needs only two terminals.
Pass current regulator: the most common type, a voltage source supplies a dynamic resistance in series with a current sensor and the load. This is a two terminal device.
So the one remaining layout is...
Shunt current regulator: a current supply (perhaps a resistor) drives a load in parallel with a dynamic resistance. The current sensor is in series with the load, so there are input, output and ground pins, a three terminal regulator.

What does this look like? Here are two representative schematics.

Click the image to open in full size.

On the left, you essentially have the Io pin driving an emitter follower (assume for a moment that Rc = infinity) driving a constant resistance load, Rt, the "long tail" resistor. As such, the incremental resistance of the Io pin will be around hFE * Rt, which is a modestly constant current, but has no offset so it's no better than a resistor. The B-E resistor Rc adds that offset by shunting base current away until about 0.7V drop across the resistor, thus setting the output current. The result is an I/V curve which looks like a typical open collector, except the knee bends over at a rather high voltage (maybe 1/3 the supply), and the linear range has a bit steeper slope. Maybe more like a pentode, if you're into that. The downside is, a large amount of current is wasted, particularly at high voltages (if the knee falls at 1/3 +V, the shunt current near +V must be twice the output current!), but this behavior is understood as the nature of any shunt regulator.

This circuit is remarkably simple, using only two resistors and one transistor -- simpler, even, than any standard [series] current source using BJTs. It suffers from many downsides, including: wide knee (due to finite transconductance), poor incremental resistance and awful tempco (current will drop by about half from R.T. to 125C).

The circuit on the right is a somewhat more refined approach. The voltage developed by the current sense/set resistor Rc is compared to a TL431 bandgap voltage reference (2.50V) using a differential pair. For the diffamp, I've chosen a tail resistor Re, which is more than sufficient as the emitter voltage doesn't vary much beyond 1.8V (above common) in use. The differential pair outputs a constant current which appears across Rb, which biases the shunt transistor, which itself outputs a constant current to Rt, shunting the requisite tail current as needed. Because transistors are themselves native CCSs, PSRR is reasonable.

I call this connection the Tail Lengthener, because you can take a regular tail resistor, put the sense resistor and circuit in series (at the load), add a supply connection and have a much "longer" tail as a result.

Tim
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Old 12th January 2009, 06:55 PM   #2
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This looks like the perfect way to waste more power in class A tube circuitry.

What a grand idea!
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