Though not difficult, driving a load from a voltage reference IC requires some attention. After determining the supply voltage and output voltage, various other design parameters need to be considered. These include the output-voltage temperature coefficient, initial accuracy, drift, noise, line and load regulation, package size and type, power consumption, stability with various capacitive loads, and the required source and sink capabilities.
The need to source or sink more current than the voltage reference can provide is a common problem. A precision unity-gain buffer amplifier offers an adequate solution in applications that can tolerate its additional drift, noise, and gain inaccuracy. One serious drawback, however, is the buffer's potential instability when driving capacitive loads (such as the well-bypassed reference inputs of an analog-to-digital or digital-to-analog converter). Attempting to guarantee the buffer's stability by introducing an isolation resistor (between op-amp output and capacitive load) further degrades the reference circuit's accuracy.
Another alternative approach is to cancel the load, that is, to make it seem like a sizable resistance. If load resistance is made to appear large, the remaining load is then composed of any capacitance that may be in parallel with the load resistance. The load may be cancelled by placing a negative resistance in parallel with the load's positive resistance. If the magnitudes of these positive and negative resistances can be made equal, the effective load resistance becomes infinite. Unlike a buffer amplifier, this negative-resistance circuit adds negligible output error (Fig. 1).
The input resistance from VIN to ground is negative, and can be calculated as follows:
Adding this circuit to the output of an ultra-stable reference able to drive ±15 mA (Fig. 2) lets the reference drive ±50 mA or more. With perfectly matched components, the reference would source negligible dc current. When 1% resistors are used in the negative-resistance circuit, the required worst-case output current is ±2 mA. Load cancellation improves the output accuracy by lowering the error created by the reference output resistance. It also minimizes any drift due to self-heating, particularly when the output current is large and the difference between the output voltage and reference-supply voltage also is substantial. In addition, the circuit is unconditionally stable with any capacitive load.
This exotic circuit solution seems to be original; actually, it is very, very old. For the first time, I saw it in the book "L’amplificateur opérationnel et ses applications Masson, Paris, 1971" by J. C. Marchais; then I was a student and was trying without success to grasp the idea behind the odd circuit). In spite of that, I have not yet seen any reasonable human friendly explanation of this clever circuit arrangement (including the book mentioned).
Believe me, it is not sufficient to say that "a negative resistance -R connected in parallel with a positive resistance R gives an infinite resistance". In order to really understand the circuit, readers (as human beings) need to know what problem the op-amp solves, why it is connected there, what it actually does in the circuit, how it does this magic, etc.
The idea is extremely simple; we may see it in the daily round when someone (something) helps someone (something) else to such extent that the latter needn't do anything. Examples of this "exact helping": parents help children to such extent that they needn't do anything, a husband supports his wife to such extent that she needn't do anything:), compensating the load by an anti-load (anti-weight) used in the lift systems and cranes, etc.
In the circuit discussed, the op-amp (or, more precisely, the whole NIC converter acting as a negative resistor) does exactly the same - it "helps exactly" the input reference voltage source by injecting the whole current needed through the load. In this way, it serves as an "exact helping" source, which has actually replaced the input source. As a result, the load does not consume any energy from the input source since it is supplied completely by the "helping" source. There is no current flowing between the output of the reference source and the load since the whole rest part of the circuit (the load + the NIC) behaves as a dynamic load with infinite internal resistance. Actually, this is the famous "bootstrapping" technique; it is put in practice for the first time by Baron Munchhausen who was using his own boot straps to pull himself out of the sea:)
In this circuit, the parallel connected negative resistor -R "neutralizes" the positive one R giving the whole load current. Then, in another implementation of this powerful idea - the famous Howland current source (and Deboo integrator as well) - the negative resistance absorbs a part of the positive one (the positive resistance dominates); it adds only a "correcting" supplementary current, in order to keep a constant current through the load. Finally, if the negative resistance dominates (it has completely absorbed the positive one and some negative resistance is left) the circuit will act as a negative resistance amplifier (in this case, the input voltage source has to have an output resistance).
If you want to know more about this topic and the basic ideas behind circuits, visit my "Circuit stories on the whiteboard" located on www.circuit-fantasia.com and the Wikipedia page about negative resistance... or just send an email to me.
email: cyril@circuit-fantasia.com
Cyril Mechkov -April 21, 2007
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