A realistic way of driving LEDs.

One concept which has existed for some time is, that LEDs can produce a variable amount of light, and this will be the case, regardless of whether that amount of light needs to be constant, modulated slowly, or modulated at very high frequencies. But, LEDs have as a property which many other components do not have, that they tend to produce a fairly constant, forward voltage drop (like any diode), but that, as the voltage increases only slightly past a certain point, current increases rapidly. And, in the History of Electronics, this has often caused circuit designers to put a resistor in series with the LED, to regulate its current accurately.

One big drawback of doing this is, that power gets wasted, as current flows through the resistor, and gets transformed into heat. The amount of power that gets wasted in that way, most strongly depends on what fraction of the supply voltage appears across the resistor, instead of across the LED. Another drawback is the fact, that the current which flows through a resistor, which has simply been connected between a supply voltage and an unpredictable component – such as the LED – is itself not constant, And, when supply voltages are low – such as 5V – small changes in supply voltage are large, in comparison to the only slightly smaller voltage-drop across the resistor. And so, technology does offer as alternative, a chip, with active components to regulate the current more precisely, and often, while wasting less energy. In principle, such a chip can also be installed by the manufacturer of LEDs, into the same package as the LED.

According to the schematic below, I have demonstrated such a circuit…

 

IMult_LED_1

 

What is happening here is, that A presumed control current is fed in to a presumed input pin, and this circuit actually doubles that current, resulting in an amount of current which will be drawn from the cathode of the LED. Additionally, more than one LED could be connected in series, to the current-sink.

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Hypothetical Variable Gain Amplifier

What I find is that in recent years, the term ‘Variable Gain Amplifier’ has changed in meaning, to correspond more to a ‘Variable Attenuation Stage’, after a fixed-gain amplifier. And this seems especially true, when applied to ‘IF Stages’ – ‘Intermediate Frequency Stages’ – Of a radio receiver. I’ve also observed that low-distortion technologies are preferred in recent years, as opposed to the high-distortion technologies that manufacturers were limited to, say, in the 1970s, when ‘AGC’ was first being marketed to consumers.

Yet, even with the technologies that are now available, there are sometimes added constraints. For example, if one wanted the variable-resistance component either to be optical – for lowest distortion – or, for that to be a JFET – easier to implement – then, this component might need to exist externally to an IC, just because the IC itself may be engineered only to allow for two complementary types of transistors, those being, an enhancement-mode N-channel MOSFET and an enhancement-mode P-channel MOSFET. Further, The properties of such MOSFETs can sometimes be inconvenient, in the form of high Threshold voltage, named ‘VT0′, which is the voltage required to make the transistors start to conduct. Practical values of VT0 may be more suited to logic circuits, than to the processing of low-amplitude, analog RF or IF frequencies. A thinner oxide layer for the entire IC can reduce the required VT0.

Yet, the possibility exists for even a MOSFET to operate in ‘Triode Mode’, which is a mode in which it is Not ‘Saturated’. This mode is achieved when:

VDS < VGS – VT0

The problem in trying to reach this mode seems to arise in the fact that if, VT0 is already a higher-than-desired voltage, VGS-VT0 is likely to be a lower-than-desired voltage-range, since VGS is also limited by the supply voltage.

In Triode Mode, a MOSFET effectively behaves like a variable resistor, which decreases in value as the Gate voltage continues to increase.

And so to summarize what form the task might take, to make the Variable Gain Amplifier monolithic with a MOSFET-based IC, I constructed the following, hypothetical diagram, which does not explicitly nail down what VT0 is supposed to be, nor the supply voltage:

Serge_VCR_3b.svg

 

What I seem to have noticed however, in order for the suggested IF stage to work, is that the actual signal should not have a ‘Peak Amplitude’ at the Gate of the last amplifier stage, greater than (0.1V). Yet, the feedback-loop itself, that adjusts attenuation, could play a role in keeping that peak amplitude close to (0.1V).

(Corrected 7/7/2019, 11h05 … )

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