## What means exist, to provide a stable voltage, in a circuit that contains an IC, and which is constrained to using a supply-voltage of only 3V.

A concept which I’ve been pondering is, that a circuit could be designed which requires a controlled voltage, negative by a stable amount from the supply voltage, yet, where the supply voltage may only be 3V. And one of the assumptions I’m making is that, once a certain number of pins on the IC have been allocated to external, discrete components, additional discrete components can be connected to the same pins, at little additional cost.

Having said that, the fact is already established in circuit design, that the supply voltage itself is a big source of errors. It could take the form of a battery-set that goes weak, or a power-supply, the output of which is fettered by unpredictable behaviours from the circuitry which it is providing the supply voltage to. And such unpredictable behaviours could include low-frequency phenomena such as widely varying current drawn, so that simply to connect a power-supply capacitor will not remedy this issue.

Thankfully, 2.4V Zener diodes are now a part of what can be mass-produced. What this means is, that the Zener diode can be connected to the supply voltage at one end, thereby not taking up any pins on the IC, but connected to one pin of the IC at the other end, which is to be at -2.4V with respect to the supply voltage. That pin of the IC could serve two purposes:

• To provide an internal, series-connected resistor, to keep the Zener diode operating within its assigned current-range,
• To provide whatever reference voltage the IC itself might require, that would be (Vcc-2.4V).

But ultimately, the point was not, that an exact (Vcc-2.4V) be made available. The point was that a Control Voltage of (Vcc-X) be made available, where (X) is supposed to remain accurate. And so, to accomplish that, a 200kΩ Trimming Potentiometer can be connected in parallel with this Zener diode, again not taking up any pins of the IC. But, the wiper of this potentiometer can in fact be connected to one additional pin of the IC, where a calibrated voltage (Vcc-X) becomes stable. This can be repeated, only taking up one additional pin of the IC each time.

Dirk

## Variable-Gain Amplifier, adapted for etching into silicon.

One of the subjects which I’ve blogged about before was, The design of a variable-gain amplifier stage, that was really a variable-attenuation stage. This stage was neither suited for direct implementation with discrete components, nor on an IC. The reason for the latter detail was, that that circuit still contained coupling capacitors. Those are difficult to implement on an IC. However, I’ve done my best to do so now, in order to design a stage, which can be etched onto an IC.

My strategy for implementing a coupling capacitor was, that I’d tie the Source, Drain and Bulk electrodes of a P-channel MOSFET together on the side of the input, and use the Gate as output. However, since the N-doped well of a P-channel MOSFET also has capacitance to the substrate, I added a schematic component, that would be a ‘Semiconductor Capacitor’ according to ‘NG-SPICE‘, and the rectangular dimensions of which would just be slightly larger in each direction, than those of the MOSFET. This is meant to simulate the added, unwanted bypass-capacitor, which the preceding transistor-stage would need to be able to overpower.

This is the schematic:

These are the model-cards used:

http://dirkmittler.homeip.net/text/NMOS2.mod.txt

http://dirkmittler.homeip.net/text/PMOS2.mod.txt

http://dirkmittler.homeip.net/text/JUNCCAP1.mod.txt

And this was the Net-List that defines both the circuit, and one of the simulations:

http://dirkmittler.homeip.net/text/Default_NM_Gain_IF_6.net.txt

Obviously, on an actual IC, the capacitor ‘C1′ would not exist either. Instead, a presumed preceding stage would have another transistor, that does what ‘MC1′ does in this stage.

The concept behind this circuit was, that ‘M1′ is a working inverting amplifier with reasonable voltage gain – in the ballpark of ~18, if there was no circuitry designed to make it attenuate a signal. Simply because the voltage-divider exists between ‘R2′ and ‘R3′ at the input, that goes down to ~9. Additionally, the fact that ‘R5′ follows ‘MC1′, brings the voltage-gain down to ~6, when the control-voltage is 3.0V. But, as ‘M3′ starts to conduct, it starts to feed the inverted signal from the coupling-capacitor back to the Gate, where the feedback competes with the current being fed by ‘R2′. The higher the gain of ‘M1′ is, the better the negation of the signal is, that results.

All outputs should have some sort of load indicated, so I added ‘R5′. In fact, I get the impression that NG-SPICE runs into difficulty simulating an output-voltage, if there is no load resistor. But in reality, the current that flows from the Source to the Drain of ‘M3′ will also see to it that any following, chained stages are biased as this stage was biased. (:1)

This circuit has a surprising, simulated behaviour, in that it will regulate the output voltage down, almost to zero, as the control voltage increases between 4.1V and 4.25V…

(Updated 7/30/2019, 10h20 … )

## Playing with NG-SPICE again, and designing two resonant-circuit bandpass filters.

NG-SPICE is a program designed to simulate circuits. The acronym stands for (Next Generation) Simulation Program, with Integrated Circuit Emphasis. While NG-SPICE is open-source, its cousins such as LT-SPICE and PSpice are proprietary. However, NG-SPICE also uses advanced Mathematical modelling of components and circuits. Sometimes I find it to be an educational toy.

A type of circuit which some people might find interesting, is the IF strip – the Intermediate Frequency stage – of a radio receiver, which receives its signal after the Radio-Frequency signal has been ‘mixed’ with a Local Oscillator, and heterodyned down to the Intermediate Frequencies. And due to modern technology, a final, intermediate frequency of 450kHz can be used both for AM and FM demodulation.

There is a type of resonant circuit that employs capacitors and inductors – i.e., coils, in order to accomplish two things:

• To act as a bandpass filter, restricting the frequency range,
• To establish a phase-shift between the incoming carrier wave, and an oscillating, derived wave, that is dependent on the momentary frequency of the carrier wave, so that later in the analog processing of the signal, a phase-discriminator can complete the task of FM demodulation. This task is also referred to as Quadrature Demodulation of an FM carrier.

This type of resonant circuit is also sometimes referred to as a “Tank Circuit”.

In short, I’ve been reinventing the wheel. But I did read an article from elsewhere on the Internet, which inspired me. The subject of that article was, how to design Varactors, which are variable-capacitance diodes, when restricted to only using CMOS transistor-pairs. These diodes would represent a good way to tune circuits and vary the frequency of oscillators, in many types of applications. But I had an application in mind, which this type of varactor would help me solve. The mentioned, “IMOS Varactors” are remarkable because they don’t actually involve any diodes. They involve a way to connect an enhancement-mode P-channel MOSFET, so that the effect of gate-voltage changes on the MOSFET’s gate capacitance, acts as a varactor.

If somebody is designing a tuned circuit using the smallest, most-modern coils, manufactured by high-tech factories, then those coils allow for a high Q-factor to exist, which is a measure of how selective the filter can become, as well as to have good thermal stability, but if they are on a budget, these components will have some amount of tolerance, meaning that in a constant way, each component’s actual inductance value will vary to some degree. This is especially unfortunate since high-quality inductors on a budget, are also unlikely to be tunable. If the inductor in question is of a better sort, that ‘only’ has 5% tolerance, this would mean that with an improperly designed radio tuned to an intended AM frequency of 800kHz, instead, the listener could end up receiving a station at 780kHz, or at 820kHz, just because this one filter’s frequency is off by 5%. Of course, real radios that are designed to any level of satisfaction don’t behave that way.

What can be done, is that in the assembly-process for the radio, some machine calibrates its tuned circuits. But again, a maximal use of the main integrated circuit is assumed, and a minimal expense of external, discrete components is assumed. Here, a trimming potentiometer is a more-affordable way to do, what back in the 1970s and 1980s, tunable inductors would have done. If the assumption was made that for reasons I won’t go in to here, The IC can hold an exact voltage steady, then this voltage can also be applied to varactors internal to the IC, in a way that corrects for whatever amount of error was present in the coil.

Even though today, tunable inductors can be bought in quantity that also offer a Q-factor of 48, those aren’t just more expensive than the fixed variety. In addition, those would be much larger components, measuring maybe ‘half a centimetre’ cubed, and requiring to be soldered in to the circuit-board, while the fixed sort can be much smaller units, soldered onto a circuit-board as a surface-mounted device.

And so, reinventing the wheel in order to educate myself, what I have done was to design two circuits, one of which tunes in to 450kHz with the aid of such monolithic varactors, and the second of which does the same at 10Mhz instead. I’m using transistors that are not the tiniest in existence, but which are still too tiny, for an implementation of these ideas to be attempted with discrete components. Capacitances in picofarads should act as a warning to any reader, not to try this with discrete components. It’s much less-risky financially, just to run some simulations using NG-SPICE…

(Updated 7/27/2019, 12h05 … )

## Another Simple Output-Amplifier, Using Discrete MOSFET Transistors

One of the facts which I’ve been writing about, is that I possess the open-source version of ‘SPICE’, that is named ‘NG-SPICE’, and that this acronym stands for ‘Simulation Program, with Integrated Circuit Emphasis’. The full, associated suite of programs allows me to edit schematic diagrams graphically, but to export ‘Netlists’, so that I can then simulate the circuit – and see if it works.

And one of the facts which I have also been contemplating, is that by default, SPICE will put transistors, which correspond to micron-sized transistors, which will therefore never be able to drive output-loads, from a hypothetical IC, unless an explicit attempt is made, to design output-buffers, which can. These output-amplifiers have as function, that they should merely follow their input voltage, but draw as little current from their respective inputs as possible – that are outputs of other, more interesting ICs – while allowing low load-resistances to be connected to their own outputs, which correspond to plausible external components, such as 100Ω load-resistors.

I had posted an earlier, conceivable design, of such an output-buffer, which had a major flaw, that I also pointed out in the preceding posting: That amplifier could only produce a range of voltages, which was a direct function of what the Gate-Source threshold voltages would be, of the component transistors used. Hence, because I had also specified low-quality, outdated MOSFET transistors with high threshold-voltages, the output-voltage-range, was also modest but reasonable. But, newer transistors will have lower threshold voltages by design, which would, oddly enough, reduce the voltage-range of that amplifier. This would be an important consideration if the transistors were not in fact discrete, but needed to be incorporated onto the IC, where low-threshold-voltage transistors are already standard. Which means, that I needed to design a better output-buffer.

So below is a better output-buffer, schematic:

And these are the SPICE definitions, of the discrete transistors which I decided to base my design on again, both enhancement-mode MOSFETs:

http://dirkmittler.homeip.net/text/2N7000.mod.txt

http://dirkmittler.homeip.net/text/BS250P.mod.txt

The main disadvantage of this latest design would be, that the transistors which I labeled ‘X2′ and ‘X3′, do in fact conduct current to their combined inputs, which makes the additional transistor ‘X1′ necessary, since this amount of current would already be excessive, to connect to an output, of any pre-existing IC circuits. But then, the advantage goes so far, that ‘X2′ now models a level-shift, which exactly mirrors the level-shift of ‘X4′, and the voltage-level-shift of ‘X3′ now mirrors ‘X5′. There is design beauty in this. But one disadvantage now is, that the Gate-Source threshold-voltage of (1) n-Channel MOSFET (2.2V) plus (1) p-Channel MOSFET (3.2V) gets subtracted from the input-voltage, so that the available voltage-range still suffers, with respect to both the supply, and the input-voltage. Input-voltage now ranges from 5.4V to approximately 12.5V, which is closer to the range of supply-voltages than what the previous circuit allowed, and the resulting output-voltages are graphed below:

(Update 06/20/2018, 0h20 : )

There is another observation which I should add:

In the days of vacuum tubes, ‘transconductance’ was measured in Amperes / Volt, and was therefore given in ‘Mhos’, which were the reciprocal of Ohms. Apparently, in modern days, the transconductance of a MOSFET, also given as its ‘KP’, is in Amperes / Volt2 . This conscious design-decision must follow the real-world behavior of MOSFETs, but makes my earlier Math, of multiplying such a component-property by the series-resistance, to arrive at gain, incorrect. Gate-Source voltage-changes lead to current-changes, but greater Drain-Source voltages, lead to greater current-gain. This is good, because the actual gain of a MOSFET, reduces the apparent capacitance at its Gate.

The low-end output-voltage came into being as follows: