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 … )

The Simplest Possible Mixer, using MOSFETs.

When a curious person searches the Internet for the circuit diagrams of (electronic) mixers, there is a certain complexity of what he or she will find. Just for people who might not know, the type of mixer I’m referring to is a component which does not add two signals together – which is what the naming might seem to suggest – but rather, which multiplies two signals. In certain cases the mixer will produce output, that contains an additive component as well as a multiplied component. But it’s the multiplied component circuit designers are interested in, because that can be used:

1. In order to produce ‘mixed frequencies’, between two input frequencies, such as between a local oscillator and a Radio Frequency, resulting in an Intermediate Frequency,
2. In order to act as a phase discriminator, the output of which will be maximally positive or negative, when two input signals are in-phase, but the output-voltage of which will be some neutral voltage, when the input waves are 90⁰ out-of-phase with each other. In this latter case, two reasonably constant input amplitudes are assumed.

What search results will often show, is somewhat complex mixers, that require either one or two balanced inputs – meaning inputs conditioned such, that they each appear differentially between two input electrodes – and which have as advantage for being designed that way, low distortion of the wave-form(s) supplied differentially in this way.

But sometimes, low distortion is not required. For example, in the case of a PLL – a “Phase-Locked Loop” – It’s assumed that the feedback voltage changes the frequency of a VCO – a “Voltage-Controlled Oscillator” – but with the intended result that two outputs lock in some phase-position, so that the two frequencies that are inputs to the phase-discriminator will be exactly the same frequency. This latter need often arises in the design of ICs. This latter application does not require that the phase-discriminator be particularly linear, nor that its output-voltages, that become feedback voltages, be in any range other than the range which the VCO requires as input.

And so the question can arise, what the simplest way might be to design a mixer, with the added detail that both inputs are unbalanced inputs – i.e., that each input appears at one terminal, and not in an opposing way, at two terminals – and for the sake of argument, our IC might be limited to using enhancement-mode, N-channel MOSFETs as the main active component. And this would be my solution:

The concept is very simple. If Vin1 and Vin2 are at 180⁰, then M1 and M2 don’t conduct simultaneously. Therefore, R1 and Vcc (the supply voltage) achieve maximally positive average output-voltage. If Vin1 and Vin2 are at 0⁰ phase-position, the two transistors will become conductive in a way that coincides. Therefore, this is actually a Coincidence Detector. And the average  output-voltage will be maximally negative in that case. And, if Vin1 and Vin2 are at a 90⁰ phase-position, then the average output-voltage will be somewhere between the two values mentioned before.

I suppose it should be mentioned that, if the circuit designer knows ahead of time that one of the two inputs has a much higher amplitude than the other, or a more predictable amplitude, then this usually stronger input should be fed to Vin1.

As part of a feedback loop, the output needs to be followed by a low-pass filter, that emulates an integrator over the time-constant which is the fastest, with which that feedback loop is supposed to be able to react to a change in one of the frequencies. The simplest low-pass filter consists of a resistor followed by a capacitor… (:1)

And so, when looking for a way to implement a phase-discriminator, the curious person needs to choose which of the following has greater priority:

• The simplest circuit-design, or
• The lowest amount of distortion.

The circuit above will certainly give the highest amount of distortion.

(Updated 7/9/2019, 16h55 … )

NG-SPICE: Biasing the Default Transistor for Ideal Linear Voltage Gain, at 3V.

In recent days and weeks, I’ve been studying some of my own ideas, concerning the creative uses of the N-Channel, Enhancement-Mode, MOSFET. And to help me explore that subject, I’ve used An Open-Source Circuit Simulation Program called ‘NG-SPICE’. One big problem with this approach is the fact that the default transistor that the software assumes the power-user wants to use, is clearly not meant for Linear Voltage Amplification in the 100kHz-1.0Mhz frequency range, and with a 3V supply voltage. This transistor type is meant to be operated at higher voltages, and mainly, for digital uses. All the software is geared for Integrated Circuit Emphasis. But, I have looked at possible ways in which the default transistor could still be used under the conditions I’m more interested in. In theory, I could change the parameters of the transistor involved as much as I like, until I’ve made a high-speed, low-voltage transistor out of it. One problem with that is the fact that I give the software the geometry of the transistor on a chip, and the software then derives many of its assumed properties. I don’t know much about IC design, so I probably would not obtain the kind of transistor I’m looking for, if I tried to invent one.

So the question comes back, what is the best way to bias this one, arbitrary transistor-type, to act as a high-impedance amplifier under the conditions written above? And how much gain does it give me? The answer seems to be, that when connected as below, the best performance I can obtain is an Alpha of (-5.25):

What I’ve also learned is, that the bias voltage associated with this circuit, with respect to ground, is (+2.14V). With respect to the supply voltage, that is (-0.86V). 3.75μV of bias current would need to flow. This information would be useful if an attempt ever came along to implement This Idea.

(Edit 7/5/2019, 17h15 : )

Doubling (VGS – VT0) of M1 would have as effect, that IDS quadruples. It would also have as effect, that equal, small changes in Gate Voltage translate into doubled changes in IDS. But, if the increase in bias current was taken into account by the circuit designer, by putting a resistor of merely 100kΩ in series with M1, thereby achieving that the supply voltage was ideally halved again as a result, then this would finally have as effect to halve the net voltage gain at the Drain of M1.

It would also have as effect, to quarter output impedance, which would be desirable from the last of a series of these stages, ending in a realistic load of some kind.

(End of Edit, 7/5/2019, 17h15.)

The Model-Card of the transistor is linked below:

http://dirkmittler.homeip.net/text/NMOS1.mod.txt

To pursue the exact subject of the earlier posting, about Variable-Gain Amplifiers, I also felt that it would be necessary to add to the circuit the components, that would transform it into a variable attenuator. And the following schematic shows how I did that:

(Updated 7/16/2019, 7h50 … )