Electronic – What does the term “slow drift” mean

Can An Operational Amplifier Circuit Be Made Entirely Out Of Diode Nand And Nor Gates?

This apparently simple-enough question is somewhat ambiguous and can be answered several ways.

• Spehro has assumed that you convert the input to a digital value and perform digital arithmetic on it. So he says the answer is yes.

• ScottMcP takes your question at face value, notes that you say "diode gates", which have no gain, assumes that you want to use the digital gate as an analog amplifier, and as diode gates have analog gains of < unity, says no.

• WhatRoughBeast also takes the "digital gate as analog amplifier" and notes that the digital gate lacks features needed to make an opamp. But he also notes that you can make a single input amplifier using a digital gate as an analog gain stage.

  • Placeholder adopts "digital gate as analog stage" approach BUT goes the second mile and shows how a "true" [tm] differential input amplifier could be constructed.

• There another option, not covered so far. That is, inputting analog signal into a digital gate which is acting as a "digital" oscillator (albeit using an analog timing delay) so that the gate outputs a digital signal, but the gate's digital operation (in this case the oscillator mark-space ratio) is affected by the analog signal, allowing amplification in the digital domain and then conversion back to audio.

Below I cover the "digital gate converted to an analog amplifier" aspects and then introduce the rarely seen [draws deep breath] "digital gate acting as an analog amplifier while producing digital output which can be easily be converted back to analog" concept. Usually more fun than useful, but potentially of value 'in an extreme situation'.

So:

Gate is digital, signals are analog"

No, not if the gates are used as digital logic gates and the amplifier deals only with analog signals.

Diode resistor logic gates with no gain.

And, also no, if the gates have no "gain".

• Diode & resistor only gates have no gain - but they are also never used in anything practical because of loading by subsequent stages.

"DTL" gates operating in analog mode.

Only "sort of".

DTL = Diode-Transistor-Logic gates can have gain and can be made into (not very good) amplifiers by using them "strangely". The series diode connected inputs place constraints on what can be achieved with analog inputs. You'd have to be very keen.

More modern digital gates in analog mode:

If you are prepared to use digital gates in analog mode, as is done in some applications, then yes, it can be done. This is relatively unusual in everyday use and is usually a compromise or "done for fun" or extreme cost cutting.

General background:

An "Op-Amp"is an analog signal amplifier with high gain (Vout/Vin = "high", a high input impedance and a low output impedance. I could explain what "analog" means in this context but, if that's unclear, it's an essential part to understanding the question, let alone the answer, and you can very easily research it yourself.

If SOME digital gates have connections made from output to input via a resistor or resistor network, they can be biased into a linear state and then used as an analog amplifier.

Some Diode AND and OR gates MAY be able to be biased in this manner and so used as an amplifier. This is not using them in their intended mode and the amplifier quality would be poor. The diodes would add very significant distortion. Even older RTL logic may work better than this but it's still not a true digital gate when used this way.

Example of a modern CMOS gate being used in an analog biased mode to create a lineara amplifier.

Circuit is from here

They say:

• CMOS inverters can be used as linear amplifiers where negative feedback is applied. Best linearity is achieved with feedback applied around three inverters which gives almost perfect linearity up to a dynamic output of 5 V peak to peak with a 10 V supply rail The gain is set by the ratio of Rl and R2 and the values are typical for a gain of 100. The high frequency response with the values given is almost flat to 20 kHz. The frequency response is determined by Cl and C2..

  This circuit is not suitable for low level signals because the signal to noise ratio is only approx 50 dB to 5 V output with the typical values shown.

Analog amplifier with purely digital gate:

You will only rarely find this concept described. It works.

BUT (and many people are not aware of this) you can use some sorts of digital gates as analog amplifiers while remaining in a digital mode. This is a rather (or very) special case.

eg if a 74C14 Schmitt triggered input inverter has a resistor connected from output to input it will oscillate (for a certain range of resistors), using stray capacitance as part of the RC timing circuit. If you now AC couple an AC signal to the input the output will be pulse width modulated. If you low pass filter the output signal you can get amplified analog. Ask me how I know :-). The secret to "keeping it digital" is the Schmitt trigger input. The gate cannot be biased into a linear mode due to hysteresis and oscillates instead.

fosc should be >> finput. As the gate input is driven above half way by the incoming AC the negative to positive transition times of the oscillation are skewed, and similarly as Von falls below half voltage, but with opposite effect on mark space. The resultant output is variable mark-space PWM driven by Vin. Filtering this AC signal recovers the amplified signal.

Diagram based on this original

What intuitions or rules of thumb would faithfully guide me on when to consider an OTA instead of a "regular" op amp; perhaps illustrated by any "classic" applications where an OTA would be preferred (and why)?

You can't really compare an OTA with a regular OpAmp. OpAmps are simple building blocks that you'll usually "configure" to do one fixed operation by adding components around it.

OTAs are similar but have the added benefit that once you've "configured" them you can still control certain aspects of the operation (lets say amplification) by applying a control current.

The key difference is, that an OTA has three inputs while your OpAmp has only two. Besides the two differential input terminals that an OTA and an OpAmp share, the OTA has a third input that lets you set the gain of the amplifier by applying a current.

This third input enables you to do things that you just can't implement with a simple OpAmp: The OTA is able to multiply two time varying signals!

The OpAmp on the other hand is able to multiply (or amplify) as well, but only one signal is time varying (the one at the differential input). The other factor that goes into the multiplication is constant and defined by the feedback resistors.

Typical use-cases of OTAs are "Voltage Controlled Amplifiers".

Lets say you want to control the volume of an audio signal. For a stereo signal you can use a stereo potentiometer, attenuate the signal and then buffer it using an OpAmp. Fine, but how would you accomplish the same thing if you are dealing with more than two channels? A 5.1 sound-system for example? You'll probably won't find potentiometers with more than two channels.

Here OTAs come to the rescue: You can use a single potentiometer to generate a control voltage and feed it to any number of voltage controlled amplifiers. With the turn of a single knob you can now control the volume of any amount of audio channels as you like.

Another common uses are automatic gain controls. Here a signal gets amplified based on it's amplitude. A signal with low amplitude gets amplified a lot while a signal with high amplitude will just get buffered. The goal here is to generate a signal with less dynamic range at the output. This may avoid clipping the signal and prevent low amplitude parts to be buried in noise. 20 years ago you found these kind of circuits in dictating machines, telephones, tape recorders etc. Nowadays the job is cheaper done in software.

Another big field where OTAs are used are "voltage controlled filters". Here you don't control the amplification of a signal but the cut-off frequency of a filter. Around the half of all analog synthesizer filters from the eighties are based on OTAs.

From the circuit design point of view OpAmps and OTAs are also used differently:

OpAmps are almost always used in closed-loop configuration. E.g. You'll almost always find a resistor or other component that goes from the output to the negative input. As you probably know this used to bring down the very high open-loop gain of an OpAmp down to some useful level.

OTAs on the other hand are very rarely used in closed loop configuration, e.g. you won't find the typical resistor from output to negative input. This is because they don't have the high open-loop gain to begin with. The gain of the OTAs are defined by the current going into the gain control input after all.

This has several consequences: Think about a voltage follower built around an OpAmp. The output of the OpAmp directly connects to the negative input. If you apply a voltage to the positive input the negative feedback makes sure that the voltage difference between the differential inputs is almost zero.

Since there is rarely negative feedback in OTA circuits there is also no mechanism to keep the differential inputs at the same voltage. Instead you'll find a huge voltage divider before the inputs that keep the maximum voltage difference of the input terminals at 10mV to 30mV (rule of thumb). If you go above this the OTA will become more and more nonlinear and will output a highly distorted signal.

Regarding your voltage regulator: This is really a bad use-case for an OTA because you don't need the gain program-ability feature. You could build one using an OTA, but the cool feature of the OTA would not be of any use.