Electronics notes/Amplifier notes

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The below is about electronic components that are amplifiers of some kind.

For amplifiers as in "the complex box that makes your audio go", see Electronics project notes/Audio notes - amps and speakers



Current amplifiers

Typically transistors, or constructions based largely on them.

Op amps (and differential amplifiers)

Side note: While most op amps are voltage amplifiers, there are some variants that are sensitive to current.

See http://en.wikipedia.org/wiki/Current-feedback_operational_amplifier


General properties and behaviour

Schematic symbol for an op amp.
Note that the + and - inputs may be swapped in schematics when it lessens line crossings.
The power supply lines (top and bottom) are often implied and omitted)
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Differential amplifiers are

  • Differential in that they amplify the difference between the voltage at the two inputs (+ and -)
  • Voltage amplifiers in that they amplify the differential voltage. They don't amplify current (like BJTs and FETs - see transistors).


Operational amplifiers are differential amplifiers of a specific design. They:

  • have very high input impedance,
  • have low output impedance, and
  • have a very high gain - that is, open loop gain; in most uses this is tamed by not using it as an open loop

Most of the below is specifically about op amps.



Behaviour tl;dr

  1. Tries to keep two two inputs at same voltage
  2. No current flows in/out of the input pins, so it does #1 by changing the output
...which it can only do when there is feedback - some circuit back into the inputs. How it's connected back there determines the behaviour.

These, and some other basic laws, help intuit what most op amp circuits do (even if detailed analysis sometimes comes down to specs).

See e.g. EEVblog #600 - OpAmps Tutorial - What is an Operational Amplifier? on a rundown of common functions and basic but pretty decent details how.




The - input is the inverting input, the + input is the non-inverting input, which mostly refers to what they are often effectively used for (not to polarity, or direct implications on how they must be connected).

The top and bottom wires, often labeled VS+ and VS-, are the power supply. These are often not shown at all and implied to be attached to something obvious, like the circuit voltage and ground, or to a present bipolar power supply. See also #Supply voltage


It can be useful to know that...

  • high gain usually means somewhere in the 10,000 ... 1,000,000+ range
and means that if not controlled, the amp will just saturate (basically act like an input-comparing on-off switch)
...so most applications reduce the gain.
  • input impedance is high
...meaning inputs draw nearly no current, often no more than a few dozen nanoamps. For most uses and calculations (but not all) this is effectively negligible, and means opamps won't noticeably alter circuits/signals they measure.
  • output impedance is low
    • ...because its output comes from a final amplification stage. They can give you a few milliamps. To drive anything more you'll probably want to control a transistor.
    • this is also an important part of their feedback behaviour; it means they are fairly good at achieving a given voltage


Feedback means the output at some point goes back into the input.

Without feedback it's basically a comparator (meaning the output saturates at one of the possible extremes, dependin on which input has higher voltage), due to the high gain making the transition area so small.

With feedback things can get more complex. Often it can help to consider that an op amp will try its best to make the difference between the inputs zero (via the feedback), so design change how it will change the output to minimize the difference between the input terminals. Op amp's high gain and low output impedance help with that.



Supply voltage

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Op amps have two power connections, the positive and negative supply, often marked V+ and V-.


Split supply setups refer to using two (non-ground) voltage rails, often from a bipolar power supply, to for example putting +12V on V+ and -12V on V- (or whatever other voltage you use).

The alternative is single supply - usually VCC on V+ and Gnd on V-.

Many amps can be used in both split supply and single supply setups, but check the data sheet to be sure.
An amp's specs will mention supply voltage restrictions, for example "±1.5V to ±16V, or +3V to +32V", meaning that in split supply, V+ must be with in 1.5V..16V and V- must be within -1.6V and -16V, while in single supply, V+ must be within 3V and 32V.
these details can particularly matter on battery-powered things (wall powered setups are less constrained, though split supply will require at least one (extra) regulator).


Rail-to-rail amps (see also mention of "wide voltage range") can output voltages very near the power supply (V+ and V-) voltages, often within a few hundred millivolts, or even a few millivolts. Amps that aren't rail to rail stop at a noticeable margin - for example, the LM324 can go almost to ground, but stops ~1.5V short of VCC.

Rail to rail amps may be preferable in single side applications, particularly when used in low-voltage applications.


To avoid getting near the limits of the voltage range, In single supply applications you would often put (particularly AC) signals on top of a basic (often resistor-divided) voltage, probably a voltage about halfway between V+ and V-.

In single supply applications you would often put a signal (particularly AC signals) on top of a basic voltage that's probably about halfway towards V+ and V-, largely because you don't want the signal to be near either of the rails (V+ or V-) as no amp is purely rail to rail, and you'ld get strange nonlinearities.

This is less of a worry in split supply applications since the signal is almost necessarily near neither of the rails.


In single supply amps you'll often have to bias the signal to add some voltage - for example, in a 0..5V application, you might put your signal on a resistor-divided ~2.5V.


Operating supply voltage is the difference between the supply terminals

  • for single supply the minimum potential at V+.
  • For split supply: V+-V-(verify))

Amps that can be used both ways, the range is often the same - for example, LM324 has a 3V to 32V span and single supply limits, which in split supply means ±1.5 to ±16.


Amps may have a minimum supply voltage: When supply voltage is below a certain point, amps may clip their output (...at a lower voltage than its usual saturation level), distort, or drop out completely. (verify)

Power-supply rejection and decoupling (noise-related)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Ideally, the output of an op amp is independent of voltage ripples it gets on its power supply.

They try to do well. The PSRR (power supply rejection ration) gives the degree to which changes in its supply voltage make it into the output, in decibel, usually something like 100dB, up to 120 or 130dB in good amps, while figures like 80, 70, or 60dB aren't so good.

You can improve somewhat it by adding decoupling capacitors on the amp's voltage supply. A 100nF (0.1uF) cap covers most applications, lower is possible depending on application (primarily speed).


Current drawn

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The two most interesting figures here are the quiescent current and the output current.


The output current is what the final amp can drive at its output. This can be a figure like 20mA, 50mA, 150mA for some.


Supply current(verify) (Is, Icc) and quiescent current (Iq) refer to the current required to operate the amp with no signal applied.

Quiescent current is the current drawn when no signal is applied, which in general means 'it won't draw less than this when you're not using it'.

  • This may be a figure like 5mA to 10mA in general amps, which matters when you want to minimize power use.
  • Low-power means something like 1mA or less, like 300uA or even 60uA(verify).
  • Low power is preferred in battery powered applications (in mains-powered applications, amps are often minor in terms of power draw).


Low-power op amps are usually slow - relatively or very (low bandwidth, low )

They are also often noisier, partly because the circuit would probably use higher-value feedback resistors to keep current low.

In some situations, you can save power by disabling an op amp while you're not using it.

Signal voltage stuff

Stability

Precision

Input offset voltage

Drift, temperature effects

Input bias current

Phase inversion

Phase inversion refers to an issue where if one of the inputs goes below the negative rail's level, the output goes to V+.

Youc can consider this a design flaw that happens in certain amps amps.


In some cases it is easily avoidable, or patched well enough with a diode, but if left to happen will put a nasty pulse-like thing over your output.

Speed

Bandwidth

Slew rate

Noise

Common mode gain/rejection

Unipolar/bipolar

Terminating op amps

If you use a dual or quad op amp and don't use all of its parts, you should terminate the rest so that it won't do parasitic nonsense.


Looking around for op amps

Some more notes...

On choosing amps

Op amps are non-trivial circuits and can be optimized for many different specific things. There are literally thousands of op different amps in production. Design choices mean tradeoffs, so while looking around it is useful to prioritize what is most important to your application.

For almost any application, only a few specs are important, while the rest just have to be vaguely good enough. Amps that are good at more than you need tend to be pricier than necessary for that application.

There may also be special-purpose amps -- things that are differential/operational amps in nature, but have a design that makes them usefuly (only) to a specific task. Consider for example current sense amplifiers.


Some amps are generic in that if you have no tight restrictions on noise, voltage, power, bandwidth or such, you can use cheap generic amps.

However, often enough you do have some restrictions. For example:

  • High frequency applications
    • will require enough bandwidth (and possibly slew rate)
  • Audio applications
    • you probably don't care much about slew
    • no real bandwidth requirements
    • may care about Input offset, sometimes drift
  • Sensor amplification
    • often require low noise
    • often require low offset
    • often require low drift (minimal temperature influence)
    • may not care about power use
  • Battery powered applications
    • wish for require a low operating voltage
    • require single supply operation (sometimes preferably rail-to-rail)
    • prefer low power
    • often have no direct care much about slew


http://www.adafruit.com/blog/2006/02/28/specifying-an-op-amp/


In some cases, two amps in a row can do better than a single fancy amp. You can't escape noise specs, but you can e.g. spread gain bandwidth(verify).


Basic notes on gain and feedback

Open-loop means no feedback circuitry. Since the gain is so high (usually something in the range 10000..1000000), this is only useful as a comparator, not to get an amplified form of a voltage signal.


For this reason, pretty much all other op amp circuits are closed-loop applications, which uses negative feedback, a.k.a. degenerative feedback. The op amp's output opposes its own input, and it will try to minimize the difference between input terminals (Horowitz & Hill voltage rule). This reduces the effective gain of the amplifier, and it makes it easy to control gain with a few components external to the amp.


This behaviour can seem a little magical at first



Positive feedback also exists, but is probably only useful for controlled oscillation. (verify)

Design and behaviour details

On temperature

On capacitors

Some uses

Op amps have a few few dozen basic setups/uses, and tons of variations.

Some of the more basic constructions follow.


Biasing an op amp

Comparator

Comparator.

The one exception in the rules in the whole set of configurations. This is the open-loop configuration of an op amp, and because there is no feedback between output and intput, the very high gain dictates that this is now a comparator.

Behaviour:

  • output = input difference times the open-loop gain. That gain is huge (order of a million), so it'll saturate to either output effectively it means:
if the + input (V2) is larger than - (V1), then Vout = VS+
if the - input (V1) is larger than + (V2), then Vout = VS-
  • in other words, it acts as a (nearly) boolean comparison between two two voltages.
  • if the inputs are microvolt different you may not see it saturated, but this is unlikely to ever happen unintentionally, and not something you'd design for


Useful e.g. for switching based on comparison to a voltage references.


Notes:

  • There are also ICs that are specifically comparators but with slightly different behaviour
  • 'nearly boolean': because the gain is not controlled, the region where the op amp does differential amplification exists, but is very small. You'll only really see a comparator saturated in either direction.
  • When you want to avoid possible high-speed output changes if the inputs are near each other, you may want a Schottky trigger instead.



Voltage follower / buffer

Voltage follower.

Vin to +, output also loops back to -.

Behaviour:

Vout = Vin

Uses the non-inverting port largely because when you want to follow a voltage, you probably don't want to invert it.

From the rules above: the output drives whatever it needs to - to make it equal to +, so will make all three equal.


Zero gain. Primarily useful as a buffer, e.g. when you want to not disturb what's on the input (inputs are high impedance), yet be able to drive a little on the output (low impedance output, does milliAmps before needing to worry).

So e.g. useful between a sensor (that would be affected by drawing current) and a component that cannot itself guarantee negligible load (e.g. and ADC)



Basic variants of amplifiers

Non-inverting amplifier

Non-inverting amplifier.

The resistors are used primarily to reduce the op amp's native huge gain to something much lower.

A single-ended amplifier, in that the - is referenced to ground, so only + is relevant (+ is the same as the difference between it and ground). You can feed AC and negative DC in, the gain just applies relative to ground.


Behaviour:

Vout = Vin * (1 + R2/R1)

(The 1 comes from a simplification: (R1 + R2)/R1 = 1+R2/R1)



Inverting amplifier

Inverting amplifier.

Behaviour:

Vout = -(R2/R1) * Vin


Note that due to the inputs-are-the-same rule, the bottom reference in the image, e.g. ground, will be the same voltage as what will be made present on the -.

This seems confusing at first in the analysis, because that means there would be no differential, and no signal to be amplified. That's mainly because you're thinking of the feedback as going through the op amp's output -- but its it's-rail, and can both sink and source voltage -- it's actually being sunk through V-, which is also why its effect is inverting.


Note that when used in a single-sided power supply, you would put half your voltage on the op-amp + side (e.g. a voltage divider between V+ and Gnd), which will offset both the input and output by that amount -- because by our rule above, the op amp will try to make the - side (still called virtual ground now) this voltage too.


Note: the point in the circuit at - is called virtual ground because it will measure as 0V (well, whatever's on +), but not actually tied there directly.




Why use inverting versus non-inverting

Differential amplifier

(Not to be confused with a differentiator)


Instrumentation amplifier

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

An in-amp turns a differential input into a single-ended output.


The basic idea is to use two amps for input buffering, and one as a differential amplifier.

You get high-impedance input - the input buffering is basically independent of the details (such as the gain) on the differential amp.


Often one of its main uses is rejecting common-mode DC voltage - including noise that is present on both inputs.

They will often also allow you to shift the voltage up or down.


An instrumentation amp can refer to an IC with this function -- or to building it yourself (mostly three op amps, and some resistors to control gain). The IC will typically offer better specs, e.g. CMRR, possible protections, and such.

You can construct it so that you can control the gain of both input buffers via a single resistor - which can be particularly handy if you use a (precision) trim potmeter. The IC variant will have two pins to connect this resistor/potmeter.



http://www.allaboutcircuits.com/vol_3/chpt_8/10.html http://en.wikipedia.org/wiki/Instrumentation_amplifier



Current to voltage amplifier

http://en.wikipedia.org/wiki/Current-to-voltage_converter

Voltage to current amplifier

Summing amplifier

Precision rectifier

Zero level detector

Schmitt trigger

Negative impedance converter

Relaxation oscillator

Wien bridge oscillator

Inverting integrator

Inverting differentiator

Miller effect

See also

Unsorted:


Operational Transconductance Amplifiers (OTA)

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Operational Transconductance Amplifiers are voltage-controlled current sources.

They are op amp's little brother, in that they also have a high impedance differential input, but their output is current rather than voltage.

They can also be seen as a variant on a transistor, but with differential input.

Most OTAs also allow tweaking on the gain via a pin.

OTA ICs tend to also have a (completely separate) output buffer amplifier, because in many applications you'll want one.


In practice, standard op amps are often more directly applicable, but in a few cases OTAs are preferred.

Example uses include tunable oscillators, tunable filters, tunable amplifiers, and voltage-controlled resistors. For some uses, not all OTAs are precise enough, and the better ones were such a niche element (e.g. useful in synthesizers) that they are now out of production.

You can build OTAs from discrete elements, but it requires more space, and some component matching.


Notes:

  • While you can use feedback on them, OTAs are regularly used open-loop.



http://www.ti.com/lit/an/sboa117a/sboa117a.pdf