Difference between revisions of "Electronics notes/Transistors"

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http://knol.google.com/k/electronic-circuits-design-for-beginners-chapter-9
 
http://knol.google.com/k/electronic-circuits-design-for-beginners-chapter-9
  
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===Compound pairs===
 
 
<!--
 
A '''Darlington pair''', and a '''Sziklai pair''' both indicate a pair of transistors essentially wired in series, with one C-E also feeding the gate of the second
 
 
The result is that their gains multiply.
 
Switching high currents, and/or a restraint from what drives the base,
 
are probably the most common reasons to use one.
 
 
 
The high gain also means they more easily react to electric fields, e.g. your fingers.
 
This sensitivity ''can'' be a problem with certain types of loads.
 
 
 
 
Notes:
 
* gain is the product of the two individual gains (so it usually acts more like a switch than an amplifier)
 
* Base turn-on voltage is higher ~1.2V due to two drops
 
* Can saturate only to 0.6V {{verify}}
 
 
 
 
A '''Sziklai pair''' (sometimes 'complementary darlington') is a similar combination, but mixing PNP and NPN
 
 
Notes:
 
* gain is the product of the two individual gains (like a darlington)
 
* Base turn-on voltage is ~0.6V
 
* Can saturate only to 0.6V {{verify}}
 
 
 
 
A similar complementary pair of FETs also exist (sometimes '''Fetlington'''),
 
as do some BJT+FET combinations.
 
They can be less parasitic, but also likely to be less stable.
 
 
 
 
See also:
 
* http://en.wikipedia.org/wiki/Darlington_transistor
 
* http://en.wikipedia.org/wiki/Sziklai_pair
 
* http://www.kpsec.freeuk.com/trancirc.htm
 
 
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==Current mirror==
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==Basic amplifiers==
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<!--
 +
Note that in these names, 'common' refers to signal common, often ground.
 +
 
 +
 
 +
'''Common emitter amplifier''' (for BJT), '''Common source amplifier''' (for FET)
 +
 
 +
Often used as voltage amplifier (inverting)
 +
 
 +
https://en.wikipedia.org/wiki/Common_emitter
 +
 
 +
 
 +
Current Follower, Common base (for BJT), common gate (for FET)
 +
 
 +
Used as a current buffer, voltage amplifier
 +
 
 +
https://en.wikipedia.org/wiki/Common_base
 +
 
 +
 
 +
Common collector (for BJT) or common drain amplifier (FET)
 +
 
 +
Often used as voltage buffer
 +
 
 +
 
 +
 
 +
 
 +
 
 +
Voltage buffer - low input impedance, high output impedance
 +
 
 +
Voltage follower -
 +
 
 +
Current buffer -
 +
 
 +
Current follower -
 +
 
 +
 
 +
 
 +
Both current buffer and voltage buffer aim to protect the first circuit from being interfered with, but they have different goals:
 +
 
 +
 
 +
A '''voltage buffer''' aims to copy a ''signal'' to a second circut without interfering much with the first,, basically without further constraints.
 +
 
 +
It's assumed the first has high output impedance so is easily interfered with, and/or/relative to the second which has relatively low imput impedance so will easily load the first.
 +
 
 +
Transistor-wise this is often implemented via common-collector setup, because it has very high input impedance. (In certain applications things like op amps make more sense, but are more complex things))
 +
 
 +
 
 +
A '''current buffer''' aims to transfer as much '''current''' to a second circuit as it needs - usually meaning the second wants nontrivial amounts (still typically specced and reasonable for the use), which means you need lower ouput impedance on this buffer.
 +
 
 +
 
 +
 
 +
 
 +
 
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Notes:
 +
* 'buffer', 'buffered input', 'buffered output' without qualifier will often refer to current buffer
 +
 
 +
* In practice you probably see voltage buffering more within and current buffering more between devices (for a number of practical reasons) {{verify}}
 +
 
 +
 
 +
-->
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 +
 
 +
<!--
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===Feedback amplifiers===
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 +
Series feedback (more commonly: emitter/source degeneration)
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 +
 
 +
Shunt feedback
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 +
-->
 +
 
 +
===On amplifier classes===
 +
 
 +
<!--
 +
While better known in audio circles, these amplifier classes are exercises in tweaking some of the basic transistor amplifiers mentioned above to minimize certain distortion - and use much more widely.
 +
 
 +
In particular, they deal with the issue that
 +
* when control signal is AC (as audio is), feeding that into a transistor as-is would cut off half the signal
 +
* there's that cutoff below ~0.6V, and some nonlinearity around it
 +
 
 +
 
 +
Class A
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* adds DC bias for the AC to sit on, enough to make the signal all-positive voltage
 +
: bias tries to stay above the nonlinearity of transistors, and below saturation
 +
* the simplest quite-decent design
 +
* but inefficient power-wise: efficiency is ~50% maximum, 25-30% typical
 +
* uses most if its power always, regardless of input signal
 +
 
 +
Class B
 +
* input AC biased to 0V
 +
* max efficiency is ~80%, typically more like 50-60%, so used when power is important
 +
* ...and reproduction of the control signal precisely is not, because you're not doing anything to solve that ~0.6V problem, which is audible/noticeable
 +
* for AC-ish signal typically seen as two complementary transistors, each doing half of the wave, then often called a  '''push-pull''' amplifier
 +
* B was mostly abandoned for AB
 +
 
 +
 
 +
Class AB
 +
* a compromise between A and B, which is
 +
** more efficient than A
 +
** avoids B's crossover distortion (depends a bit on proper design - the nonlinear part is entered, but controlled and avoids the cutoff)
 +
* lower cost than A at comparable power
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* efficiency beween that of class A and B
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* quite a few decent-quality audio amplifiers use AB, and have for a pretty long time now {{verify}}
 +
 
 +
 
 +
 
 +
Class C is specialised - good at pulses, and efficient, but poor at signal reproduction
 +
* biased well below the cutoff
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* it amplifies little of the input signal, and gives a distorted version
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: yet good at pulses, so good for some specific functions that use those (oscillators, some RF)
 +
* not worth mentioning in audio context
 +
 
 +
 
 +
In audio circles this goes on, like how class D is filtered PWM, and a bunch of tweaks (and/or marketing terms) on top of one of the amove, mostly D.  You can find mention of at least classes E, G, G, H, I, S, and T, if you look hard enough.
 +
 
 +
 
 +
https://www.electronics-tutorials.ws/amplifier/amplifier-classes.html
 +
 
 +
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1970/15-20W%20Class%20AB%20Audio%20Amplifier.pdf
 +
 
 +
-->
 +
 
 +
==Multiple transistor==
 +
 
 +
 
 +
===Compound pairs===
 +
 
 +
====Darlington pair====
 +
<!--
 +
A '''Darlington pair''' indicate a pair of transistors essentially wired in series, with one C-E also feeding the gate of the second.
 +
 
 +
The result is that their gains multiply. Basically it ensures the second always saturates,
 +
which is something you want to ensure e.g. when switching high currents, and/or have constraints from the current you can drive into the base (e.g. microcontrollers).
 +
 
 +
 
 +
The high gain also means they more easily react to electric fields, e.g. your fingers.
 +
This sensitivity ''can'' be a problem with certain types of loads,
 +
and may involve some extra care.
 +
 
 +
 
 +
Notes:
 +
* gain is the product of the two individual gains (so it usually acts more like a switch than an amplifier)
 +
* Base turn-on voltage is higher, around 1.2V, due to ''two'' drops
 +
* Can saturate only to 0.6V {{verify}}
 +
 
 +
See also:
 +
* http://en.wikipedia.org/wiki/Darlington_transistor
 +
* https://electronicsclub.info/transistorcircuits.htm
 +
 
 +
-->
 +
 
 +
====Sziklai pair====
 +
<!--
 +
A '''Sziklai pair''', also '''complementary feedback pair''' (sometimes 'complementary darlington') is a similar combination, but mixing PNP and NPN
 +
 
 +
 
 +
Notes:
 +
* gain is the product of the two individual gains (like a darlington)
 +
* Base turn-on voltage is ~0.6V
 +
* Can saturate only to 0.6V {{verify}}
 +
 
 +
https://en.wikipedia.org/wiki/Complementary_feedback_pair
 +
 
 +
====Other pairs====
 +
A similar complementary pair of FETs also exist (sometimes '''Fetlington'''),
 +
as do some BJT+FET combinations.
 +
These can be less parasitic, but also likely to be less stable.
 +
 
 +
-->
 +
 
 +
 
 +
===Cascode===
 +
<!--
 +
Probably best considered a tweak of the common-base design.{{verify}}
 +
 
 +
A common-emitter stage feeding into a common-base stage,
 +
usually done for higher input–output isolation,
 +
higher input impedance, high output impedance, or higher bandwidth.
 +
 
 +
https://en.wikipedia.org/wiki/Cascode
 +
-->
 +
 
 +
===Current mirror===
 +
 
 +
<!--
 +
 
 +
A current mirror aims to have a current in one part of a circuit control and equal current elsewhere, regardless of some basic loading.
 +
 
 +
 
 +
This can be used
 +
* to creat bias currents
 +
* active loads [https://en.wikipedia.org/wiki/Active_load]
 +
* act as a switch of sorts
 +
* amplify the difference between two inputs (basic differential amplifier)
 +
: op amps often havev current mirrors
 +
 
 +
When made for signals it is often used to create filters
 +
 
 +
Such uses are easier to put on a wafer (than e.g. resistors) so current mirrors
 +
are actually all over the place.
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 +
 
 +
 
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 +
 
 +
 
 +
 
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While invented in the vacuum tube days, it's now typically seen in BJT or FET form.
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 +
-->
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====Long tailed pair====

Latest revision as of 15:04, 1 December 2019

This is for beginners and very much by a beginner. It's meant to try to cover hobbyist needs, and as a starting point to find out which may be the relevant details for you, not for definitive information.

Some basics and reference: Volts, amps, energy, power · Ground · batteries · resistors · changing voltage · transistors · fuses · diodes · varistors · capacitors · inductors · transformers · baluns · amplifier notes · frequency generation · skin effect


And some more applied stuff:

IO: wired local IO wired local-ish IO · · · · Shorter-range wireless (IR, ISM RF, RFID) · bluetooth · 802.15 (including zigbee) · 802.11 (WiFi) · cell phone


Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer

Audio notes: basic audio hacks · microphones · amps and speakers · device voltage and impedance, audio and otherwise ·

Less sorted: Common terms, useful basics, soldering · Microcontroller and computer platforms · Arduino and AVR notes · ESP series notes · Electronics notes/Phase Locked Loop notes · mounts, chip carriers, packages, connectors · signal reflection · pulse modulation · electricity and humans · Unsorted stuff


See also Category:Electronics.


BJT family

Transistor behaviour (BJT)

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)

Transistors have four distinct modes of operation:

  • Cutoff
NPN: Vb < Ve and Vb < Vc
PNP: Vb > Ve and Vb > Vc
C-E is an open circuit, nothing happens.
Maximum VCE (VCE = VCC, the least flow through the collector (verify))
  • Active (a.k.a. Forward active)
When
NPN: Vc > Vb and Vb > Ve
PNP: Vc < Vb and Vb < Ve
The C-E currrent is hFE*IB i.e. proportional to the current into the base, amplifying current
  • Saturation
When
NPN: Vb > Ve and Vb > Vc
PNP: Vb < Ve and Vb < Vc
The C-E connection is essentially a short circuit already. The transistor acts like a switch
Saturation also describes the fact that increasing the current on the base no longer has an effect on the C-E current (because it's already maximum).
  • Reverse (a.k.a. Reverse Active)
When
NPN: Vb > Vc and Ve > Vb
PNP: Vb < Vc and Ve < Vb
the gain in this direction will be much smaller
this is rarely used intentionally


When amplifying signals, note that transistors are linear in a region, but nonlinear overall, and you often want to avoid 0V with a little biasing.


https://learn.sparkfun.com/tutorials/transistors/operation-modes


Gain

A BJT's (forward) current gain, hFE, is ΔIC/ΔIB, a dimensionless value.

If the input and output impedance is equal (it often is), this can be simplified to Iout/Iin, and can be given in dB

There is also a reverse gain, which will be much smaller, and is rarely characterized because it's not typically used.



Oscillators

As a diode

Identifying a bipolar transistor's legs

FET family

Voltage controlled resistor

Transistor behaviour (FET)

Insulated-gate bipolar transistors (IGBT)

A hybrid of the above, basically the high-current ruggedness of a Bipolar with the sensitivity of a FET

https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor


Phototransistor / optocouple / opto-isolator

A phototransistor is a transistor with amount of light being the base.

Uses:

  • switching things on at night.
  • galvanically isolated switching
  • galvanically isolated communication - then often IR (and often modulated, to avoid environment light being confusing)


Optocouples are essentially a LED plus phototransistor isolated in an IC. These are typically used for their galvanic isolation, e.g. avoiding ground loops, and are also useful when you want simple (one-way) interactions between circuits at different voltages.


Often appear as 4-pin or 6-pin ICs.

  • The 4-pin variants give you the LED (cathode and anode) on one side, and the transistor's (collector and emitter) on the other.
  • The 6-pin variant use 5 pins; it adds the transistor's base, which just works as an extra (non-isolated) trigger. In practice it may often be left unconnected

ICs with multiple optocouplers also exist.


Specs vary in details such as:

  • current use
  • output voltage
  • how much voltage difference can be isolated
  • added components -- may e.g. be a darlington setup

...and more.


See also:


Transistor behaviour, circuit styles (shared, differences)

Simple logic

BJTs and FETs as a switch

High side versus low side switching/driving
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)
Some limits and choices

Basic amplifiers

On amplifier classes

Multiple transistor

Compound pairs

Darlington pair

Sziklai pair

Cascode

Current mirror

Long tailed pair