Difference between revisions of "Electronics notes/Transistors"

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{{Electronics notes}}
 
{{Electronics notes}}
  
==BJTs and FETs==
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<!--
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BJT is called bipolar because current flows
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both in response to electron and hole carriers,
 +
FET unipolar because it's only electron.
 +
(huh?)
 +
 
 +
 
 +
-->
 +
 
 +
==BJT family==
 
<!--
 
<!--
 
A transistor is, at the core, a current amplifier.
 
A transistor is, at the core, a current amplifier.
  
The basic idea is that the current across C and E is the gain time the current flowing into B
+
 
 +
A BJT is a current controlled current amplifier.
 +
The current across C(ollector) and E(mitter) is the gain time the current flowing into B(ase)
 +
 
 +
FETs, inc contrast, are voltage controlled current amplifier (see below for more detail)
  
  
'''Gain''' is fixed for a particular transistor, and is  
+
'''Gain''' (DC current gain, really) is fixed for a particular transistor, and is  
 
: on the order of a few dozen for power transistors
 
: on the order of a few dozen for power transistors
 
: over a hundred for signal transistors
 
: over a hundred for signal transistors
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With smaller base current, you amplify a signal fairly linearly (with footnotes).
 
With smaller base current, you amplify a signal fairly linearly (with footnotes).
  
{{comment|(gain is also known as 'hFE' (apparently short for "Hybrid parameter forward current gain", a measure of DC gain) and 'beta' (presumably because it's shorter to say)}}
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{{comment|(gain is often named 'hFE' in a datasheet, short for "Hybrid parameter forward current gain", where h parameters are a set of simplified characterizations) and also known as beta (presumably because it's shorter to say)}}
  
  
Beyond a certain amount of base current,
+
At a certain height of base current,
 
the transistor saturates and the current between C and E, whatever it is, just passes it as comes in.
 
the transistor saturates and the current between C and E, whatever it is, just passes it as comes in.
  
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===Bipolar family===
 
  
 
<!--
 
<!--
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====Transistor behaviour, circuit styles====
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====Transistor behaviour (BJT)====
 
{{stub}}
 
{{stub}}
  
Transistors are linear in a region, but nonlinear overall.
+
Transistors have four distinct modes of operation:
 
+
They have four distinct modes of operation:
+
 
+
 
* Cutoff
 
* Cutoff
 
:: NPN: Vb < Ve and Vb < Vc
 
:: NPN: Vb < Ve and Vb < Vc
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: the gain in this direction will be much smaller  
 
: the gain in this direction will be much smaller  
 
: this is rarely used intentionally
 
: 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.
  
  
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There is also a reverse gain, which will be much smaller,
 
There is also a reverse gain, which will be much smaller,
 
and is rarely characterized because it's not typically used.
 
and is rarely characterized because it's not typically used.
 
  
  
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-->
 
-->
  
===FET family===
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==FET family==
 
<!--
 
<!--
'''FET''' stands for Field-Effect Transistor.
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'''FET''' stands for Field-Effect Transistor, pointing out at ''how'' the gate controls the flow.
 +
 
 +
A FET is a voltage controlled current amplifier with very high input impedance,
 +
though its function can be rather similar to a BJT in a various situation.
 +
 
 +
Due to the different workings the pins are are called D(rain), S(ource), and G(ate).
 +
 
 +
 
 +
 
 +
[https://en.wikipedia.org/wiki/Field-effect_transistor#Types There are a bunch of types] of FET, though most are specialized, and the more generic components ones are often JFET or MOSFET (mostly the latter because they're easier to produce and cheaper).
 +
 
 +
https://www.rfwireless-world.com/Terminology/BJT-vs-FET.html
 +
 
  
[https://en.wikipedia.org/wiki/Field-effect_transistor#Types There are a bunch of types] of FET, though most are specialized, and the more generic components ones are often JFET or MOSFET (mostly the latter because they're cheaper - they're easier to produce).
 
  
  
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-->
 
-->
  
===Insulated-gate bipolar transistors (IGBT)===
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===Transistor behaviour (FET)===
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 +
<!--
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 +
Note that source and drain are basically interchangeable (whereas in BJT the direction is fixed)
 +
 
 +
 
 +
 
 +
 
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 +
 
 +
-->
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 +
==Insulated-gate bipolar transistors (IGBT)==
  
 
A hybrid of the above, basically the high-current ruggedness of a Bipolar with the sensitivity of a FET
 
A hybrid of the above, basically the high-current ruggedness of a Bipolar with the sensitivity of a FET
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https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor
 
https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor
  
===Transistor behaviour, circuit styles===
+
 
 +
==Transistor behaviour, circuit styles (shared, differences)==
 
<!--
 
<!--
  
  
 +
BJT, FET differences
  
* [http://en.wikipedia.org/wiki/Common_collector Common-collector] (BJT) / [http://en.wikipedia.org/wiki/Common_draincommon-drain] (FET)
 
  
* [http://en.wikipedia.org/wiki/Common_emitter Common-emitter] (BJT) / [http://en.wikipedia.org/wiki/Common_source Common-source]    (FET)
+
* BJT is a current controlled device, FET is so high impedance that it's effectively voltage controlled
  
* [http://en.wikipedia.org/wiki/Common_base Common-base] (BJT) / [http://en.wikipedia.org/wiki/Common_gate Common-gate] (FET)
+
* FETs can be used in two directions, unlike BJT
  
 +
* BJTs are a little noisier (due to the junction)
  
 +
* FET is a better amplifier, BJT arguably a better switch{{verify}}
  
* Buffer amplifier [http://en.wikipedia.org/wiki/Buffer_amplifier]
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* FETs are frequently more power efficient{{verify}}
** voltage buffer (/ voltage follower)
+
 
** current buffer
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* BJT has more [[gain bandwidth product]]
 +
 
 +
https://www.rfwireless-world.com/Terminology/BJT-vs-FET.html
  
  
Other notes:
 
* You can parallelize transistors (BJT and FETs. FETs may be a little easier). However, note that variations in transistors (even in the same batch) means this makes more sense for some purposes (e.g. driving more current) than others (dealing with subtle signals -- but there are cases in which it can actually help){{verify}}
 
  
  
Amplifiers can be described as current-controlled or voltage-controlled, and voltage-sourced or current-sourced.
+
Amplifiers in general can be described as current-controlled or voltage-controlled, and voltage-sourced or current-sourced.
* current controlled current source (CCCS) is a current amplifier
+
* current controlled current source (CCCS) is a current amplifier (e.g. BJT, FET)
 
* voltage controlled voltage source (VCVS) is a voltage amplifier
 
* voltage controlled voltage source (VCVS) is a voltage amplifier
* current controlled voltage source (CCVS) is a transresistance amplifier
+
* current controlled voltage source (CCVS) is a transresistance amplifier (implemented with Opamps{{verify}})
* voltage controlled current source (VCCS) is a transconductance amplifier
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* voltage controlled current source (VCCS) is a transconductance amplifier (e.g. FET, OTA{{verify}})
  
 +
FET sort of qualifies for both CCCS and VCCS{{verify}} depending on how it is used{{verify}}
  
  
  
 +
All are sensitive to ESD, but FETs generally more so{{verify}}
  
 +
FET has high input impedance (order of megaohm) than BJT (?),
 +
which is a large part of ''why'' it acts voltage-controlled in practice.
  
 +
 +
 +
 +
You can consider the gate as isolated from the channel, and acting as a capacitor, so that voltage on it essentially controls the active width of the current channel -- hence [https://en.wikipedia.org/wiki/Field_effect_(semiconductor) Field Effect] Transistor.
 +
 +
MOSFET is the common type{{verify}} of IGFET ("Isolated Gate"),
 +
meaning the gate is insulated from the current channel (with thin silicon dioxide),
 +
and its impedance is higher than many other FET types.
 +
 +
 +
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==="Common collector" / "Common Drain"===
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 +
* [http://en.wikipedia.org/wiki/Common_collector Common-collector] (BJT) / [http://en.wikipedia.org/wiki/Common_draincommon-drain] (FET)
 +
 +
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==="Common Emitter" / "Common Source"===
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* [http://en.wikipedia.org/wiki/Common_emitter Common-emitter] (BJT) / [http://en.wikipedia.org/wiki/Common_source Common-source]    (FET)
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 +
==="Common Base" / "Common Gate"===
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* [http://en.wikipedia.org/wiki/Common_base Common-base] (BJT) / [http://en.wikipedia.org/wiki/Common_gate Common-gate] (FET)
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 +
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===Buffer amplifier===
 +
 +
Buffer amplifier [http://en.wikipedia.org/wiki/Buffer_amplifier]
 +
* voltage buffer (/ voltage follower)
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* current buffer
 +
 +
 +
===Parallel transistors===
 +
Other notes:
 +
* You can parallelize transistors (BJT and FETs. FETs may be a little easier).
 +
However, note that variations in transistors (even in the same batch) means this makes more sense for some purposes (e.g. driving more current) than others (dealing with subtle signals -- but there are cases in which it can actually help){{verify}}
 +
 +
 +
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===See also===
 
See also:
 
See also:
 
* [http://www.allaboutcircuits.com/vol_3/chpt_4/index.html allaboutcircuits.com - Bipolar Junction Transistors]
 
* [http://www.allaboutcircuits.com/vol_3/chpt_4/index.html allaboutcircuits.com - Bipolar Junction Transistors]
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-->
 
-->
  
====Compound pairs====
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===Compound pairs===
  
 
<!--
 
<!--
A '''Darlington pair''', and a '''Sziklai pair''' both indicate a pair of transistors.
+
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
  
These setups mostly just give a larger current gain.
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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.
  
The gain function is also a lot steeper, which is why they are typically used as a switch to drive some high-current power.
 
  
It also means that a Darlington needs to be fed less base current than a single power transistor would to drive the same amount of output current (and show less input following as a result).
 
 
The high gain means they more easily react to your fingers.
 
This sensitivity ''can'' even be a problem with certain types of loads.
 
 
 
A '''Darlington transistor''' refers to two transistors (two NPNs or two PNPs) wired together with the emitter from the first into the basis of the second.
 
  
 
Notes:
 
Notes:
 
* gain is the product of the two individual gains (so it usually acts more like a switch than an amplifier)
 
* 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 ~1.2V
+
* Base turn-on voltage is higher ~1.2V due to two drops
 
* Can saturate only to 0.6V {{verify}}
 
* Can saturate only to 0.6V {{verify}}
  
  
A '''Sziklai pair''' (sometimes 'complementary darlington') is a similar combination, using one PNP and one NPN
+
 
 +
A '''Sziklai pair''' (sometimes 'complementary darlington') is a similar combination, but mixing PNP and NPN
  
 
Notes:
 
Notes:
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A similar complementary pair of FETs also exist (sometimes '''Fetlington'''), as do some BJT+FET combinations.
+
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.
 
They can be less parasitic, but also likely to be less stable.
 
 
  
  
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==Some practical notes==
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==Phototransistor / optocouple / opto-isolator==
  
  
===Tri-state buffer===
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A phototransistor is a transistor with amount of light being the base.
  
==Phototransistor / optocouple / opto-isolator==
+
Uses:
 +
* switching things on at night.
 +
* galvanically isolated switching
 +
* galvanically isolated communication - then often IR (and often modulated, to avoid environment light being confusing)
  
  
These can be seen as transistors that are triggered via light (LED-and-phototransistor combination)
 
instead of using conductors.
 
  
 
+
Optocouples are essentially a LED plus phototransistor isolated in an IC.
Often used for isolation of currents,  
+
These are typically used for their galvanic isolation, e.g. avoiding ground loops,
to avoid communicating some of the voltage noise,  
+
and are also useful when you want simple (one-way) interactions between circuits  
or to have simple (one-way) interaction between circuits at different voltages.
+
at different voltages.
  
  
  
 
Often appear as 4-pin or 6-pin ICs.
 
Often appear as 4-pin or 6-pin ICs.
* The 4-pin variants give you the LED cathode and anode, and the transistor's collector and emitter.  
+
* 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, but occasionally it's rather convenient to be able to trigger both ways (e.g. from the same and from the isolated circuit).
+
* 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.
 
ICs with multiple optocouplers also exist.
 +
 +
  
 
Specs vary in details such as:
 
Specs vary in details such as:

Revision as of 12:07, 14 July 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: IO and wired communication · localish communication · wireless (ISM RF, GSM, RFID, more) · 802.11 (WiFi) · 802.15 (including zigbee)


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 · Bluetooth serial · JY-MCU · DMX · ESC/POS notes

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.



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)


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


Transistor behaviour, circuit styles (shared, differences)

Compound pairs

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: