Type of transistors

Types of transistor

There are two types of standard transistors, NPN and PNP, with
different circuit symbols. The letters refer to the layers of semiconductor
material used to make the transistor. Most transistors used today are NPN
because this is the easiest type to make from silicon. This page is mostly about
NPN transistors and if you are new to electronics it is best to start by
learning how to use these first. The leads are labelled base (B),
collector (C) and emitter (E). These
terms refer to the internal operation of a transistor but they are not much help
in understanding how a transistor is used, so just treat them as labels!
A Darlington pair is two transistors connected together to give a very high
current gain. In addition to standard (bipolar junction) transistors, there are
field-effect transistors which are usually referred to as FETs.
They have different circuit symbols and properties and they are not (yet)
covered by this page.

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Transistor currents

The diagram shows the two current paths through a transistor. You can build this
circuit with two standard 5mm red LEDs and any general purpose low power NPN
transistor (BC108, BC182 or BC548 for example).
The small base current controls the larger
collector current.
When the switch is closed a small current flows into the base (B) of
the transistor. It is just enough to make LED B glow dimly. The transistor
amplifies this small current to allow a larger current to flow through from its
collector (C) to its emitter (E). This collector current is large enough to make
LED C light brightly.
When the switch is open no base current flows, so the transistor
switches off the collector current. Both LEDs are off.
A transistor amplifies current and can be used as a switch.

This arrangement where the emitter (E) is in the controlling circuit
(base current) and in the controlled circuit (collector current) is called
common emitter mode. It is the most widely used arrangement for transistors
so it is the one to learn first.

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Functional model of an NPN transistor

The operation of a transistor is difficult to explain and understand in terms of
its internal structure. It is more helpful to use this functional model:

• The base-emitter junction behaves like a diode.
• A base current I_B flows only when the voltage V_BE
  across the base-emitter junction is 0.7V or more.
• The small base current I_B controls the large collector
  current Ic.
• Ic = h_FE × I_B   (unless the transistor is
  full on and saturated)
  
  h_FE is the current gain (strictly the DC current gain), a typical
  value for h_FE is 100 (it has no units because it is a ratio)
  
• The collector-emitter resistance R_CE is controlled by the
  base current I_B:
  
  
  
  • I_B = 0   R_CE = infinity   transistor off
  • I_B small   R_CE reduced   transistor partly on
    
  • I_B increased   R_CE = 0   transistor full on
    ('saturated')
    
  
  
  

Additional notes:

• A resistor is often needed in series with the base connection to limit
  the base current I_B and prevent the transistor being damaged.
  
• Transistors have a maximum collector current Ic rating.
• The current gain h_FE can vary widely, even for
  transistors of the same type!
• A transistor that is full on (with R_CE = 0) is said to
  be 'saturated'.
• When a transistor is saturated the collector-emitter voltage V_CE
  is reduced to almost 0V.
• When a transistor is saturated the collector current Ic is determined by
  the supply voltage and the external resistance in the collector circuit, not
  by the transistor's current gain. As a result the ratio Ic/I_B for
  a saturated transistor is less than the current gain h_FE.
• The emitter current I_E = Ic + I_B, but Ic is much
  larger than I_B, so roughly I_E = Ic.
  

There is a table showing technical data for some popular transistors on the
transistors
page.

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Darlington pair

This is two transistors connected together so that the current amplified by the
first is amplified further by the second transistor. The overall current gain is
equal to the two individual gains multiplied together:
Darlington pair current gain, h_FE = h_FE1 × h_FE2


(h_FE1 and h_FE2 are the gains of the individual
transistors)
This gives the Darlington pair a very high current gain, such as 10000, so
that only a tiny base current is
required to make the pair switch on.
A Darlington pair behaves like a single transistor
with a very high current gain. It has three leads (B, C and E)
which are equivalent to the leads of a standard individual transistor. To turn
on there must be 0.7V across both the base-emitter junctions which are connected
in series inside the Darlington pair, therefore it requires 1.4V to turn on.
Darlington pairs are available as complete packages but you can make up your
own from two transistors; TR1 can be a low power type, but normally TR2 will
need to be high power. The maximum collector current Ic(max) for the pair is the
same as Ic(max) for TR2.
A Darlington pair is sufficiently sensitive to respond to the small current
passed by your skin and it can be used to make a touch-switch as shown in
the diagram. For this circuit which just lights an LED the two transistors can
be any general purpose low power transistors. The 100k
resistor protects the transistors if the contacts are linked with a piece of
wire.

Touch switch circuit

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Using a transistor as a switch

When a transistor is used as a switch it must be either OFF or fully
ON. In the fully ON state the voltage V_CE across the transistor
is almost zero and the transistor is said to be saturated because it
cannot pass any more collector current Ic. The output device switched by the
transistor is usually called the 'load'.
The power developed in a switching transistor is very small:

• In the OFF state: power = Ic × V_CE, but Ic = 0, so the
  power is zero.
• In the full ON state: power = Ic × V_CE, but V_CE
  = 0 (almost), so the power is very small.
  

This means that the transistor should not become hot in use and you do not need
to consider its maximum power rating. The important ratings in switching
circuits are the maximum collector current Ic(max)
and the minimum current gain h_FE(min).
The transistor's voltage ratings may be ignored unless you are using a supply
voltage of more than about 15V. There is a table showing technical data for some
popular transistors on the
transistors
page.
For information about the operation of a transistor please see the
functional model
above.

Protection diode

If the load is a motor,
relay or
solenoid (or any other device with a coil) a
diode must be
connected across the load to protect the transistor from the brief high voltage
produced when the load is switched off. The diagram shows how a protection diode
is connected 'backwards' across the load, in this case a relay coil.
Current flowing through a coil creates a magnetic field which
collapses suddenly when the current is switched off. The sudden collapse of the
magnetic field induces a brief high voltage across the coil which is very likely
to damage transistors and ICs. The protection diode allows the induced voltage
to drive a brief current through the coil (and diode) so the magnetic field dies
away quickly rather than instantly. This prevents the induced voltage becoming
high enough to cause damage to transistors and ICs.

When to use a
relay

Transistors cannot switch AC or high voltages (such as mains electricity) and
they are not usually a good choice for switching large currents (> 5A). In these
cases a relay
will be needed, but note that a low power transistor may still be needed to
switch the current for the relay's coil!
Advantages of relays:

• Relays can switch AC and DC, transistors can only switch
  DC.
• Relays can switch high voltages, transistors cannot.
  
• Relays are a better choice for switching large currents
  (> 5A).
• Relays can switch many contacts at once.

Disadvantages of relays:

• Relays are bulkier than transistors for switching small
  currents.
• Relays cannot switch rapidly, transistors can switch many
  times per second.
• Relays use more power due to the current flowing through
  their coil.
• Relays require more current than many ICs can provide, so
  a low power transistor may be needed to switch the current for the relay's
  coil.

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Connecting a transistor to the output from an IC

Most ICs cannot supply large output currents so it may be necessary to use a
transistor to switch the larger current required for output devices such as
lamps, motors and relays. The 555 timer IC is unusual because it can supply a
relatively large current of up to 200mA which is sufficient for some output
devices such as low current lamps, buzzers and many relay coils without needing
to use a transistor.
A transistor can also be used to enable an IC connected to a low voltage
supply (such as 5V) to switch the current for an output device with a separate
higher voltage supply (such as 12V). The two power supplies must be linked,
normally this is done by linking their 0V connections. In this case you should
use an NPN transistor.
A resistor R_B is required to limit the current flowing into the
base of the transistor and prevent it being damaged. However, R_B must
be sufficiently low to ensure that the transistor is thoroughly saturated to
prevent it overheating, this is particularly important if the transistor is
switching a large current (> 100mA). A safe rule is to make the base current I_B
about five times larger than the value which should just saturate the
transistor.

Choosing a suitable NPN transistor

The circuit diagram shows how to connect an NPN transistor, this will
switch on the load when the IC output is high. If you need the opposite
action, with the load switched on when the IC output is low (0V) please
see the circuit for a PNP transistor below.
The procedure below explains how to choose a suitable switching transistor.

NPN transistor switch

(load is on when IC output is high)

Using units in calculations

Remember to use V, A and

or

V, mA and k.
For more details

please see the Ohm's Law
page

1. The transistor's maximum collector current Ic(max)
   must be greater than the load current Ic.
   
   
   

   load current Ic =	supply voltage Vs
   	load resistance RL

   
   
   
2. The transistor's minimum current gain h_FE(min)
   must be at least five times the load current Ic divided by the
   maximum output current from the IC.
   
   
   

   hFE(min)  >   5 ×	load current Ic
   	max. IC current

   
   
   
3. Choose a transistor which meets these
   requirements and make a note of its properties: Ic(max)
   and h_FE(min).
   
   There is a table showing technical data for some popular transistors
   on the
   transistors page.
   
    
   
   
4. Calculate an approximate value for the base
   resistor:
   
   
   

   RB =	Vc × hFE	where Vc = IC supply voltage   (in a simple circuit with one supply this is Vs)
   	5 × Ic

   For a simple circuit where the IC and the load share the same
   power supply (Vc = Vs) you may prefer to use: R_B = 0.2 × R_L × h_FE
   
   Then choose the nearest standard value for the base
   resistor.
    
   
   
   
5. Finally, remember that if the load is a motor or relay coil a
   protection diode is required.
   

Example

The output from a 4000 series CMOS IC is required to operate a relay with
a 100
coil.

The supply voltage is 6V for both the IC and load. The IC can supply a maximum
current of 5mA.

1. Load current = Vs/R_L = 6/100 = 0.06A = 60mA, so
   transistor must have Ic(max) > 60mA.
   
2. The maximum current from the IC is 5mA, so transistor must have
   h_FE(min) > 60 (5 × 60mA/5mA).
   
3. Choose general purpose low power transistor
   BC182 with Ic(max) = 100mA and
   h_FE(min) = 100.
4. R_B = 0.2 × R_L × h_FE = 0.2 × 100
   × 100 = 2000.
   so choose R_B = 1k8 or 2k2.
   
5. The relay coil requires a protection diode.
   

Choosing a suitable PNP transistor

The circuit diagram shows how to connect a PNP transistor,
this will switch on the load when the IC output is low (0V). If you need
the opposite action, with the load switched on when the IC output is high
please see the circuit for an NPN transistor above.

The procedure for choosing a suitable PNP transistor is exactly
the same as that for an NPN transistor described above.

PNP transistor switch

(load is on when IC output is low)

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Using a transistor switch with sensors

LED lights when the LDR is dark

The top circuit diagram shows an LDR (light sensor) connected so that the LED
lights when the LDR is in darkness. The variable resistor adjusts the brightness
at which the transistor switches on and off. Any general purpose low power
transistor can be used in this circuit.
The 10k
fixed resistor protects the transistor from excessive base current (which will
destroy it) when the variable resistor is reduced to zero. To make this circuit
switch at a suitable brightness you may need to experiment with different values
for the fixed resistor, but it must not be less than 1k.

If the transistor is switching a load with a coil, such as a motor or relay,
remember to add a protection diode across the load.
The switching action can be inverted, so the LED lights when the LDR
is brightly lit, by swapping the LDR and variable resist
LED lights when the LDR is bright

                                                                             

or.
In this case the fixed resistor can be omitted because the LDR resistance cannot
be reduced to zero.
Note that the switching action of this circuit is not particularly good
because there will be an intermediate brightness when the transistor will be
partly on (not saturated). In this state the transistor is in danger of
overheating unless it is switching a small current. There is no problem with the
small LED current, but the larger current for a lamp, motor or relay is likely
to cause overheating.
Other sensors, such as a thermistor, can be used with this circuit, but they
may require a different variable resistor. You can calculate an approximate
value for the variable resistor (Rv) by using a multimeter to find the minimum
and maximum values of the sensor's resistance (Rmin and Rmax):
Variable resistor, Rv = square root of (Rmin × Rmax)
For example an LDR: Rmin = 100,
Rmax = 1M,
so Rv = square root of (100 × 1M) = 10k.

You can make a much better switching circuit with sensors connected to a
suitable IC (chip). The switching action will be much sharper with no partly on
state.

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A transistor inverter (NOT gate)

Inverters (NOT gates) are available on logic ICs but if you only require one
inverter it is usually better to use this circuit. The output signal (voltage)
is the inverse of the input signal:

• When the input is high (+Vs) the output is low (0V).
• When the input is low (0V) the output is high (+Vs).
  

Any general purpose low power NPN transistor can be used. For general use R_B = 10k
and R_C = 1k,
then the inverter output can be connected to a device with an input impedance
(resistance) of at least 10k
such as a logic IC or a 555 timer (trigger and reset inputs).
If you are connecting the inverter to a CMOS logic IC input (very high
impedance) you can increase R_B to 100k
and R_C to 10k,
this will reduce the current used by the inverter.