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Transistor CircuitsThis page explains the operation of transistors in circuits.
Practical matters such as testing, precautions when soldering and identifying
leads are covered by the Transistors page.
General: Types | Currents | Functional model | Darlington pair
Switching: Introduction | Use relay? | IC output | for NPN | and PNP | Sensors | Inverter
Next Page: Analogue and
Also See: Transistors
(soldering, lead identification)
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
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.
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
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
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.
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
- The base-emitter junction behaves like a diode.
- A base current IB flows only when the voltage VBE
across the base-emitter junction is 0.7V or more.
- The small base current IB controls the large collector current
- Ic = hFE × IB (unless the transistor
is full on and saturated)
hFE is the current gain
(strictly the DC current gain), a typical value for hFE is 100 (it
has no units because it is a ratio)
- The collector-emitter resistance RCE is controlled by the base
- IB = 0 RCE = infinity transistor off
- IB small RCE reduced transistor
- IB increased RCE = 0 transistor full
There is a
table showing technical data for some popular transistors on the transistors
- A resistor is often needed in series with the base connection to limit the
base current IB and prevent the transistor being damaged.
- Transistors have a maximum collector current Ic rating.
- The current gain hFE can vary widely, even for
transistors of the same type!
- A transistor that is full on (with RCE = 0) is said to
- When a transistor is saturated the collector-emitter voltage
VCE 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/IB for a
saturated transistor is less than the current gain hFE.
- The emitter current IE = Ic + IB, but Ic is much
larger than IB, so roughly IE = Ic.
|Touch switch circuit|
Darlington pairThis 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
Darlington pair current gain, hFE = hFE1 ×
(hFE1 and hFE2 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.
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 VCE
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:
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 hFE(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
- In the OFF state: power = Ic × VCE, but Ic = 0, so the
power is zero.
- In the full ON state: power = Ic × VCE, but
VCE = 0 (almost), so the power is very small.
For information about the operation of a transistor please see the functional model
Protection diodeIf 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
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:
Disadvantages 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
- Relays can switch many contacts at once.
- Relays are bulkier than transistors for switching small currents.
- Relays cannot switch rapidly, transistors can switch many times per
- Relays use more power due to the current flowing through their
- 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.
Connecting a transistor to the output from an ICMost 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 RB is required to limit the current flowing into the
base of the transistor and prevent it being damaged. However, RB 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
IB about five times larger than the value which should just saturate
Choosing a suitable NPN transistorThe 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
The procedure below explains how to choose a suitable switching transistor.
- 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|
- The transistor's minimum current gain
hFE(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|
- Choose a transistor which meets these
requirements and make a note of its properties: Ic(max) and hFE(min).
There is a table showing technical data for some popular
transistors on the transistors
- Calculate an approximate value for the base
||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:
RB = 0.2 × RL × hFE
Then choose the nearest standard value for the base
- Finally, remember that if the load is a motor or relay coil a protection diode is required.
The output from
a 4000 series CMOS IC is required to operate a relay with a 100 coil.
supply voltage is 6V for both the IC and load. The IC can supply a maximum
current of 5mA.
- Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must
have Ic(max) > 60mA.
- The maximum current from the IC is 5mA, so transistor must have hFE(min) > 60 (5 × 60mA/5mA).
- Choose general purpose low power transistor BC182
with Ic(max) = 100mA and hFE(min) = 100.
- RB = 0.2 × RL × hFE = 0.2 × 100 × 100 =
2000. so choose RB = 1k8 or 2k2.
- The relay coil requires a protection diode.
|PNP transistor switch|
(load is on when IC
output is low)
Choosing a suitable PNP transistorThe 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
The procedure for choosing a suitable PNP transistor is exactly the same as
that for an NPN transistor described above.
Using a transistor switch with sensors
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.
|LED lights when the LDR is
|LED lights when the LDR is
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
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 resistor. In this case the
fixed resistor can be omitted because the LDR resistance cannot be reduced to
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
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:
purpose low power NPN transistor can be used. For general use
RB = 10k and
RC = 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).
- When the input is high (+Vs) the output is low (0V).
- When the input is low (0V) the output is high (+Vs).
If you are connecting the inverter to a CMOS logic IC input (very high
impedance) you can increase RB to 100k and RC to
10k, this will reduce the current used by the inverter.
Next Page: Analogue
and Digital Systems | Studying Electronics
© John Hewes 2008, The Electronics Club, http://www.kpsec.freeuk.com/