AUSTRIA, DIANA ELAINE G.
31-ECE-01
1. DARLINGTON PAIR
Sidney Darlingtons name is well known to electronic circuit
designers. He is credited with the discovery and initial demonstration
in the early 1950s of what ever since has been known as the
Darlington transistor pair, or simply the Darlington transistor or
Darlington pair.
A darlington pair is used to amplify weak signals so that they can
be clearly detected by another circuit or a computer/microprocessor.
It is a compound structure consisting of two bipolar transistors
connected in such a way that the current amplified by the first
transistor is amplified further by the second one. This configuration
gives a much higher current gain than each transistor taken separately
and, in the case of integrated devices, can take less space than two
individual transistors because they can use a shared collector.
Integrated Darlington pairs come packaged singly in transistor-like
packages or as an array of devices (usually eight) in an integrated
circuit. A Darlington pair behaves like a single transistor with a
high current gain (approximately the product of the gains of the two
transistors).
A general relation between the compound current gain and the
individual gains is given by:
If 1 and 2 are high enough (hundreds), this relation can be
approximated with:
Fig. 1. Darlington transistor pair. Resistors as shown are usually
included to
reduce the switching delay when turning off a conducting pair.
Fig. 1 shows a schematic diagram with transistors T1 and T2 connected as a
Darlington transistor pair.
The resistors shown are not essential, but are usually included to
permit independent design of bias currents and to reduce the time
required to turn off a conducting pair. They reduce the current gain
particularly at low currents. If we neglect the current flowing in the
resistors and define the common-emitter current gain for a single
transistor a
= Ic/IB, simple analysis shows that the overall dc or low
frequency current gain for the Darlington pair is Iout/Iin=
2.A
2. Current Mirror
A current mirror is a circuit block which functions to produce a
copy of the current in one active device by replicating the current in
second active device. An important feature of the current mirror is a
relatively high output resistance which helps to keep the output
current constant regardless of load conditions. Another feature of the
current mirror is a relatively low input resistance which helps to
keep the input current constant regardless of drive conditions. The
current being 'copied' can be, and often is, a varying signal current.
Conceptually, an ideal current mirror is simply an ideal current
amplifier with a gain of -1. The current mirror is often used to
provide bias currents and active loads in amplifier stages.
Current mirrors are basic building blocks of analog design. Fig. 1(a)
shows the basic npn current mirror. For its analysis, we assume
identical transistors and neglect the Early effect, i.e. we assume
VA. his makes the saturation current IS and current gain
independent of the collector-base voltage VCB. he input current to the
mirror is labeled IREF. his current might come from
a resistor connected to the positive rail or a current source realized
with a transistor or another current mirror. The emitters of the two
transistors are shown connected to ground. These can be connected to a
dc voltage, e.g. the negative supply rail.
The
basic
circuit
is
shown
in
the
diagram:It comprises two transistors, one of
which
has
together.
transistors
emitters.
the
base
The
are
and
base
then
collector
connections
linked,
as
connected
of
are
both
the
In
terms
of
the
operation
of
the
circuit,
the
base
emitter
junction of T1 acts like a diode because the collector and base are
connected together.
The current into TI is set externally by other components, and as a
result there is a given voltage built up across the base emitter
junction of T1. As the base emitter voltage on both transistors is the
same, the current in one transistor will exactly mirror that of the
second,
assuming
that
both
transistors
are
accurately
matched.
Therefore the current flowing into T1 will be mirrored into T2 and
hence into the load R1.
Circuit limitations
The circuit shown above is often quite adequate for most applications.
However
the
circuit
has
some
noticeable
limitations
under
many
circumstances:
Current matching dependent on transistor matching:
mirroring
is
dependent
upon
the
matching
of
The current
the
transistors.
Often the transistors need to be on the same substrate if they
are to accurately mirror the current.
Current
varies
with
change
in
output
voltage:
This
effect
occurs because the output impedance is not infinite. This is
because there is a slight variation of Vbe with the collector
voltage at a given current in T2. Often the current may vary by
about 25% the output compliance range.
3. CURRENT SOURCE
A
current
source
is
an
electronic
circuit
that
delivers or absorbs an electric current which is
independent of the voltage across it.
A
current
source.
The
source
term
is
the
dual
of
constant-current
voltage
'sink'
is
sometimes used for sources fed from a negative voltage supply. Figure
1 shows the schematic symbol for an ideal current source, driving a
resistor load. There are two types - an independent current source (or
sink) delivers a constant current. A dependent current source delivers
a current which is proportional to some other voltage or current in
the circuit.
In circuit theory, an ideal current source is a circuit element
where the current through it is independent of the voltage across it.
It is a mathematical model, which real devices can only approach in
performance. If the current through an ideal current source can be
specified independently of any other variable in a circuit, it is
called
an
independent
current
source.
Conversely,
if
the
current
through an ideal current source is determined by some other voltage or
current in a circuit, it is called a dependent or controlled current
source. Symbols for these sources are shown in Figure 2.
The internal resistance of an ideal current source is infinite. An
independent current source with zero current is identical to an ideal
open circuit. The voltage across an ideal current source is completely
determined by the circuit it is connected to. When connected to a
short circuit, there is zero voltage and thus zero power delivered.
When connected to a load resistance, the voltage across the source
approaches infinity as the load resistance approaches infinity (an
open circuit). Thus, an ideal current source, if such a thing existed
in reality, could supply unlimited power and so would represent an
unlimited source of energy.
4. FEEDBACK PAIR
The feedback pair connection is a two-transistor circuit that operates
like
the
Darlington
circuit.As
with
Darlington
connection,
the
feedback pair provides very high current gain.A typical application
uses a Darlington connection and a feedback pair connection to provide
complementary transistor operation.
In electronics, the Sziklai pair (also known as a "complementary
feedback pair" (CFP) or "compound transistor") is a configuration of
two bipolar transistors, similar to a Darlington pair.[1] In contrast to
the Darlington arrangement, the Sziklai pair has one NPN and one PNP
transistor, and so it is sometimes also called the "complementary
Darlington". Current gain is similar to that of a Darlington pair,
which
is
the
product
of
the
gains
of
the
two
transistors.
The
configuration is named for its early popularizer, George C. Sziklai.
In a typical application the Sziklai pair acts somewhat like a
single transistor with the same type (e.g. NPN) as Q1 and with a very
high current gain (). The emitter of Q2 acts the role of a collector.
Hence the emitter of Q2 is labeled "C" in the figure to the right.
Likewise, in a typical application the collector of Q2 (also connected
to the emitter of Q1) plays the role of an emitter and is thus labeled
"E."
The figure at the right illustrates a NPN-PNP pair that acts like a
single NPN transistor overall. By replacing Q1 with a PNP transistor
and Q2 with a NPN transistor the pair will act like a PNP transistor
overall. (Just reverse the two arrows in the figure to visualize the
PNP-NPN pair.)
One advantage over the Darlington pair is that the base turn-on
voltage is only about 0.6V or half of the Darlington's 1.2V nominal
turn-on voltage. Like the Darlington, it can saturate only to 0.6V,
which is a drawback for high-power stages.
