The operational amplifiers are shortly
termed as Op-Amps and are also called as differential amplifiers. The
operational amplifier is typically used as a differential amplifier in
various electrical and electronic circuits. These operational amplifiers
can be used for performing filtering, signal conditioning and
mathematical operations. The electrical and electronics components
such as resistors and capacitors are used at the input or/and output
terminals of the operational amplifier. Such that, the amplifier
function results, benefit of the resistive feedback or capacitive
feedback configurations are regulated by these components. Thus, the
amplifier can achieve various operations, hence, it is termed as
operational amplifier.
Differential Amplifier
The electronic amplifier
used for amplifying the difference between two input signals can be
called as a differential amplifier. In general, these differential
amplifiers consist of two terminals namely inverting terminal and
non-inverting terminal. These inverting and non-inverting terminals are
represented with – and + respectively.
Differential Amplifier Circuit
The differential amplifier can be
considered as an analog circuit which consists of two inputs and one
output. The differential amplifier circuit can be represented as shown
in the figure below.
Differential Amplifier
Where, A= gain of the amplifier.
Differential Amplifier Circuit using Transistors
The differential amplifier circuit using transistors
can be designed as shown in the figure below which consists of two
transistors T1 and T2. These transistors and resistors are connected as
shown in the circuit diagram.
Differential Amplifier using Transistors
There are two inputs I1 & I2 and two
outputs V1out & V2out in the differential amplifier circuit. The
input I1 is applied to the transistor T1 base terminal, input I2 is
applied to the transistor T2 base terminal. The emitter terminals of
transistor T1 and transistor T2 are connected to a common emitter
resistor. Thus, the two input signals I1 & I2 will affect the
outputs V1out & V2out. The differential amplifier circuit consists
of two supply voltages Vcc and Vee but there is no ground terminal. Even
with single voltage supply also circuit can be operated fine as it is
intended (similarly while using two supply voltages). Hence, the
opposite points of positive voltage supply & negative voltage supply are connected to the ground.
Differential Amplifier Working
The differential amplifier working can
be easily understood by giving one input (say at I1 as shown in the
below figure) and which produces output at both the output terminals.
Differential Amplifier Working
If the input signal (I1) is supplied to
the base of transistor T1, then a high voltage drop appears across the
resistor connected to the transistor T1 collector terminal which will
get less positive. If no input signal (I1) is supplied to the base of
transistor T1, then a low voltage drop appears across the resistor
connected to the transistor T1 collector terminal which will get more
positive. Thus, we can say that the inverting output appearing across
collector terminal of transistor T1 is based on the input signal I1
supplied at the base terminal of T1.
If T1 is turned ON by applying the
positive value of I1, then the current passing through emitter
resistance increases as the emitter current & collector current are
almost equal. Thus, if the voltage drop across emitter resistance increases,
then the emitter of both the transistors goes in a positive direction.
If the transistor T2 emitter is positive, then the base of T2 will be
negative and in this condition, current conduction is less.
Thus, there will be less voltage drop
across resistor connected at the collector terminal of transistor T2.
Hence, for the given positive input signal collector of T2 will go in a
positive direction. Thus, we can say that the non-inverting output
appearing across collector terminal of transistor T2 is based on the
input signal applied at the base of T1.
The amplification can be driven
differentially by taking output between the collector terminals of
transistors T1 and T2. From the above circuit diagram, assuming all the
characteristics of transistors T1 & T2 are identical and if base
voltages Vb1 is equal to Vb2 (base voltage of transistor T1 is equal to
base voltage of transistor T2), then emitter currents of both
transistors will be equal (Iem1=Iem2). Thus, total emitter current will
be equal to the sum of emitter currents of T1 (Iem1) and T2 (Iem2).
Thus, emitter current can be driven as
Thus, the emitter current remains
constant independent of the hfe value of transistors T1 and T2. If
resistances connected to the collector terminals of T1 & T2 are
equal, then their collector voltages are also equal.
Differential Amplifier Applications
There are numerous differential amplifier applications
in practical circuits, signal amplification applications, controlling
of motors & servo motors, input stage emitter coupled logic, switch,
and so on are common applications of the differential amplifier
circuit.
For more information regarding amplifier
circuits and differential amplifier applications you can approach us by
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