A differential amplifier is kind of amplifier which uses two mirrored amplifiers to amplify the difference between two input signal applied at each amplifier. It is a basic building used in operational amplifier(op-amps) used inside integrated circuit(IC). Besides using it inside integrated circuit, a differential amplifier can also be build with discrete BJT(Bipolar Junction Transistors) or FET(Field Effect Transistor) transistors. In this blog post, we will discuss the basic principles of the BJT differential amplifier, its construction, and its applications.
A BJT differential amplifier is used in a variety of applications, such as audio amplifiers, differential amplifier modulator, instrumentation amplifiers, voltage controlled oscillator(VCO) etc. The differential amplifier is used to amplify the difference between two input signals, while rejecting any common-mode signal that is present in both input signals. This makes the differential amplifier an important component in many circuits, as it allows for accurate measurements and efficient signal processing.
Principles of BJT Differential Amplifier
The BJT differential amplifier is a two-input circuit that amplifies the difference between two input signals, V1 and V2 applied to the base terminals of two BJTs (Bipolar Junction Transistors), which are configured in a differential pair configuration(see figure below). The basic differential pair configuration consists of two BJTs wherein the two inputs V1 and V2 are applied to the bases of the two BJTs with base resistors R1 and R2, their collectors are connected to the power supply Vcc via collector resistors R1 and R2 and their emitter are connected to a common emitter resistor RE.
Following is circuit diagram of basic BJT differential amplifier which is of called dual input balanced output differential amplifier.
Dual Input Balanced Output differential amplifier circuit diagram
There are actually 4 typologies or types of differential amplifiers which are:
(1) Dual inputs balanced output
(2) Dual inputs unbalanced output
(3) Single input balanced output
(4) Single input unbalanced output
The above circuit shown is dual input balanced output since two inputs are applied at the two bases of BJT transistors Q1 and Q2 and two outputs are taken from the collectors of the transistors. Furthermore, it is an emitter coupled differential amplifier.
The following shows the V1, V2 input waveform and the resulting waveform at the output Vout1 and Vout2.
The operation of the BJT differential amplifier is based on the fact that the current flowing through the two BJTs is proportional to the difference between the two input voltages, V1 and V2. When the input signals are equal, the current through both BJTs is equal, and the output voltage is zero. However, when the input signals are different, the current through the BJTs is different, and this causes a voltage drop across the load resistor, resulting in an output voltage.
The operation of the BJT amplifier can be analyzed using DC and AC analysis. This gives us the operating point(Vceq and Icq) and the input, output resistances and the differential and common mode gain.
DC Analysis
Determination of Q-point collector current(Icq)
The q-point collector current(Icq) is equal to the emitter current(Ie). Thus to derivation and expression for emitter current(Ie) is equivalent to finding expression for collector current(Icq).
Under DC condition we can ground the input V1 and follow the base to emitter loop to derive expression for the emitter current Ie.
Using KVL around the base-emitter loop we have,
AC Analysis
Applications of BJT Differential Amplifier
The BJT differential amplifier has many applications in electronic circuits. Some of the most common applications include:
Audio Amplifiers: The BJT differential amplifier is used in audio amplifiers to amplify the difference between the left and right audio signals, resulting in a stereo output.
Instrumentation Amplifiers: The BJT differential amplifier is used in instrumentation amplifiers to amplify small differential signals, such as those generated by sensors, and reject common-mode signals.
Operational Amplifiers: The BJT differential amplifier is used in operational amplifiers to provide a high input impedance and low output impedance, making them suitable for use in a variety of circuits.
Conclusion
In conclusion, a BJT differential amplifier is constructed using discrete and individual BJTs, resistors, and capacitors. Alternatively, it can be built using integrated circuits (ICs), which are available in a variety of packages, including dual-in-line (DIP) and surface-mount (SMT). The BJT differential amplifier is a versatile circuit that is used in a wide range of applications, from audio amplifiers to instrumentation amplifiers. Its ability to amplify the difference between two input signals, while rejecting any common-mode signal, makes it an important component in many circuits. With the widespread availability of discrete components and integrated circuits, it is now easier than ever to incorporate a BJT differential amplifier into your own electronic projects.
The dc analysis Q-point for dual input balanced output are:
(1) Icq = (Vee-Vbe)/2Re
(2) Vceq = Vcc+Vbe-IcqRc
The ac analysis differential mode gain, common mode gain, the input and output resistance are as follows:
(1) Ad = -hfe*Rc/(hie+Rs)
(2) Acm = -(hfe*Rc)/(Rs+hie+2*Re(1+hfe))
(3) Ri = Vin/Ib = 2(Rs+hie)
(4) Ro = Rc
