A differential amplifier is an amplifier that amplifies the difference between two inputs. A BJT differential amplifier is a differential amplifier constructed with identical pair of BJT(Bipolar Junction Transistor). By identical BJT we mean identical value of current gain beta, base to emitter voltage(Vbe) etc. Also a differential amplifier needs identical biasing resistors for both of the BJT. Differential amplifier can are usually constructed inside an integrated circuit(IC) but one can also construct differential amplifier with discrete BJT, JFET, MOSFET transistors. Also one can construct differential amplifier with combination of BJT and FET transistors which are called BiCMOS differential amplifier. A differential amplifier can be configured as dual input balanced output, dual input unbalanced output, single input balanced output and single input unbalanced output. This different differential amplifier typologies was explained in the tutorial types of differential amplifier and how they work. The basic differential amplifier is the dual input balanced output differential amplifier which was explained in the tutorial Basic BJT Differential Amplifier Construction and Analysis. In this BJT differential amplifier tutorial it is explained how the dual input unbalanced output BJT differential amplifier works.
Dual Input Unbalanced Output BJT Differential Amplifier
A dual input unbalanced output BJT differential amplifier has two inputs and a single output. It is commonly used in analog circuit design, particularly in audio applications, because it can amplify small signals and reject common-mode noise. The circuit diagram of dual input unbalanced output differential amplifier is shown below.
Dual input single output BJT differential amplifier analysis
Here analysis of the dual input single output BJT differential amplifier is done using 2N3904 NPN bipolar transistor.
Consider the following circuit.
The power supply is +12V/-12V. Both the collector resistor RC1 and RC2 is of 100Ohm and both the base resistor RB1 and RB2 is of 1KOhm. The common emitter to both transistor is 100Ohm. The input V1 is of 50mVpk amplitude and frequency of 1KHz. The input V2 is of 25mVpk and frequency of 1KHz. The output is taken from the transistor Q2 collector.
The following shows the V1, V2 and Vout waveform.
From the graph the output signal Vout has amplitude of 100mV.
DC operating point:
The dc operating point of dual input unbalanced output BJT differential amplifier are:
\(I_{CQ}(=I_E)\approxeq \frac{V_{EE}-V_{BE}}{2R_E}\)
\(V_{CEQ} = V_{CC}+V_{BE}-I_{CQ}R_C\)
AC analysis parameters:
(a) Differential gain, \(A_{dm}=\frac{h_{fe} R_C}{2(R_B+h_{ie})}\)
(b) Input Resistance, \(R_i=2(R_B+h_{ie})\)
(c) Output Resistance, \(R_o=R_C\)
Applications of Dual Input Unbalanced Output(DIUO) BJT Differential Amplifier
Here are some applications of DIUO BJT Differential Amplifiers:
- Audio Amplification: DIUO BJT Differential Amplifiers can be used to amplify audio signals with high accuracy and low distortion. They are commonly used in audio preamplifiers, mixing consoles, and audio power amplifiers.
- Instrumentation Amplifiers: DIUO BJT Differential Amplifiers can be used in instrumentation amplifiers to measure small signals with high accuracy. They are commonly used in industrial applications, such as temperature and pressure sensors.
- Biomedical Applications: DIUO BJT Differential Amplifiers are used in biomedical applications, such as electrocardiograms (ECGs) and electroencephalograms (EEGs), to amplify small signals from the body with high accuracy.
- Communication Systems: DIUO BJT Differential Amplifiers are used in communication systems to amplify, modulate, and filter signals in radio frequency (RF) circuits, such as in antenna circuits, voltage controlled oscillator, differential amplifier modulator and mixers.
- Control Systems: DIUO BJT Differential Amplifiers are used in control systems to measure and amplify small signals from sensors and transducers with high accuracy, such as in feedback control systems and motor control circuits.
Overall, Dual Input Unbalanced Output(DIUO) BJT Differential Amplifiers are useful in a wide range of applications that require accurate amplification of small signals while rejecting common-mode noise.
References:
[1] Differential and Common Mode Operation of BJT Differential Amplifier
