How to build a Differential Amplifier Circuit using Transistors?

The short form of the operational amplifier is op-amp and it also works as a differential amplifier.An Op-Amp is an essential component in various electronic circuits. Op-Amps are linear devices which are used to do mathematical operations and filtering, signal conditioning. These devices are designed to use in external electrical and electronic components like resistors, capacitors and its i/p and o/p terminals. These components regulates the operation of the amplifier and function results and benefit of the altered feedback configurations like resistive or capacitive, the amplifier can achieve different operations, and this is termed as an operational amplifier. An op-amp comprises of two terminals namely inverting and non-inverting which are represented with + &-. The main function of this amplifier is, it strengthens the changes between the two input voltages. But, defeats any voltage mutual to the two inputs.

Differential Amplifier
Differential Amplifier

Differential Amplifier

All op-amps are differential amplifiers due to their i/p configuration.If the first voltage signal is connected to the i/p terminal and the remaining signal is connected to the opposite i/p terminal, then the result o/p voltage is related to the difference between two i/p voltage signals.The output voltage can be calculated by connecting each input to 0 Volts ground using superposition theorem.

Differential Amplifier Circuit
Differential Amplifier Circuit

Differential Amplifier Circuit by Using Transistors

A Differential amplifier circuit using BJTs is explained in detail and the circuit diagram with appropriate equations are provided for a better understanding. The following circuit is designed with transistors to give the difference between two i/p signals.

Differential Amplifier Circuit  using BJTs
Differential Amplifier Circuit using BJTs

As shown in the above circuit diagram, the circuit consists of two inputs and two outputs, namely I/P1, I/P2 and O/P1, O/P2. The input I/P1 is applied to the base terminal of the T1 transistor and IP2 is applied to the base terminal of the T2 transistor. The emitter terminals of the two transistors are connected to a mutual emitter resistor so that the two o/p terminals get damaged by the two i/p signals. The two supply voltages of the circuit are Vcc & Vss. The circuit also works with a single voltage supply and we can observe that the circuit doesn’t have the ground terminal.

Working of Differential Amplifier

The working of differential amplifier with transistors is shown below.

When the first input signal is applied to the T1 transistor, then there will be a high voltage drop across the collector resistance (RCOL1) and the collector of transistor T1 will be less positive. When the input1 is negative, transistor T1 will be turned OFF & the voltage drop across the collector resistor RCOL1 becomes very low & the collector of transistor T1 will be more positive

Working of Differential Amplifier Circuit using BJTs
Working of Differential Amplifier Circuit using BJTs

Thus, can conclude that the o/p which is inserted will be appears on the collector of transistor T1 for applying the signal at input1. When transistor T1 is turned ON by the positive value of the input1, the current through the resistor REM increases the emitter current is equal to the collector current.

So the voltage drop across resistor REM increases and & makes the emitter of both T1, T2 transistors flow in a positive direction. Making the transistor T2 is the same as making of the transistor’s base negative, in that condition the T2 transistor will behave less current which in turn will cause less voltage drop in RCOL2 and thus the collector of transistorT2 will go in a +Ve direction for the +Ve i/p signal. Thus, we can conclude that the o/p of the non-inverting terminal appears on the collector of the T2 transistor for input at the base of T1. The amplification of the transistor can be driven differently by taking o/p b/n the collector of both transistors T1 & T2 shown in the above circuit.

If the both transistors are assumed to be equal in all characteristics, and if the voltages are identical (VBASE1 = VBASE2), then the emitter current of the transistors can also be said to be identical.

The total emitter current (IE) =IEM1+IEM2
                      VEM = VBASE – VBASE EM
                       IEM = (VBASE – VBASE EM) / REM

The emitter current of the transistor remains almost constant irrespective of the hfe value of the transistor. Since ICOL1 IEM1, &   ICOL2 IEM2, ICOL1 ICOL2.
Also, VCOL1 = VCOL2 = VCC – ICOL RCOL, assuming the collector resistance RCOL1 = RCOL2 = RCOL.

The differential amplifier circuit is a closed loop amplifier which increases the variance between two signals. Such a circuit is very suitable in instrumentation systems. Differential amplifiers have high CMRR (common mode rejection ratio) & a high i/p impedance. Differential amplifiers can be designed using one or two op-amps.

Thus, this is all about differential amplifier circuit using a BJT transistor. We believe that you have got a better understanding of this concept. Furthermore, any doubts regarding this topic, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what are the applications of a differential amplifier?

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