A basic differential amplifier is an electronics circuit that produces output(s) which is proportional to the two input signal. Differential amplifier are also called difference amplifier. They can be constructed using discrete BJT(Bipolar Junction Transistor) transistors or FET(Field Effect Transistor) or fabricated in a single Integrated Circuits(IC) chips.
Differential amplifier forms the basic building blocks for
Operational
Amplifiers(op-amps). In op-amps, different stages of differential
amplifiers are used to provide high gain and high input impedance and
high common-mode rejection ratio. The symbolic representation of
differential amplifier is shown below.
Internally the basic differential amplifier circuit constructed using BJT transistors is shown below.
As shown in the above differential amplifier circuit diagram, two BJT transistors Q1 and Q2 are used. Their BJT transistor emitter are tied up and connected to -Vee power supply via RE resistor. Similarly their collectors are connected to the Vcc source via collector resistors Rc1 and Rc2. The input to the differential amplifier can be two different signals applied on both the bases of the two transistors or the input can be applied to one of the base while the other is grounded. Similarly the output can be taken from either the collector output or from both. Hence we can have single ended output or dual output.
Thus we have four possible configuration for differential amplifier as follows.
1. Single Input Unbalanced Output
2. Single Input Balanced Output
3. Dual Inputs Unbalanced Output
4. Dual Inputs Balanced Output
1. Single Input Unbalanced Output Differential Amplifier
In the Single Input Unbalanced Output difference amplifier, the input is applied to one of the base of the two transistor and the other base is grounded. The output can be taken from both or either of the collector. The output is measured w.r.t the ground and therefore called unbalanced output.
The below circuit diagram shows input signal applied to the base of Q1 and the other input is grounded.
Similarly, the following circuit diagram shows input applied to second transistor Q2 and the other input is grounded.
2. Single Input Balanced Output Differential Amplifier
The circuit diagram of Single Input Balanced Output Differential Amplifier is shown below.
In this case, one of the input signal is applied to one of the transistor base and the other input terminal is is grounded. The output is measured between the two collector and is called balanced output. It is called balanced because the both the collectors are at the same dc potential with respect to ground.
Below diagram shows single-ended differential signal waveform applied at input 1 and the waveforms at the two outputs.
The output 1 waveform from the first transistor is out of phase with the applied input 1 while the output 2 waveform is in phase with the input 1.
Similarly if the input signal is applied at transistor 2, the output waveform at output 2 is out of phase with respect to the input signal while the signal waveform at output 1 is in phase with respect to input waveform.
3. Dual Inputs Unbalanced Output Differential Amplifier
The circuit schematic of dual inputs unbalanced output differential amplifier is shown below.
4. Dual Inputs Balanced Output Differential Amplifier
The circuit schematic of dual inputs balanced output differential amplifier is shown below.
Types of input signal to difference amplifier
The input signals for an ideal differential amplifier can be of the following types:
1. Differential input signal
2. Common mode signal
1. Differential input signal
If the two input signals are \(V_{in1}\) and \(V_{in2}\) then the difference input signal \(V_{id}\) is the difference between the two input signals \(V_{in1}\) and \(V_{in2}\). That is,
\(V_{id} = V_{in1} - V_{in2}\)
The output from the difference signal will amplify the difference signal, that is,
\(V_{out} = k (V_{in1} - V_{in2})\)
that is, \(V_{out} = k V_{id}\)
Thus the output from the differential amplifier is non-zero and amplifies the difference between the two inputs only if there exists some difference in the input signals otherwise the output is zero.
2. Common mode signal
If same signal is applied to both the inputs that is when \(V_{in} = V_{in1} = V_{in2}\) then this signal \(V_{in}\) is called common mode signal. In this case, the output produced by ideal differential amplifier is zero.
The tutorial Basic BJT Differential Amplifier Construction and Analysis shows how to perform DC analysis, AC analysis to obtain the Q-point collector current and the collector emitter voltage, the differential mode gain, the common mode gain, the input and output resistances.
