A differential amplifier is type of amplifier that amplifies the difference of two inputs. A basic differential amplifier can be constructed using two identical transistors which could be a BJT, FET or BiCMOS and can be implemented either with discrete transistors or inside an integrated circuit(IC). Differential amplifier are however usually implemented inside an IC rather than with discrete individual transistors and with an IC the transistors can be closely matched with each other and an actual differential amplifier requires many transistors such as current source and current mirror, level shifter etc. Differential amplifier are usually used to make operational amplifier which are in turn used in many electronics circuit application. There are 4 types of differential amplifier configuration according to the number of inputs and outputs. The working principle of the basic dual input balanced output BJT differential amplifier and dual input unbalanced output BJT differential amplifier was explained earlier. Besides these the other two configuration are the single input balanced output and the single input unbalanced output BJT differential amplifier. In this article it explained how the single input balanced output BJT differential amplifier works.
Single Input Balanced Output(SIBO) BJT Differential Amplifier
A single input balanced Output(SIBO) BJT Differential Amplifier is a differential amplifier that amplifies the difference between two signal inputs and the amplified output is taken from the two collectors of the differential pair transistors. Since one of the input is grounded we can say the SIBO amplifies the single input to produce balanced output.
A BJT differential amplifier is constructed using identical pair of bipolar junction transistors and identical biasing resistors. The following shows the circuit diagram of single input balanced output BJT differential amplifier.
In the circuit diagram above, the two transistors Q1 andQ2 are identical(beta, emitter-base voltage etc) and their collector resistors RC1 and RC2 are equal to RC. A single input V1 is applied to base of the BJT transistor Q1 while the other transistor base is grounded. We can also apply input signal to base of Q2 and ground the Q1 base. The emitter resistor RE is common to both.
Circuit Analysis
Using DC and AC analysis using h-parameter model we can arrive at the following equations for single input balanced output differential amplifier
The Q-point current and voltages 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 derived parameters are:
(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, \(R_o=R_C\)
Consider that we apply signal V1 which has 50mV peak and frequency of 1KHz to the base of Q1. Let us use the 2N3904 bipolar transistor. Let us consider that the collector resistors are 100Ohm, emitter resistor is 100Ohm and that the base resistor is 1KOhm. The following is the circuit diagram.
The output is dc coupled differential output and the following graph shows the input signal waveform and the differential output waveform.
That is the differential pair signal output from the single input balanced output is a DC biased and has peak to peak voltage of approximately 2.28V or peak voltage of 2.28/2=1.14V.
The differential voltage gain is,
\(A_{dm} = \frac{V_{out}}{V_{in}}=\frac{1.14V}{50mV}\approx 23\)
We can add coupling capacitor to obtain ac coupled differential signal output. The following shows the circuit diagram where 0.1uF capacitor are connected to both the outputs.
The following shows the waveform of the input and the ac coupled differential output from the single input balanced output BJT amplifier.
Application of BJT differential amplifiers
BJT differential amplifiers are used in construction of op-amps, are used in communication electronics circuits such as voltage controlled oscillator, BJT differential amplifier modulator etc.
