Differential amplifier are type of amplifier that uses two identical transistors to amplify difference between two input signal. The transistors can be bipolar junction transisotors(BJT), field effect transistors(FET) or BiCMOS transistors. Usually differential amplifiers are implemented within an integrated Circuit(IC) because when fabricated on single IC, the transistors are more closely tailored and saves size and power. Despite this, differential amplifier may also be constructed using individual BJT, FET or BiCMOS transistors. There are 4 different types of differential amplifier configuration depending upon the number of inputs and output taken:
(1) dual input balanced output BJT amplifier
(2) dual input unbalanced output BJT amplifier
(3) single input balanced output BJT amplifier
(4) single input unbalanced output BJT amplifier
In this blog post, the single input unbalanced output BJT amplifier working principle is explained.
Single input unbalanced output BJT differential amplifier
In single input unbalanced output BJT differential amplifier, the input is applied to one of the differential pair transistor and output is taken from either of the two transistor collector. Following shows the circuit diagram of single input unbalanced output(SIUO) BJT differential amplifier wherein the input signal is applied to the base of the transistor Q1 and output is taken from the collector of the second transistor Q2 and measured with respect to the ground.
Circuit Analysis
The DC analysis and AC analysis of the single input unbalanced output BJT differential amplifier gives us the Q-point or the quscient opertating point, the differential gain, the input and output resistances. Following are the DC and AC analysis result.
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, \(A_{dm}=\frac{h_{fe} R_C}{R_B+h_{ie}}\)
(b) Input Resistance, \(R_i=2(R_B+h_{ie})\)
(c) Output Resistance, \(R_o=R_C\)
Consider that we are using 2N3904 NPN bipolar transistor. Let the collector resistors be 100Ohm, the base resistor be 1KOhm and the emitter resistor be 100Ohm. Let the input signal applied to Q1 be 50mV peak with frequency of 1KHz. The circuit diagram is shown below.
For this circuit, following shows the input and the dc coupled single ended output waveform.
The single ended dc coupled output has amplitude of 1.13V peak to peak and is dc biased at around 2.3V. That is it has 1.13/2=0.56V peak amplitude.
We can add a coupling capacitor to the output to get ac coupled singled ended output as shown in the circuit diagram below and plot the waveform again to see comparison of the input signal and the singled ended output waveform more clearly.
We can calculate the differential gain of this single ended unbalanced output amplifier as follows.
\(A_{dm} = \frac{V_{out}}{V_{in}}=\frac{0.56V}{50mV}\approx 11\)
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.References:
[1] Differential and Common Mode Operation of BJT Differential Amplifier
