These configurations are shown in fig. 2, and are defined by number of input signals used and
the way an output voltage is measured. If use two input signals, the configuration is said to be
dual input, otherwise it is a single input configuration. On the other hand, if the output voltage is
measured between two collectors, it is referred to as a balanced output because both the
collectors are at the same dc potential w.r.t. ground. If the output is measured at one of the
collectors w.r.t. ground, the configuration is called an unbalanced output.
A multistage amplifier with a desired gain can be obtained using direct connection between
successive stages of differential amplifiers. The advantage of direct coupling is that it removes
the lower cut off frequency imposed by the coupling capacitors, and they are therefore, capable
of amplifying dc as well as ac input signals.
Dual Input, Balanced Output Differential Amplifier:
The circuit is shown in fig. 1, v1 and v2 are the two inputs, applied to the bases of Q1 and Q2
transistors. The output voltage is measured between the two collectors C1 and C2 , which are at
same dc potentials.
To obtain the operating point (ICC and VCEQ) for differential amplifier dc equivalent circuit is
drawn by reducing the input voltages v1 and v2 to zero as shown in fig. 3.