It is amazing that so much of what is in use today was designed so long ago. It is subject to drift due to aging, has fair noise rejection and relatively low gain. Distortion can be substantially reduced by adding a cathode resistor, but at the expense of gain and increased output impedance. To achieve higher gains, two such stages can be put in tandem by the use of a coupling capacitor. Such an amplifier is known as a triode cascade amplifier.
Differential amplifiers have a number of advantages over conventional single-ended amplifiers.
Differential amplifiers are a type of balanced amplifier, although certainly not the only type. This means that there two inputs to a differential amplifier and two outputs. Any signal at the input of one side of the amplifier will result in two outputs, equal but opposite of each other. Differential amplifiers present a constant load to the power supply, resulting in less noise in the power supply. Differential amplifiers have roughly 6 dB lower noise then the same circuit executed in a single-ended manner.