From an audio signal, this circuit gets two signals of the same amplitude and opposite phases. This setting is widely used in bridge amplifiers (BTL). Transistors may be commonly used as BC series.
This circuit is from 1977 documentation, but can be assembled with BC548 for Q1, Q2 and Q4, BC558...
Found in American documentation from the 90s, this circuit uses components that are still common...
This high impedance input amplifier is suitable for the use with piezoelectric sources such as...