What is OTL

OTL is the abbreviation of Output Transformer Less, which means no output transformer. An OTL power amplifier is a power amplifier circuit without an output coupling transformer. Most OTL power amplifiers use a complementary push-pull output stage circuit.

As shown in the figure, this is a complementary symmetrical OTL power amplifier circuit. T2 is an NPN power transistor, T3 is a PNP transistor, they form a complementary push-pull output tube, and T1 is a voltage amplifier excitation tube. After the signal is coupled to T1 through C1 for amplification, the signal generated from the collector of T1 turns on T2 in the positive half cycle, and turns on T3 in the negative half cycle. The amplified signal is output to the speaker through capacitor C3.
Capacitor C2 in the circuit is a bootstrap capacitor, which forms a bootstrap circuit with R2 and R3, so that the potential of point B increases with the increase of output voltage, expanding the dynamic range of the circuit.