Push–pull output#Digital circuits

A totem-pole output or a totem-pole circuit is a push -pull output stage of bipolar or field effect transistors. It is optimized for high switching speed and unsuitable due to their nonlinear transmission characteristics for analogue signals. The special feature of this circuit is that it is not realized as conventional push-pull stages with a pnp and an npn transistor (complementary output), but generally with two NPN transistors. The name of the circuit refers to the symbol arrangement for the output stage transistors in the circuit diagram. These are normally drawn one above the other, like a totem pole of the Indians (English: totem pole ).

Logic circuits

One of the main applications is transistor-transistor logic. There, the totem pole TTL output is the actual gate downstream to permit higher currents at the output.

A normal TTL gates with resistor and transistor in series ( in the logical output between the transistor and resistor is tapped ), can only very limited supply higher currents because the resistance virtually acts as a resistor for the output and correspondingly small for a high current would have to be. But then the transistor ( in series) through a useless high current would flow only through the ( now mentally low ) resistance and the switched transistor.

To work around this problem, you still connected to the output a push-pull stage. It consists of two series-connected transistors of the same polarity, either one or the other by operating, but not at the same time, both transistors ( indicated in the push-pull). Only when the output status changes, turn briefly to both transistors. To limit this short-circuit current, the upper transistor, a resistor is connected in series. It would also be possible that both transistors lock, then the output is high impedance ( see Tri -State).

The totem-pole output is considered normal output, but should not be merged with other TTL outputs, as otherwise there is a danger of an association of logical 0 and 1, which can possibly lead to a high current (see web link).

Inverter

The totem-pole output is also used in inverter circuits. There are inverters in the form of a half bridge with a totem-pole output, inverter in the form of a full bridge with two totem-pole outputs and three-phase inverter with three totem-pole outputs.