Totem pole exit
A totem pole output or a totem pole circuit is a push-pull output stage made of bipolar or field effect transistors . It is optimized for high switching speeds and, due to its non-linear transmission properties, is unsuitable for analog signals . The special feature of this circuit is that they do not like conventional push-pull stage having a PNP and an NPN transistor ( complementary output is realized), 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 over the other, comparable to a totem pole of the Indians ( engl. : Totem pole ).
Logic circuits
One of the main uses of totem pole circuits is in transistor-transistor logic . There the totem pole output is connected downstream of the actual TTL gate in order to enable higher currents at the output.
A normal TTL gate with resistor and transistor in series (in which the logical output between transistor and resistor is tapped) can only deliver higher currents to a very limited extent, because the resistor acts as a series resistor for the output and is correspondingly small for a high current would have to be. But if the transistor then switches through (in series), a useless high current would only flow through the (now conceptually low-ohmic) resistor and the through-connected transistor.
To get around this problem, a push-pull stage is connected to the output. This consists of two series-connected transistors of the same polarity, with either one or the other switching through, but not both transistors at the same time (in counter- clock). Both transistors only switch briefly when the state of the output changes. In order to limit this short-circuit current, a resistor is connected upstream of the upper transistor. It would also be possible for both transistors to block, then the output becomes high-impedance ( see tri-state ).
The totem pole output is considered a normal output, but must not be connected to other TTL outputs, as otherwise there is a risk of logic 0 and 1 combining, which may 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 one totem pole output, inverters in the form of a full bridge with two totem pole outputs and three-phase inverters with three totem pole outputs.