Figure is a TTL level / disability l2V level conversion circuit . Circuit , VTl is the base grounded switch circuit. When74LSO4 output goes high , VTl conduction , its collector current, into VT2 base current , VT1 is also conducting , the output U. Level Into -l2V and with the VT2 the collector saturation voltage addition. The circuit itself is not high-speed switching . If with Rl in parallel a capacity of several hundred pF of capacitance Cl, and VT2 of the base – collector indirect Ruxiaoteji diode VD1, can VT2 conduction during not completely saturated , and therefore to some extent will increase the switching frequency .