Schmitt flip-flops

Schmitt trigger

The Schmitt trigger is a comparator, for which the positive and negative transitions of the output occur at different levels of the input voltage. Their difference is characterized by the hysteresis DU_i. As described in earlier sections, Schmitt triggers can be realized by transistors, but in the following section some designs involving comparators are discussed.

Inverting Schmitt trigger

In the Schmitt trigger of Fig. 2.70 the hysteresis is produced by a positive feedback of the comparator, via the voltage divider R₁, R₂. If a large negative voltage U_i is applied, U₀ = U_{0 max}. At the P-input, the potential is then given as

If the input voltage is changed towards positive values, U₀ does not change at first; only when U_i reaches the value V_Pmax, does the output voltage reduce and therefore also V_p. The difference U_D= V_P – V_N becomes negative. Due to the positive feedback, U₀ falls very quickly to the value U_0min. The potential V_P assumes the value

Fig. 2.70 - Inverting Schmitt trigger. Fig. 2.71 - Transfer characteristic.

Switch-on level:

Switch-off level:

Hysteresis:

U_D is negative and large, this resulting in a stable state. The output voltage changes to U_0max only when the input voltage has reached V_Pmin. The corres­ponding transfer characteristic is shown in Fig. 2.71. The circuit has two stable states only if the loop gain

Figure 2.72 shows the output of the Schmitt trigger for a sinusoidal input voltage.

Fig. 2.72 - Voltage waveshapes of the inverting Schmitt trigger.

Date: 2015-01-12; view: 888