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Schmitt triggers find a wide range of uses mostly as logic inputs. The operation of the non-inverting comparator is similar – the output again changes the configuration of a resistor network to change the threshold to prevent unwanted oscillations or noise. Tracing a line from x to y, we find that once the lower threshold has been crossed, the hysteresis goes high and vice versa. This can be understood in the usual sense – the x axis is the input and y axis is the output. This can be summarised in the form of a graph: HYSTERESIS CURVE The circuit now has two effective thresholds or states – it is bistable. The input has to cross the threshold just once resulting in a single clean transition. Once crossed, the output goes high and the circuit is ‘reset’ to the initial configuration. To cause the output to go high, the input must now cross the new lower threshold. Since the reference voltage is lowered, there is no chance of a small change in input causing multiple transitions – in other words, there is no longer a dead zone. Normally this shouldn’t affect the reference voltage in any way, but since there’s a feedback resistor, the reference voltage drops slightly below the nominal value because the feedback and the lower reference resistor are now in parallel with respect to ground (since a low output shorts that terminal of the resistor to ground). When the input goes above the reference voltage, the output goes low. Since the output is high through the pullup resistor, this creates a current path through the feedback resistor, slightly increasing the reference voltage. V* is the reference input voltage which creates a fixed bias at the non-inverting input. This reinforcing property is useful – it makes the comparator decide the state of the output it wants, and makes it stay there, even within what would normally be the dead zone.Ĭonsider this simple circuit: INVERTING COMPARATOR WITH HYSTERESISĪssume the input voltage is lower than the reference voltage at the non-inverting pin and the output is therefore high. CLEAN TRANSITION USING HYSTERISISĪgain, note the unstable reference voltage.Ī Schmitt trigger makes use of positive feedback – it takes a sample of the output and feeds it back into the input so as to ‘reinforce’, so to speak, the output – which is the exact opposite to negative feedback, which tries to nullify any changes to the output. The difference is marked, again from the figure. This is something that can be remedied using hysteresis – in this case with the addition of a single resistor between the inverting terminal (which in this case is the reference) and the output. If there was any logic connected to the output (which in most cases is true), it would detect the multiple transitions and cause havoc – flip flops would toggle multiple times, maybe causing something important to reset. If you notice carefully, the input signal varies with the output swing and there’s a lot of noise on the supply rail (as seen on the output through the pullup resistor), which is a result of poor decoupling! MULTIPLE OUTPUT TRANSITIONS WITHOUT HYSTERESIS (BLUE INPUT, YELLOW OUTPUT) This problem also occurs with signals that have a slow transition time – the input signal spends enough time in the dead zone (with reference to the reference voltage, of course) to create multiple output transitions, as shown in the figure below. Within this narrow range, the comparator has no idea what to do with its output – which leads to something called motorboating, which is the output oscillating. This problem gets worse when the differential input signals reach the dead zone, that is, the minimum input differential voltage required to maintain a stable output. Comparators also have especially sensitive inputs because of their very high gain – even tiny changes in the input can cause instant change of state on the output. Comparators are not limited by output slew rate and transition times are in the order of nanoseconds. Schmitt (whose legacy is somewhat understated) who called it a ‘thermionic trigger’.Ĭomparators by nature are very fast, since they lack the compensating capacitor found in their op-amp cousins. It was invented way back in 1937 by Otto H. INVERTING AND NONINVERTING SCHMITT TRIGGERS USING AN LM193 (LM393 RELATIVE) A Schmitt trigger is a comparator (not exclusively) circuit that makes use of positive feedback (small changes in the input lead to large changes in the output in the same phase) to implement hysteresis (a fancy word for delayed action) and is used to remove noise from an analog signal while converting it to a digital one.
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