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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. 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.
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. The difference is marked, again from the figure. 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.
Assume the input voltage is lower than the reference voltage at the non-inverting pin and the output is therefore high. Since the output is high through the pullup resistor, this creates a current path through the feedback resistor, slightly increasing the reference voltage. When the input goes above the reference voltage, the output goes low. 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.
To cause the output to go high, the input must now cross the new lower threshold. The input has to cross the threshold just once resulting in a single clean transition. The circuit now has two effective thresholds or states — it is bistable. This can be understood in the usual sense — the x axis is the input and y axis is the output.
Tracing a line from x to y, we find that once the lower threshold has been crossed, the hysteresis goes high and vice versa. 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. Schmitt triggers find a wide range of uses mostly as logic inputs. Having two thresholds gives Schmitt triggers the like ability to act like predictable oscillators.
The capacitor begins charging thought the resistor R. Once the upper threshold is reached, the gate flips to output low, discharging the capacitor to the low threshold, providing a predictable frequency output. Mechanical switches as logic inputs are not exactly the best idea. The switch contacts tend to be somewhat springy, causing a lot of unwanted jitter, which again can cause multiple transitions and glitches further down the line.
Using a Schmitt trigger with a simple RC circuit can help mitigate these problems. When the switch is pressed, it discharges the capacitor and causes the output to go high for a moment till the capacitor charges up again, creating a clean pulse on the output. Schmitt triggers are better known as buffers or inverters in the logic world — but beware, not all gates are Schmitt triggers. The positive feedback is introduced by adding a part of the output voltage to the input voltage.
There are three specific techniques for implementing this general idea. The first two of them are dual versions series and parallel of the general positive feedback system. In these configurations, the output voltage increases the effective difference input voltage of the comparator by 'decreasing the threshold' or by 'increasing the circuit input voltage'; the threshold and memory properties are incorporated in one element.
In the third technique , the threshold and memory properties are separated. Dynamic threshold series feedback : when the input voltage crosses the threshold in some direction the circuit itself changes its own threshold to the opposite direction. For this purpose, it subtracts a part of its output voltage from the threshold it is equal to adding voltage to the input voltage. Thus the output affects the threshold and does not impact on the input voltage. These circuits are implemented by a differential amplifier with 'series positive feedback' where the input is connected to the inverting input and the output - to the non-inverting input.
In this arrangement, attenuation and summation are separated: a voltage divider acts as an attenuator and the loop acts as a simple series voltage summer. Examples are the classic transistor emitter-coupled Schmitt trigger , the op-amp inverting Schmitt trigger , etc. Modified input voltage parallel feedback : when the input voltage crosses the threshold in some direction the circuit changes its input voltage in the same direction now it adds a part of its output voltage directly to the input voltage.
Thus the output augments the input voltage and does not affect the threshold. These circuits can be implemented by a single-ended non-inverting amplifier with 'parallel positive feedback' where the input and the output sources are connected through resistors to the input. The two resistors form a weighted parallel summer incorporating both the attenuation and summation. Examples are the less familiar collector-base coupled Schmitt trigger , the op-amp non-inverting Schmitt trigger , etc.
Some circuits and elements exhibiting negative resistance can also act in a similar way: negative impedance converters NIC , neon lamps , tunnel diodes e. In the last case, an oscillating input will cause the diode to move from one rising leg of the "N" to the other and back again as the input crosses the rising and falling switching thresholds.
Two different unidirectional thresholds are assigned in this case to two separate open-loop comparators without hysteresis driving a bistable multivibrator latch or flip-flop. The trigger is toggled high when the input voltage crosses down to up the high threshold and low when the input voltage crosses up to down the low threshold. Again, there is a positive feedback but now it is concentrated only in the memory cell. Examples are the timer and the switch debounce circuit.
The symbol for Schmitt triggers in circuit diagrams is a triangle with a symbol inside representing its ideal hysteresis curve. The original Schmitt trigger is based on the dynamic threshold idea that is implemented by a voltage divider with a switchable upper leg the collector resistors R C1 and R C2 and a steady lower leg R E. Q1 acts as a comparator with a differential input Q1 base-emitter junction consisting of an inverting Q1 base and a non-inverting Q1 emitter inputs.
The input voltage is applied to the inverting input; the output voltage of the voltage divider is applied to the non-inverting input thus determining its threshold. The comparator output drives the second common collector stage Q2 an emitter follower through the voltage divider R 1 -R 2. The emitter-coupled transistors Q1 and Q2 actually compose an electronic double throw switch that switches over the upper legs of the voltage divider and changes the threshold in a different to the input voltage direction.
This configuration can be considered as a differential amplifier with series positive feedback between its non-inverting input Q2 base and output Q1 collector that forces the transition process. There is also a smaller negative feedback introduced by the emitter resistor R E. Thus less current flows through and less voltage drop is across R E when Q1 is switched on than in the case when Q2 is switched on. Initial state. For the NPN transistors shown on the right, imagine the input voltage is below the shared emitter voltage high threshold for concreteness so that Q1 base-emitter junction is reverse-biased and Q1 does not conduct.
The Q2 base voltage is determined by the mentioned divider so that Q2 is conducting and the trigger output is in the low state. The two resistors R C2 and R E form another voltage divider that determines the high threshold. Neglecting V BE , the high threshold value is approximately. The output voltage is low but well above ground. It is approximately equal to the high threshold and may not be low enough to be a logical zero for next digital circuits.
This may require additional shifting circuit following the trigger circuit. Crossing up the high threshold. When the input voltage Q1 base voltage rises slightly above the voltage across the emitter resistor R E the high threshold , Q1 begins conducting. Its collector voltage goes down and Q2 begins going cut-off, because the voltage divider now provides lower Q2 base voltage.
The common emitter voltage follows this change and goes down thus making Q1 conduct more. The current begins steering from the right leg of the circuit to the left one. This avalanche-like process continues until Q1 becomes completely turned on saturated and Q2 turned off. Now, the two resistors R C1 and R E form a voltage divider that determines the low threshold. Its value is approximately. Crossing down the low threshold. With the trigger now in the high state, if the input voltage lowers enough below the low threshold , Q1 begins cutting-off.
Its collector current reduces; as a result, the shared emitter voltage lowers slightly and Q1 collector voltage rises significantly. The R 1 -R 2 voltage divider conveys this change to the Q2 base voltage and it begins conducting.
The voltage across R E rises, further reducing the Q1 base-emitter potential in the same avalanche-like manner, and Q1 ceases to conduct. Q2 becomes completely turned on saturated and the output voltage becomes low again.
Non-inverting circuit. The classic non-inverting Schmitt trigger can be turned into an inverting trigger by taking V out from the emitters instead of from a Q2 collector. In this configuration, the output voltage is equal to the dynamic threshold the shared emitter voltage and both the output levels stay away from the supply rails. Another disadvantage is that the load changes the thresholds so, it has to be high enough.
The base resistor R B is obligatory to prevent the impact of the input voltage through Q1 base-emitter junction on the emitter voltage. Direct-coupled circuit. To simplify the circuit, the R 1 —R 2 voltage divider can be omitted connecting Q1 collector directly to Q2 base. The base resistor R B can be omitted as well so that the input voltage source drives directly Q1's base. Only Q2 collector should be used as an output since, when the input voltage exceeds the high threshold and Q1 saturates, its base-emitter junction is forward biased and transfers the input voltage variations directly to the emitters.
As a result, the common emitter voltage and Q1 collector voltage follow the input voltage. This situation is typical for over-driven transistor differential amplifiers and ECL gates. Like every latch, the fundamental collector-base coupled bistable circuit possesses a hysteresis. So, it can be converted to a Schmitt trigger by connecting an additional base resistor R to one of the inputs Q1 base in the figure. The two resistors R and R 4 form a parallel voltage summer the circle in the block diagram above that sums output Q2 collector voltage and the input voltage, and drives the single-ended transistor "comparator" Q1.
Thus the output modifies the input voltage by means of parallel positive feedback and does not affect the threshold the base-emitter voltage. The emitter-coupled version has the advantage that the input transistor is reverse biased when the input voltage is quite below the high threshold so the transistor is surely cut-off.
It was important when germanium transistors were used for implementing the circuit and this advantage has determined its popularity. The input base resistor can be omitted since the emitter resistor limits the current when the input base-emitter junction is forward-biased. An emitter-coupled Schmitt trigger logical zero output level may not be low enough and might need an additional output shifting circuit.
The collector-coupled Schmitt trigger has extremely low almost zero output at logical zero. Schmitt triggers are commonly implemented using an operational amplifier or a dedicated comparator. Due to the extremely high op-amp gain, the loop gain is also high enough and provides the avalanche-like process. In this circuit, the two resistors R 1 and R 2 form a parallel voltage summer. It adds a part of the output voltage to the input voltage thus augmenting it during and after switching that occurs when the resulting voltage is near ground.
This parallel positive feedback creates the needed hysteresis that is controlled by the proportion between the resistances of R 1 and R 2. The output of the parallel voltage summer is single-ended it produces voltage with respect to ground so the circuit does not need an amplifier with a differential input. Since conventional op-amps have a differential input, the inverting input is grounded to make the reference point zero volts. The output voltage always has the same sign as the op-amp input voltage but it does not always have the same sign as the circuit input voltage the signs of the two input voltages can differ.
When the circuit input voltage is above the high threshold or below the low threshold, the output voltage has the same sign as the circuit input voltage the circuit is non-inverting. It acts like a comparator that switches at a different point depending on whether the output of the comparator is high or low. When the circuit input voltage is between the thresholds, the output voltage is undefined and it depends on the last state the circuit behaves as an elementary latch. The input voltage must rise above the top of the band, and then below the bottom of the band, for the output to switch on plus and then back off minus.
The Schmitt Trigger allows input buffers to respond to slow input edge rates with a fast output edge rate. Most importantly, Schmitt Triggers provide. The Schmitt trigger input buffer has similar V IL and V IH as the LVTTL I/O standard but with better noise immunity. The Schmitt trigger. The Schmitt trigger gives proper results even if the input signal is noisy. It uses two threshold voltages; one is the upper threshold voltage .