In electronics, the Schmitt trigger is a hysteresis comparator circuit applied by applying positive feedback to the non-inverting input of the comparator or differential amplifier. This is an active circuit that converts analogue input signals into digital output signals. Circuits are called "triggers" because the output maintains its value until the input changes enough to trigger the change. In non-inverting configuration, when input is higher than the choice threshold, the result is high. When inputs are below the different (lower) selection threshold, the output is low, and when the input is between two levels, the output will retain its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger has memory and can act as a batchable multifibrator (latch or flip-flop). There is a close relationship between two types of circuits: Schmitt triggers can be converted into latches and latches can be converted into Schmitt triggers.
Schmitt triggering devices are commonly used in signal conditioning applications to remove noise from signals used in digital circuits, especially mechanical contact reflections on the switch. They are also used in closed loop negative feedback configurations to implement relaxation oscillators, which are used in function generators and switching power supplies.
Video Schmitt trigger
Discovery
Schmitt trigger was created by American scientist Otto H. Schmitt in 1934 when he was a graduate student, later described in his doctoral dissertation (1937) as a "thermionic trigger". It is a direct result of Schmitt's study of the propagation of neural impulses on the squid nerve.
Maps Schmitt trigger
Implementation
Fundamental idea
Circuits with hysteresis are based on a fundamental positive feedback idea: each active circuit can be made to behave as a Schmitt trigger by applying positive feedback so that the loop gain is more than one. Positive feedback is introduced by adding part of the output voltage to the input voltage. This circuit contains 'attenuator' (box B in the picture on the right) and 'summer' (circle with "" inside) next to the amplifier which acts as a comparison. There are three specific techniques for implementing this general idea. The first two of them are the double (series and parallel) versions of the general positive feedback system. In this configuration, the output voltage increases the input voltage of the effective difference of the comparator by 'lowering the threshold' or by 'increasing the input voltage of the circuit'; Threshold and memory properties are inserted in one element. In the third technique, 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 in the opposite direction. For this purpose, it reduces some of its output voltage from the threshold (equal to adding a voltage to the input voltage). Thus the output affects the threshold and does not affect the input voltage. This circuit is implemented by a differential amplifier with a 'positive feedback series' where the input is connected to the inverting input and output - to the non-inverting input. In this setting, attenuation and summing are separated: the voltage divider acts as an attenuator and the loop acts as a simple series summer voltage. An example is a classical transistor trigger paired with Schmitt, an inverting trigger op-amp, etc.
Modified input voltage (parallel feedback): when the input voltage across the threshold in some direction the circuit converts its input voltage in the same direction (now adding a portion of the the output voltage is directly to the input voltage). Thus the output adds to the input voltage and does not affect the threshold. This circuit can be implemented by a single non-inverting amplifier ending with a 'parallel positive feedback' where the input and the output source are connected through the resistor to the input. The two resistors form a weighted parallel summer that combines both attenuation and summation. Examples are less familiar collectors combined with Schmitt triggers, non-inverting Schmitt triggers, etc.
Some circuits and elements that exhibit negative resistance can also act in the same way: negative impedance converter (NIC), fluorescent lamp, tunnel diode (for example, diodes with current-shape characteristics "N" in the first quadrant) etc. In the latter case , the oscillating input will cause the diode to move from one leg up from "N" to the other and back again when the input crosses the threshold up and down.
Two different unidirectional thresholds are assigned in this case to two separate open-loop compasses (without hysteresis) moving bistable multivibrators or flip-flops. The trigger is toggled high when the input voltage crosses down to the high and low threshold when the input voltage crosses down to the low threshold. Again, there is positive feedback but now it is just concentrated inside the memory cells. Examples are the 555 timer and the debounce switch circuit.
The symbol for the Schmitt trigger in the circuit diagram is a triangle with symbols representing the ideal hysteresis curve.
Schmitt Transistor Trigger
Classical pairs-emitter class
Schmitt's original trigger is based on the idea of ​​a dynamic threshold implemented by a voltage divider with a replaceable upper leg (collector resistor R C1 and R C2 ) and stable lower leg (R < sub> E ). Q1 acts as a comparator with a differential input (Q1 base-emitter junction) consisting of an inverting input (Q1 basis) and a non-inverting (Q1 emitter). The input voltage is applied to the inverting input; the voltage divider voltage is applied to the non-inverting input thereby determining the threshold. The comparator output drives the second common collector stage Q2 (emitter follower ) through the voltage divider R 1 -R 2 . The emitter-coupled transistors Q1 and Q2 actually form a double throw electronic switch that switches over the upper leg of the voltage divider and changes the threshold in different directions (to the input voltage).
This configuration can be regarded as a differential amplifier with a positive feedback circuit between the non-inverting input (base Q2) and output (collector Q1) that forces the transition process. There is also a smaller negative feedback introduced by the emitter resistor R E . To make a positive feedback dominate over the negative and to obtain hysteresis, the proportion between two collector resistors is selected R C1 & gt; R C2 . So less current flows through and decreases less voltage at R E when Q1 is activated than in case when Q2 is enabled. As a result, the circuit has two different thresholds in terms of ground (V - in the image).
Operation
Initial status. For the NPN transistors shown to the right, imagine the input voltage under the emitter voltage (high threshold for concrete) so that the base-emitter intersects Q1 is reversed and Q1 does not hold. The Q2 base voltage is determined by the divider until Q2 performs and the trigger output is low. Two resistor R C2 and R E form another voltage divider that determines the high threshold. Ignoring V BE , a high threshold value is around .
The output voltage is low but far above ground. This is roughly equal to a high threshold and may not be low enough to be a logical zero for the next digital circuit. This may require additional shift circuits following the trigger circuit.
Cross the high threshold. When the input voltage (base voltage Q1) rises slightly above the voltage at the emitter resistor R E (high threshold), Q1 starts doing. The collector voltage drops and Q2 begins to break, because the voltage divider now provides a lower base voltage of Q2. The general emitter voltage follows this change and goes down so as to make Q1 do more. The steering current starts from the right leg of the circuit to the left circuit. Although Q1 does more, it passes less current through R E (since R C1 & gt; R C2 ); the emitter voltage continues to decrease and the effective base-emitter Q1 voltage continues to increase. Processes like this avalanche continue until Q1 becomes fully turned on (saturated) and Q2 is turned off. The trigger is switched to high state and the output voltage (Q2 collector) is close to V. Now, two resistors R C1 and R E form a voltage divider that determines the low threshold. Its value is roughly
- .
Cross the low threshold. With the trigger now in high state, if the input voltage is low enough (below the low threshold), Q1 starts to break. Collector current is reduced; as a result, the joint emitter voltage decreases slightly and the collector voltage Q1 rises significantly. The voltage divider R 1 -R 2 conveys this change to the base voltage Q2 and starts operating. The voltage at R E rises, further reducing the potential of the Q1-base emitter in the same way as an avalanche, and Q1 stops functioning. Q2 becomes fully turned on (saturated) and the output voltage becomes low again.
Variations
Non-inverting circuit. The irreversible classic Schmitt trigger can be converted into an inverting trigger by taking V out of the emitter instead of the collector Q2. In this configuration, the output voltage is equal to the dynamic threshold (the shared emitter voltage) and the second level of output away from the supply rail. Another disadvantage is that the load turns the threshold so, it should be quite high. The resistor base R B is mandatory to prevent the impact of the input voltage through the base-emitter junction Q1 at the emitter voltage.
Direct paired circuits. To simplify the circuit, the voltage divider R 1 -R 2 can be removed connecting the collector Q1 directly to base Q2. The base resistors R B can be removed as well so that the input drive voltage source is directly base Q1. In this case, the common emitter voltage and collector voltage Q1 are not suitable for output. Only the Q2 collector should be used as output because, when the input voltage exceeds the high threshold and Q1 is saturated, the emitter base connection is forward biased and transfers the input voltage variation directly to the emitter. As a result, the general emitter voltage and collector voltage Q1 follow the input voltage. This situation is typical for differential amplifier transistors and excessively driven ECL gates.
Collector-base combinations
Like each latch, the base collector base coupled with a bistable circuit has hysteresis. So, it can be converted to a Schmitt trigger by connecting an additional base resistor R to one of the inputs (base Q1 on the image). Two resistors R and R 4 form a summer of parallel voltages (the inner circle of the ballock diagram above) that adds the output (Q2 collector) voltage and input voltage, and drives a single-ended transistor comparator "Q1. the base crossing the threshold (V BE0 0.65 V) in some direction, part of the collector voltage Q2 is added in the same direction to the input voltage Thus the output modifies the input voltage by means of positive feedback parallel and does not affect the threshold (base-emitter voltage).
Comparison between emitter and collector-coupled circuits
The emitter-coupled version has the advantage that the input transistor is reverse-biased when the input voltage is just below the high threshold so the transistor must be disconnected. That's important when germanium transistors are used to implement circuits and this advantage has determined its popularity. The input base of the resistor can be removed because the emitter resistor limits the current when the junction input-emitter junction is biased forward.
Schmitt triggers paired with the emitter do not have a low enough level at logical zero output and require additional output transfer circuits. Schmitt trigger coupled with collector has a very low output level (almost zero) at the output of zero logic .
Implementation Op-amp
Schmitt triggers are generally implemented using operational amplifiers or special comparators. Op-amps and open-loop comparators can be considered as analog-digital devices that have analog and digital outputs extracting the voltage difference marks between the two inputs. Positive feedback is applied by adding part of the output voltage to the input voltage in series or parallel. Since the gain of the op-amp is very high, the loop gain is also quite high and provides a process like avalanche.
Non-inverting Schmitt trigger
In this circuit, two resistors R 1 and R 2 form a parallel voltage summer. This adds part of the output voltage to the input voltage so as to add it during and after the switching that occurs when the voltage is generated near the ground. This parallel positive feedback creates the required hysteresis controlled by the proportion of resistance R 1 and R 2 . The output of the summer parallel voltage is one-ended (generating voltage to ground) so the circuit does not require amplifier with differential input. Since the conventional op-amp has 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 input voltage of the op-amp but does not always have the same sign as the input voltage of the circuit (the signs of the two inputs) the voltage may vary). When the input voltage of the circuit is above the high threshold or below the low threshold, the output voltage has the same sign as the input voltage of the circuit (non-reversing circuit). It works like a comparator that switches at different points depending on whether the output of the comparator is high or low. When the input voltage of the circuit is between thresholds, the output voltage is undefined and it depends on the last state (the circuit acts as the bottom latch).
Misalnya, jika pemicu Schmitt saat ini dalam keadaan tinggi, output akan berada di rel catu daya positif ( V S ). Tegangan output V dari musim panas resistif dapat ditemukan dengan menerapkan teorema superposisi:
The unique properties of circuits with parallel positive feedback is the impact on the input source. In circuits with negative parallel feedback (for example, an inverting amplifier), the virtual ground at the inverting input separates the input source from the op-amp output. Here there is no virtual ground, and the stable output voltage of the op-amp is applied over the network R 1 -R 2 to the input source. The op-amp output passes the opposite current through the input source (it injects the current into the source when the input voltage is positive and draws current from the source when it is negative).
Practical Schmitt trigger with the appropriate threshold shown in the picture on the right. The transfer characteristics have exactly the same shape from the previous basic configuration, and the threshold value is the same. On the other hand, in the previous case, the output voltage depends on the power supply, while the current is determined by the Zener diode (which can also be replaced by a single double-Zode diode anode). In this configuration, the output level can be modified with an exact choice of Zener diodes, and these levels are resistant to fluctuations in power supplies (ie, they increase the PSRR of the comparator). The resistor R 3 exists to limit the current through the diode, and the resistor R 4 minimizes the input offset voltage caused by the current leaked comparator input (see real op-amp limitations ). Inverting Schmitt trigger
In inverting version, attenuation and addition are separated. Two resistors R 1 and R 2 act only as "pure" attenuator (voltage divider). The loop input acts as a simple series summer voltage which adds part of the output voltage in series to the input voltage of the circuit. the positive feedback of this series creates the required hysteresis controlled by the proportion between the resistance R 1 and all the resistances (R 1 and R) 2 ). The effective voltage applied to the op-amp input is floating so that the op-amp must have a differential input.
Circuit is called inverting because the output voltage always has the opposite sign to the input voltage when it exits the hysteresis cycle (when the input voltage is above the high threshold or below the low threshold). However, if the input voltage is in the hysteresis cycle (between high and low thresholds), the circuit can be inverted and non-inverting. The output voltage is undefined and it depends on the last state so the circuit behaves like a base latch.
To compare two versions, network operations will be considered under the same conditions as above. If the Schmitt trigger is currently in high state, the output will be in the positive power supply rail (V S ). The output voltage V of the voltage divider is:
Komparator akan beralih ketika V dalam = V . Jadi harus melebihi di atas tegangan ini untuk mendapatkan output untuk beralih. Setelah output komparator beralih ke - V S , ambang batas menjadi untuk beralih kembali ke tinggi. Jadi rangkaian ini menciptakan sebuah band switching yang berpusat pada nol, dengan tingkat pemicu (dapat digeser ke kiri atau ke kanan dengan menghubungkan R 1 ke tegangan bias). Tegangan input harus naik di atas bagian atas band, dan kemudian di bawah bagian bawah band, untuk output untuk mematikan (minus) dan kemudian kembali (plus). Jika R 1 adalah nol (yaitu, hubungan pendek) atau R 2 tidak terbatas, band ini runtuh ke lebar nol, dan berperilaku sebagai pembanding standar.
Unlike the parallel version, this circuit has no impact on the input source because the source is separated from the voltage divider output by high op-amp input impedance.
In the inverting amplifier the voltage drop across the resistor R1 decides the reference voltage ie, the upper threshold voltage (V) and the threshold voltage V () for comparison with the applied input signal. This voltage is defined as the output voltage and the resistor value is set.
so by changing the drop at (R1) the threshold voltage may vary. By adding a bias voltage in series with the resistor (R1) the drop through it can vary, which can change the threshold voltage. The desired value of the reference voltage can be obtained by varying the bias voltage.
Persamaan di atas dapat dimodifikasi sebagai
Aplikasi
Schmitt triggers are typically used in open loop configurations for noise immunity and closed loop configurations to implement function generators.
- Analogue to digital conversion : Schmitt trigger effectively is one bit analog to digital converter. When the signal reaches a certain level, it will move from low to high.
- Detect level : The Schmitt trigger circuit can provide level detection. When performing this app, it is necessary that the hysteresis voltage be taken into account so that the circuit switches to the required voltage.
- Acceptance of the line : When running a data line that might have picked up voice to logic gates, it is important to ensure that the logic output level only changes as data is changed and not as a result of a false sound that may have been taken. Using Schmitt triggers extensively allows peak to peak noise to reach hysteresis levels before a false trigger can occur.
Kekebalan suara
One of the Schmitt trigger applications is to increase the noise immunity in the circuit with only one input threshold. With only one input threshold, a noisy input signal near the threshold can cause the output to switch back and forth from noise alone. A Schmitt Trigger input signal that is noisy near a threshold can cause only one switch in the output value, after which it must move beyond the other threshold to cause another switch.
For example, amplified infrared photodiodes can generate electrical signals that often switch between the absolute lowest value and the absolute highest value. This signal then low-pass filtered to form a smooth signal that rises and falls in accordance with the relative amount of time the switching signal is on and off. The filtered output passes to the Schmitt trigger input. The net effect is that the output of the Schmitt trigger only passes from low to high after the received infrared signal excites the photodiode longer than some known period, and once the Schmitt trigger is high, it moves only low after the infrared signal stops the excite of the photodiode longer than a similar known period. While photodiodes are susceptible to fake switching due to noise from the environment, delays are added by the filter and the Schmitt trigger ensures that the output only switches when there is an input that stimulates the device.
Schmitt triggers often occur in many switching circuits for the same reason (for example, to switch debouncing).
Use as oscillator
Schmitt trigger is a bistable multivibrator, and can be used to implement other types of multivibrator, relaxation oscillator. This is achieved by connecting one circuit integrating the RC between the output and the input of the Schmitt inverting trigger. The output will be a continuous square wave whose frequency depends on the R and C values, and the Schmitt trigger threshold point. Because some Schmitt trigger circuits can be provided by one integrated circuit (eg 4000 CMOS device type 40106 contains 6 of them), the backup part of the IC can be quickly incorporated into the service as a simple and reliable oscillator with only two external components.
Here, Schmitt-based comparator triggers are used in the reversion configuration. In addition, the slow negative feedback is added to the integrated RC network. The result, shown on the right, is that the output automatically oscillates from V SS to V DD as the capacitor load from one Schmitt trigger threshold to the other.
See also
- The operational amplifier app
- Shared multivibrator circuits
- Threshold detector with hysteresis
Note
References
External links
- Inverting Schmitt Trigger Calculator
- Non-Inverting Schmitt Trigger Calculator
Source of the article : Wikipedia
0 Comments