First, the characteristics:

The (1) circuit has two stable states. The maintenance and transition of the two stable states depend entirely on the external trigger signal. When the level value of the input signal reaches a certain voltage, the output state suddenly changes and the output is a pulse signal. Trigger mode: level trigger.

⑵ Voltage hysteresis transmission: The circuit has two switching levels (upper trigger switching level UT+ and lower trigger switching level UT-). When the input signal rises from a low level, the corresponding input level when the circuit state transitions is different from that when the input signal falls from a high level.

(a) Co-directional output characteristics (a) Reverse output characteristics

Figure 1 1 Schmidt circuit transmission characteristics

Forward threshold voltage? :? When it rises, why? Corresponding mutation? Value.

Reverse threshold voltage? :? When you fell, why? Corresponding mutation? Value.

(3) There is a positive feedback process when the state is reversed, thus outputting a rectangular pulse with steep edges.

2. Schmidt trigger composed of gate circuits

1. Circuit composition:

Two CMOS inverters and two voltage dividing resistors. The voltage dividing resistor feeds back the voltage at the output end to the input end, thus affecting the circuit.

(a) circuit (b) logic symbol

Figure 12 Schmidt trigger composed of gate circuits

2. Working principle

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Set the threshold voltage of CMOS inverter? ,? ,? This is a triangular wave.

We can know from the circuit: What is the input level of G 1? Determine the state of the circuit.

What time? G 1 off, G2 on. At this time? .

What time? Ascension ambassador? When, make the circuit produce the following positive feedback

Soon? At this time? (Forward threshold voltage).

What time? When is the circuit state maintained? there will be no changes.

What time? After rising to the maximum, it began to decline, and then fell to? , the circuit produces the following positive feedback:

So that the circuit can be quickly converted into? The state at this time? Down to? (negative threshold voltage).

Is that enough? , the circuit is stable in? The state of.

The return difference voltage is:?

Returns the sum of differential voltages? In direct proportion, change? This ratio can adjust the size of the return voltage. The working waveform and transmission characteristics of the circuit are shown in figure 13.

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Figure 13 Working Waveform and Voltage Transmission Characteristics of Schmidt Trigger

3. Application of Schmidt trigger: It is often used for signal waveform shaping (that is, transforming a slowly changing signal waveform into a rectangular wave with steep edges) and amplitude discrimination.

(1) Converts a slowly changing waveform into a rectangular wave (for example, converts a triangular wave or a sine wave into a rectangular wave with the same period).

Figure 14 converts a slowly changing waveform into a rectangular wave.

(2) pulse shaping

In digital system, rectangular pulse often appears waveform distortion or edge oscillation after transmission. Schmidt trigger shaping can obtain ideal rectangular pulse waveform.

Figure 15 pulse shaping

(3) pulse amplitude identification

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A series of pulse signals with different amplitudes are added to the input of Schmidt trigger, and only those pulses with amplitudes greater than UT+ will produce output signals at the output. It can be seen that Schmidt trigger has the ability of pulse amplitude discrimination.

Figure 16 pulse amplitude discrimination

Example 3 Known? Half wave? , the circuit as shown in figure 17? ,? ,? . Trying to draw? Waveform.

(a)(b)

Figure 17

Solution:? The waveform of is shown in figure 18.

Figure 18

Example 4 The circuit that can play the role of timing is ().

A. Schmidt trigger;

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B. decoder;

C. multivibrator;

D. monostable trigger;

The answer is D.

Analysis: (1) Schmitt trigger is often used for signal waveform shaping (that is, transforming a slowly changing signal waveform into a rectangular wave with steep edges) and amplitude discrimination.

(2) decoding by a decoder;

(3) the multivibrator is used to generate a pulse square wave;

(4) Monostable trigger: shaping → outputting rectangular wave. Timing → outputting a rectangular wave with a certain width; Delay → Extend the output signal for a certain time and then output it.

Example 5(2006) The component that can improve the timing accuracy is ().

A. Schmidt;

B. bistable;

C. monostable;

D. multivibrator

The answer is D.

Analysis: Oscillator is the core of digital clock. The stability of oscillator and the accuracy of oscillation frequency determine the timing accuracy of digital clock. The higher the frequency of the oscillator, the higher the timing accuracy.

Three. Schmitt trigger composed of 555 timer

Figure 19555 timer composed of Schmidt trigger

1. Working principle:

What time? v，？ Output high level.

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