Schematic Volt: January 2015

Monday, January 26, 2015

Basics of Schmitt Trigger Circuits – Part 3

All Schmitt triggers are active devices relying on positive feedback to achieve their hysteresis action. The output goes to high whenever the input rises above a certain preset upper threshold limit, and goes to low whenever the input drops below a lower threshold limit.

The output retains its previous value (low or high), when the input is between the two threshold limits. This type of circuit is often used to clean up noisy signals, and convert an analogue waveform into a digital waveform (1’s and 0’s) with clean, fast edge transitions.

There are three methods typically used in implementing positive feedback to form a Schmitt trigger circuit. These methods are Parallel Feedback, Series Feedback, and Internal Feedback, and are discussed as follows. The parallel and series feedback techniques are actually dual versions of the same feedback circuit type.

Parallel Feedback

A parallel feedback circuit is sometimes called a modified input voltage circuit. In this circuit, the feedback is added directly to the input voltage, and does not affect the threshold voltage. As the feedback is added to the input when the output changes state, the input voltage has to shift by a greater amount in the opposite direction to cause further change in output.

If the output is low, and the input signal increases to the point where it crosses the threshold voltage and the output changes to high. Part of this output is applied directly to the input through a feedback loop, which “helps” the output voltage stay in its new state. This effectively increases the input voltage, which has same effect as lowering the threshold voltage. The threshold voltage itself is not changed, but the input now has to move farther in the downward direction to change the output to a low state. Once the output is low, this same process repeats itself to get back to the high state.

This circuit does not have to use a differential amplifier, as any single-ended non-inverting amplifier will work. Both the input signal and the output feedback are applied to the non-inverting input of the amplifier through resistors, and these two resistors form a weighted parallel summer. If there is an inverting input, it is set to a constant reference voltage. Examples of parallel feedback circuits are a collector-base coupled Schmitt trigger circuit or a non-inverting op-amp circuit, as shown:

Series Feedback

A dynamic threshold (series feedback) circuit operates in basically the same way as a parallel feedback circuit, except that the feedback from the output directly changes the threshold voltage instead of the input voltage. The feedback is subtracted from the threshold voltage, which has the same effect as adding feedback to the input voltage. As soon as the input crosses the threshold voltage limit, the threshold voltage changes to the opposite value. The input now has to change to a greater extent in the opposite direction to change the output state again.

The output is isolated from the input voltage and only affects the threshold voltage. Therefore, the input resistance can be made much higher for this series circuit compared to a parallel circuit.

This circuit is usually based on a differential amplifier where the input is connected to the inverting input and the output is connected to the non-inverting input through a resistor voltage divider. The voltage divider sets the threshold values, and the loop acts like a series voltage summer. Examples of this type are the classic transistor emitter-coupled Schmitt trigger and an inverting op-amp circuit, as shown here:

Internal Feedback

In this configuration, a Schmitt trigger is created by using two separate comparators (without hysteresis) for the two threshold limits. The outputs of these comparators are connected to the set and reset inputs of an RS flip-flop. The positive feedback is contained within the flip-flop, so there is no feedback to the comparators. The output of the RS flip-flop toggles high when the input goes above the upper threshold, and toggles low when the input goes below the lower threshold. When the input is between upper and lower thresholds, the output retains its previous state.

An example of a device that uses this technique is the 74HC14 made by NXP Semiconductors and Texas Instruments. This part consists of an upper threshold comparator and a lower threshold comparator, which are used to set and reset an RS flip-flop. The 74HC14 Schmitt trigger is one of the most popular devices for interfacing real world signals with digital electronics. The two threshold limits in this device are set at a fixed ratio of Vcc. This minimises the part count and keeps the circuit simple, but sometimes the threshold levels need to be changed for different kinds of input signal conditions. For example, the input signal range might be smaller than the fixed hysteresis voltage range. The threshold levels can be changed in the 74HC14 by connecting a negative feedback resistor from the output to input, and another resistor connecting the input signal to the input. This effectively reduces the fixed 30% positive feedback to some lower value, such as 15%. It is important to use high-value resistors for this (Mega-Ohm range) in order to keep the input resistance high.