In comparators, ideally the transition from one state to another state should be instant, but practically it will take certain time to switch from one state to another state. There are slanted edges observed at the transitions. These transitions are more noticeable at high frequencies or even greater than the input signal period itself. Thus there is upper limit of the operating frequency for the comparator. This maximum operating frequency is dependent on the slew rate of the op-amp. Higher the slew rate, higher is the operating frequency.
In comparator applications, op-amp is operated in open loop configuration, so frequency compensation is not required. Thus uncompensated op-amps are preferred in comparator applications.
Since the output of op-amp goes into saturation, there are some compatibility problems with the comparator. The saturation voltage is high (= 0.9VCC). For e.g. With TTL logic, two levels are defined: +5V (logic 1) and 0V (logic 0). Thus to get the output level within specified limit, additional components are required like zener diodes. To avoid the false triggering apply a positive feedback and the circuit is called as Schmitt trigger.
1] Symmetrical Inverting Schmitt Trigger:
A part of output is fed back to the non-inverting (positive) input of the op-amp, hence called as positive feedback comparator. The triggering point VT is calculated as,
VT=R2/(R1+R2 ) Vout
Thus when output is +Vsat, the upper threshold point is given as
VUT=R2/(R1+R2 ) [+Vsat ]
And when output is -Vsat, the lower threshold point is given as
VLT=R2/(R1+R2 ) [-Vsat ]
The operation of the above circuit can be explained with the two conditions
When input voltage Vin is less than upper threshold VUT, the output is in positive saturation +Vsat. When input crosses the upper threshold VUT, output is changed to negative saturation –Vsat. This output state is maintained till the next threshold level i.e. VLT. When input signal crosses the lower threshold VLT, output is changed to positive saturation.
Thus output state is changed only when the two thresholds are crossed. This is shown in the transfer characteristics. Between the VLT and VUT, output (±Vsat) remains constant i.e output is not responding to any changes in the input signal. Thus output is dead between VLT and VUT and called as dead band. It is also referred as hysteresis width, denoted by 'H'. Thus in transfer characteristics we get a rectangle. This is called as hysteresis loop. The graph indicates that the output remains in the state indefinitely until input voltage crosses the any of the threshold levels. The Width of Hysteresis Loop is calculated as H=VUT-VLT
∴H= R2/(R1+R2 ) [+Vsat ]-R2/(R1+R2 ) [-Vsat ]
∴H= (2R2)/(R1+R2 ) [Vsat ]
2] Asymmetrical Inverting Schmitt Trigger:
In previous section we have seen that the triggering points VLT and VUT are having same magnitudes. If we want to the upper and lower threshold values to be different then an additional battery of potential 'V' is added.
∴VUT=R2/(R1+R2 ) [+Vsat ]+R1/(R1+R2 ) V —–Upper Threshold
∴VLT=R2/(R1+R2 ) [-Vsat ]+R1/(R1+R2 ) V —–Lower Threshold