Comparator encyclopedia topics

Input voltage range

The input voltages must not exceed the power voltage range : $V_{S-} le V_+,V_- le V_{S+}$

In the case of TTL/CMOS logic output comparators, negative inputs are not allowed: $0 le V_+,V_- le V_{cc}$

Op-amp implementation of voltage comparator

A standard op-amp operating without negative feedback can be used as a comparator. When the non-inverting input (V+) is at a higher voltage than the inverting input (V-), the high gain of the op-amp causes it to output the most positive voltage it can. When the non-inverting input (V+) drops below the inverting input (V-), the op-amp outputs the most negative voltage it can. Since the output voltage is limited by the supply voltage, for an op-amp that uses a balanced, split supply, (powered by ± V_S) this action can be written:

$V_{out}=V cdot sgn(V_+ - V_-)$

where sgn(x) is the sign function. Generally, the positive and negative supplies V_S will not match absolute value: '''

$V_{out}le V_{S+}$ when $V_+>V_-,!$ else $V_{S-},!$ when $V_+$

Equality of input values is very difficult to achieve in practice. The speed at which the change in output results from a change in input (often called the slew rate in operational amplifiers) is typically in the order of 10ns to 100ns, but can be as slow as a few tens of μs.

Dedicated voltage comparator chips

A dedicated voltage comparator chip, such as the LM339, is designed to interface directly to digital logic (for example TTL or CMOS). The output is a binary state, and it is often used to interface real world signals to digital circuitry (see analog to digital converter). If one of the voltages is fixed, for example because a DC adjustment is possible in a device earlier in the signal path, a comparator is just a cascade of amplifiers. When the voltages are nearly equal, the output voltage will not fall into one of the logic levels, thus analog signals will enter the digital domain with unpredictable results. To make this range as small as possible the cascade is long and high gain, that is bipolar transistors instead of field effect transistors are used, except sometimes for the first stage. For high speed the input impedance of the stages is made low. This already reduces the saturation of the slow, large P-N junction of the bipolar transistors, which would otherwise lead to long recovery times. Fast that is small Schottky diodes as in binary logic are applied to improve matters even further. Also like in binary logic the speed is not as high as if the amplifiers would be used for analog signals. Slew rate has no meaning for these devices. For the application in flash ADCs after each amplifier the signal can be fanned out over 8 ports matched to the voltage and current gain and resistors are used as level-shifters.

The LM339 accomplishes this with an open collector output. When the inverting input is at a higher voltage than the non inverting input, the output of the comparator is connected to the negative power supply. When the non inverting input is higher than the inverting input, the output is floating (has a very high impedance to ground).

Inputs	Output
− > +	Negative
+ > −	Floating

With a pull-up resistor and a 0 to +5V power supply, the output takes on the voltages 0 or +5 and can be interfaced to TTL logic:

$V_mathrm{out} le V_mathrm{CC}$ when $V_+ ge V_-$ else $0$ .

Precautions in use

When comparing a noisy signal to a threshold, the comparator may switch rapidly from state to state as the signal crosses the threshold. If this is unwanted, positive feedback forming a Schmitt trigger can be used to provide hysteresis and a cleaner output signal.