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I'm building an audio circuit and need a comparator. I've explored using an op amp for this purpose but my TL072 doesn't seem to have a high enough slew rate which resulted in latency. Instead, I've been investigating using the ubiquitous 555. It has two internal comparators and by tying the positive input of the upper one to Vcc (pin 6) I can manipulate the digital output by sending a signal to the lower one (pin 2). 555 Internal Schematic My problem is this: The input signal begins to clip when the voltage reaches the transition point (1/3 VCC). I've attempted remedying this with an emitter follower between the signal output and pin 2 but without luck. Any ideas? The 555 is an NE555.

Edit: I misidentified the op amp I was using. From memory I wrote it above as a 'TL072' but it's actually a 'TLC27'. This is at least consistent with @WhatRoughBeast's point about the TL072's slew rate. At any rate, I swapped it out for a TL082 (and I checked the labelling on that one) yesterday and it worked fine. Given that this method is simpler than the 555 route (no worrying about varying input impedance/flip-flop logic) I'll shift my approach and use the TL082 instead.

For posterity, however, here's what my issue with the TLC27 looked like: TLC27 Comparator Notice how the high and low trigger points are different. It may actually have been windowing.

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  • \$\begingroup\$ If a TL072 will not do the job then a 555 will not help either. I suspect that you are searching in the wrong direction. Show us what audio circuit you're trying to build then perhaps we can determine why the TL072 doesn't do what you think it should do. \$\endgroup\$ Dec 15 '15 at 20:21
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    \$\begingroup\$ The 555 is featured in www.badbeetles.com I would have thought that connecting pin 2 to 6 would have given a comp with deadband that runs between 1/3 and 2/3 Vcc .Do keep trying with feeding with low source impedence ,Surely a complimentry follower would work . \$\endgroup\$
    – Autistic
    Dec 15 '15 at 20:23
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    \$\begingroup\$ Have you given thought to the also-ubiquitous LM339? ~1 microsecond response time ought to be adequate, and no input problems. \$\endgroup\$ Dec 15 '15 at 20:28
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    \$\begingroup\$ The TL072 could be a lot faster if you prevent it from clipping to the power supply rails by using some negative feedback through diodes or zeners. \$\endgroup\$
    – jippie
    Dec 15 '15 at 21:07
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    \$\begingroup\$ Have you considered using comparators? \$\endgroup\$ Dec 15 '15 at 21:43
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The problem you describe is caused by the fact that the input impedance of the NE555 is asymmetric around its 1/3 × Vcc threshold level. The impedance for higher input voltages is much higher than for lower voltages.

Look at this NE555 internal circuit diagram I found on upload.wikimedia.org.

The internal resistive divider is in the middle, in green. It divides the power supply in three equal shares, 1/3 × Vcc and 2/3 × Vcc.

The comparator you are trying to use is the red'ish block on the left of it. It is built from four transistors (and a current source from Vcc).

found on commons.wikimedia.org

It can be seen from the circuit that the comparator's emitters tied to the long tail current source will be at approximately 1/3 × Vcc + 2 × 0.6V. Following the two emitter-base diodes further to the left brings you to the trigger input. Two diode drops the voltage back to 1/3 × Vcc.

In other words, the trigger input will have a preference to be at the same 1/3 × Vcc as it's mirror tranistor in the differential amplifier, which is tied to the resistive divider.

  • So the trigger input is high impedance for voltages higher than Vcc as the base-emitter diodes will stop conducting;
  • and much lower impedance for voltages below that as the base to emitter diodes start conducting.

Next look at a datasheet of the NE555. Page 4 lists I(TRIG) for 0V input as 0.5 ~ 2.0 µA. This means your input signal will be loaded with a (worst case) -2µA current. Negative as the NE555 is sourcing current.

A CMOS variant (7555) wouldn't have this problem. Likewise a proper comparator wouldn't have it either. Another option is to use the TL072 as unity gain input buffer. Keeping it in its linear region it should be pretty fast. If the combination of the 555 and TL072 improves performance remains to be seen. The final alternative to improve performance of the TL072 if you prevent it from clipping to the power supply rails by using some negative feedback through diodes or zeners.

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  • \$\begingroup\$ Nice answer. One thing I'd like to point out though is that my input signal only distorts when the voltage rises above 1/3 Vcc. Beyond that it behaves in a similar fashion to what you mention, "disappearing" into a straight line at 1/3 Vcc. I'm going to think this through a bit more but I think you are generally correct. I'll also take a look into going back to the TL072 given that I may have an open half of one in my design somewhere. \$\endgroup\$ Dec 15 '15 at 22:07

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