555 pulse generator - voltage drop on output

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moretti1993

Joined Sep 11, 2016
6
Hello there... :)

Im currently building a pulse generator using 555 timer. The attached photo shows how its setup.
The frequency changes works just fine BUT the output voltage varies when i change the freqency. When set to low frequence (1-20Hz) i measure 5V peak to peak but when i turn up the frequency it gradually decrease the voltage and ends up with 4.4V at 2k Hz, even without load.

I guess thats not supposed to happen.

circuit is from this site http://www.electronics-tutorials.ws/waveforms/555_oscillator.html
 

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ian field

Joined Oct 27, 2012
6,536
Hello there... :)

Im currently building a pulse generator using 555 timer. The attached photo shows how its setup.
The frequency changes works just fine BUT the output voltage varies when i change the freqency. When set to low frequence (1-20Hz) i measure 5V peak to peak but when i turn up the frequency it gradually decrease the voltage and ends up with 4.4V at 2k Hz, even without load.

I guess thats not supposed to happen.

circuit is from this site http://www.electronics-tutorials.ws/waveforms/555_oscillator.html
The CMOS 555 can hold its output amplitude to a higher frequency - but the output current is only about half what you get from a bipolar 555.
 

Alec_t

Joined Sep 17, 2013
14,280
The output from a 555 is always somewhat less than the supply voltage because of the way its output stage is constructed. As you've found, the more current it sources the lower the output voltage.
 

ian field

Joined Oct 27, 2012
6,536
The output from a 555 is always somewhat less than the supply voltage because of the way its output stage is constructed. As you've found, the more current it sources the lower the output voltage.
The bipolar 555 never gets nearer to the rails than the VCEsat of the output transistors, the CMOS 555 does a lot better - until you load it.

There's a list of reasons why the output amplitude diminishes as you approach the device frequency limit - with the CMOS part; its primarily capacitance. Mostly parasitic capacitance of anything connected to the output.

Its slightly less with bipolar, but there's a time delay associated with injecting charge carriers into the base emitter junctions in the output transistors. AFAICR: its worse when you want to purge carriers from the junction to turn the transistor off.
 
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