I am teaching electronics a few people as a hobby. I wanted to show them that as the frequency increases output voltage of low pass filter(simple first order RC filter) decreases. I don't have a signal generator to produce frequencies. I am using 555/556 timers for this purpose. But for correct filter output calculations I need to know what is the output impedence of these ICs. But those information is not present in the datasheet. How do I calculate this output impedence?
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\$\begingroup\$ put an op amp follower on your output stage, and then the output impedance is a few ohms and you don't need to worry about it. \$\endgroup\$– Scott SeidmanCommented May 19, 2016 at 14:05
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1\$\begingroup\$ Surely it's best to use sinewaves to demonstrate filters and the 555 produces a square wave (rich in harmonics). \$\endgroup\$– Andy akaCommented May 19, 2016 at 14:07
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\$\begingroup\$ @ScottSeidman: But to get the cutoff frequency don't I need the exact resistance value? \$\endgroup\$– Sreeraj ChundayilCommented May 19, 2016 at 14:09
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\$\begingroup\$ An op-amp's output impedance (as a unity gain buffer) is going to be an ohm or so at audio so, add the 1 ohm to the filter resistance of (say) 1 kohm and you get a 0.1% error. At 1 MHz the op-amp has to be chosen carefully but less than 10 ohms is definitely realistic. \$\endgroup\$– Andy akaCommented May 19, 2016 at 14:11
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1\$\begingroup\$ Output impedance is, as always, voltage drop/lift divided by current. \$\endgroup\$– Ignacio Vazquez-AbramsCommented May 19, 2016 at 14:17
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2 Answers
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You shouldn't require this number for a well-designed circuit. For light loads on a CMOS output timer you might be able to find a number, but generally it's variable with power supply voltage, state (it's asymmetrical) load, temperature, unit-to-unit etc. and there will be a big nonlinear component in bipolar output timers even with a light load.
If you want a controlled output impedance add some resistance such as a few K ohms to the unknown (but very low) resistance of the timer output.
A TLC555 has "typical" output resistance of about 80 ohms (high, at 10mA) and 12 ohms (low, at 10mA) with a 15V supply. With a 5V supply it's 200 ohms (high, at 1mA) and 25 ohms (low, at 3.2mA). If you connect a capacitor directly to such an output your predictions will be way off if based on those resistance values since the MOSFETs do not behave like resistors when their drain-source voltage gets close to the threshold voltage (they behave more like constant current sources).
I generally use 50-100 ohms for the open-loop output impedance of an op-amp. Sometimes a typical value is given on the datasheet.
answered May 19, 2016 at 15:40
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Output impedance is not usually given in logic IC datasheets, because this is not important for digital signals.
However, you can approximate it with some information given in the datasheet: take the voltage drop at high level (Vcc - Voh) and divide it by the current for which the Voh is specified. You'll get an idea of the output impedance for the high level. You can do the same for the low-level: Vol / I (it may be different than for high level, for older chips)
For the NE555, it gives, for a 5V supply (based on ti datasheet):
- for high level: (5V - 3.3V) / 100mA = 17Ohm
- for low level: 0.15V / 8mA = 18.75Ohm
