Understanding the use of diodes for amplitude limiting in Op Amp feedback loops - Page 1
Forum section: electronics / beginners

author: zfzackfrost
author_rank: Newbie
timestamp: January 14, 2021, 02:18:22 am
content: Hello all! This is my first post to the forum, so please bear with me.The other day I was researching some wien-bridge oscillator circuits to use as references for a simple audio oscillator. I came across this variant:https://www.multisim.com/content/tzBJdSz8w6uNPGM3vvhfGk/wien-bridge-oscillator-diode-limiter-potentiometer-gain-adjust/The diode amplitude limiting is the part that stumped me.I plugged this circuit into a spice simulation and lo and behold, it works as advertised. Also, I managed identical amplitude characteristics putting that same fragment in the feedback loop ofan op amp inverting amplifier, non-inverting amplifier, and summing amplifier.So my question is, how and why does this work? I know that diodes can be used a voltage regulators, but this seems different to me. What I really want to be able to do is gain a better understandingof how this witchcraft operates so that I can make actual use of it in my designs, and not just by trial and error.Sorry for the rambling and thanks for any help.A screenshot of the schematic from the above link is in the attachments, if you prefer.

author: fourfathom
author_rank: Super Contributor
timestamp: January 14, 2021, 04:55:35 am
content: Look at that circuit without the diodes. You will see that the gain is set by Rfand the setting of Rg. Now, add the diodes. These diodes will not conduct until the voltage across them exceeds about 0.6V -- and beyond this output voltage the diodes are effectively shunting Rf, thus reducing the gain when the output exceeds +/-0.6V (1.2V P-P). This limits the output amplitude.

author: zfzackfrost
author_rank: Newbie
timestamp: January 14, 2021, 06:15:04 am
content: Quote from: fourfathom on January 14, 2021, 04:55:35 amAhhh. That makes more sense now. Much appreciated.

author: MathWizard
author_rank: Super Contributor
timestamp: January 14, 2021, 09:55:03 am
content: Ah yes, when looking at op-amp circuits with diodes before, I never thought of the actual effect on gain that way, by DC clamping. But at AC of course a cap can change the gain too. But yup, now I'm be looking fot that next time.

author: thinkfat
author_rank: Unknown
timestamp: January 14, 2021, 10:02:25 am
content: Typically the idea of those diodes is to limit the output swing so that the opamp doesn't go into saturation, as this will affect the bandwidth.

author: fourfathom
author_rank: Super Contributor
timestamp: January 14, 2021, 04:15:22 pm
content: Quote from: thinkfat on January 14, 2021, 10:02:25 amIn this oscillator circuit, I think the diodes are there more to maintain the amplitude in a controlled manner, rather than have it determined by the opamp input and output characteristics or power supply variations. Of course there will now be the diode temperature characteristics (2mV per Deg C) affecting the output level.

author: David Hess
author_rank: Super Contributor
timestamp: January 17, 2021, 05:40:48 pm
content: As the voltage across the feedback resistor approaches the forward voltage of the parallel diodes, the diodes begin to conduct which has the same effect as if the feedback resistor was passing more current because it was a lower value. The diodes are adding non-linearity to the feedback network which lowers the gain by lowering the feedback resistance as the output amplitude increases.The old alternative way is to place a resistor with a large positive temperature coefficient in the common leg of the feedback network in the form of a small incandescent lamp. Using a lamp produces less distortion because its resistance depends on temperature instead of instantaneous voltage.