Single Supply Op amp bias voltage question

Gsatterw

Member
Somewhat of a technical question: every op amp tutorial always talks about how both inverting and non-inverting pins are at the same voltage and the op amp itself works to ensure that. I get that they are always talking about ideal op amps and real will differ a bit. I've been working with typical dual op amps. Every op amp OD/distortion circuit that I build when set up in a non-inverting configuration with Vref = 1/2 VCC and a bias resistor on the + pin always measures around 0.1 - 0.5 V lower than my output and inverting pins measure (which are only 0.001 V different). The non inverting pin is always a good bit less than Vref, and approaches Vref as I reduce the bias resistor size (values between 2 M and 470k). I even measured some of my assembled OD pedals and observe the same behavior. What's going on here? Is the current into pin 3 really that high that would cause that kind of voltage drop across a 470k bias resistor?

Another related question: if my pin 3 measures 4.3 VDC and pin 2 measures 5 VDC and my gain is 100, why doesn't the op amp drive to the positive rail? 100*(5-4.3)=70. Really trying to understand this stuff and can't find any resource that addresses these questions. Thanks!
 
According to this book (Electrical Engineering 101: Everything You… on Scribd. Check it out: https://www.scribd.com/book/239466990), the idea that the condition of the inverting and non-inverting pins being equal is a special case. It’s an interesting read anyway, focusing more on what components do in practical terms than the usual ‘learn Ohm’s law, Thevenin’s theorem, and so on.’ Simon Monk’s Electronics for Inventors is also a useful, practical read (with theory and practice well-integrated). Shout if you need more detail.
 
The appropriate size of the bias resistor is sort of dependent on your op amp. JFET input ones usually work with 1M or higher, some BJT input ones don't like that so you'd be better off biasing them with a smaller resistor, 470-510K is the standard.

Now, the size of the biasing resistor also depends on the impedance at the specific stage you're at. If you're at the start of the circuit, you want to keep the biasing resistor as high as possible for better impedance. Without going too much into detail (since I'm also not pretending I understand all of it), you can get away with smaller resistor values between 10K and 100K in the middle of the circuit. It's all just ballpark values I'm giving here, but usually they'll work.

As for what you're describing at the end is basically asymmetrical clipping of the Op amp. Some designs, like pretty much everything based off the Paul Cochrane Timmy, consciously incorporate this as a design choice by using 2 Vref voltage divider resistors of different values. Similar to asymmetrical clipping diodes in theory this should lead to less regular, thus more organic gain sounds. You wouldn't want it in clean applications where you want your signal as unadulterated as possible (clean boosts, buffers, compressors) but for dirt circuits it can work nicely. If you go very overboard with it you can turn an op amp overdrive or distortion into straight up fuzz!
 
What I have set up on my breadboard at the moment, my Vref rail is exactly half of my positive rail (resistors both 22k), but there is a 0.1-0.2 V drop across my biasing resistor, depending on value. Since I have a DC blocking capacitor and nothing else going to ground between it and my op amp on my input, that means there is a decent amount of current going into the op amp, right?

Even on my Timmy pedal, while yes, Vref isnt exactly half of Vcc, there is a fair amount of voltage drop across the biasing resistor on the first op amp.

This seems to be a common thing with non inverting differential op amp configurations in pedals, but i can't find any explanations for it.
 
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