Op Amp Question - Vref

[drummer4gc]

In trying to learn all I can about using op amps as buffers or boosters, I think I have a good understanding now of the purpose of a reference voltage. However, I am seeing two different ways used to provide Vref to the input of an opamp.

Take a look at the basic buffers page on muzique for an example: http://www.muzique.com/lab/buffers.htm

Look at the first two op amp buffers. In the first one, Vref is created with a voltage divider (referenced at the top of the page) and then travels through a 1M resistor on its way to the non-inverting input.


Below, Vref is created again from a voltage divider, but is then presented directly to the input.


Out of curiosity, I tried both ways on a breadboard, and I seemed to notice clearer high end when using the 1M resistor as in the first method. What is the difference between these two methods? What is preferable? I'm guessing that the 1M resistor limits the current while preserving the voltage reference, but I'm truly just guessing, and I am not sure why that would even be preferable.

Thanks!

[tubegeek]

Both +9V and ground can be considered "ground" from the perspective of AC signals.
Therefore Circuit 2 (direct from divider) presents a 1M||1M = 500K input impedance.

Circuit 1 (1M in series with divider) presents a (1M||1M) + 1M = 1.5M input impedance.

The notation 1M||1M means "1M in parallel with 1M."

Guitar source impedance is fairly high; the larger your input impedance, the more highs are preserved (up to a point) - so what you are hearing is just what you might expect to hear. It's the reason we use buffers on guitars in the first place: to preserve the full range of tone by presenting a high input impedance to the guitar.

Both +9V and ground can be considered "ground" from the perspective of AC signals because both spots present a very small impedance to AC signals: ground = 0 ohms (we hope) and +9V, the battery or power supply impedance = very very small, (we hope.)

[MrStab]

if you add a pulldown resistor at some point, this could further affect the impedance. not really relevant atm, just thought i'd mention this in case it saves you any hassle further down the line.

http://www.muzique.com/news/pulldown-resistor-vs-input-impedance/

[duck_arse]

in the first circuit, Vref can be formed by 2 low value resistors, like 10k instead of 1M, with a large  value cap at their junction. this is often called noiseless biasing, it reduces the thermal noise generated by high value resistors. the input impedance is then set by the value of R1.

[teemuk]

What is the difference between these two methods?

Obvious difference is that input impedance of the single-resistor method is opamp's input impedance (usually high enough to ignore) in parallel with R1. In the second one it's opamp's input impedance in parallel with R1 in parallel with R2.

The second difference is power supply rejection ratio: First circuit uses a voltage reference point, which is likely well-filtered (no voltage fluctations) and noise free. Note that we are talking about reference for an amplifier input, which preferably should be as constant as possible, after all it is a reference.

In second example the voltage reference is derived directly without filteration. If supply voltage fluctuates (e.g. noise, ripple, poor decoupling from surrounding stages, etc.) all of this will be directly reflected to the reference point because the reference is simply derived with a simple resistive divider and voltage potential in this refence point must therefore track all voltage fluctuations in power supply voltage. This may work with battery-powered circuits and with extra good AC power supply and noding designs but generally it's considered a somewhat bad design. ...But it can save a few parts and works decently in a handful of applications so it has its place.

What is preferable?

Unless you can ensure an admirably stable and noise-free power supply the first method. The first method also requires just one reference for a multitude of stages. The applications of the second circuit are practically limited to stage its in.

[drummer4gc]

Thank you! Those were all excellent explanations.