[sdiy] buffer opamp configuration [was: 1/8th inch questions]

[Seb Francis]

Hi All,

As I've come to expect, nothing's ever as simple as it first appears.  It's been really interesting reading all your comments about protection/stabilising resistors and buffer opamp configuration.  Sometimes I wonder why I bother trying to make an accurate, fully featured MIDI2CV when perhaps I should just buy something like the MOTM-650 and save all the hard work.  But then I remember how much I've learnt in the process (and had a lot of fun :)  Also, my MIDI2CV will have some unique software features which I'm fairly sure you just can't buy in any commercial product.  Like I've said before, I'm good with software .. but have a lot to learn about electronics ;-)

Anyway, I'm going to summarise (as much for my own benefit as for the list) the points which have been made and what I've learnt from other reading  ...


A resistor in between the buffer opamp output and the output jack is a good (necessary) thing for the following reasons:

(1) It limits the current which can be drawn from the opamp - even with opamps which are "short circuit protected", drawing a lot of current can overheat the chip, leading to potential premature failure.  Additionally, large current transients can cause glitches on the power rail which can result in audible pops and clicks.

(2) It also protects against shorting opamp output to other voltages, or to other opamp outputs.

(3) Where the opamp output is used internally in other parts of a module (e.g. a VCO saw wave used to make other waveforms), the resistor stops a temporary short to ground causing too much of an audible glitch on other outputs.

(4) It helps prevent oscillations when driving capacitive loads such as patch cords.

Typical output resistor values are from 100R to 1K.  The former being more accurate in terms of load effect on CV level, the latter being more safe.


The problem with using a resistor on the output for accurate pitch CVs is that even low loads can produce noticeable detuning.  However it seems clear from the above points that it is pretty much essential to use a resistor on the output.  The solution is to place the output resistor within the feedback loop.

For a normal inverting opamp configuration, a small resistor (e.g. 100R->1K) should be put betwen the opamp output and the output jack, with a larger resistor (choose size to set required gain) between the output jack and inverting input.  Additionally put a small capacitor between the opamp output and inverting input for extra stability.

Unity gain (non-inverting) buffers can be problematic since opamps are less stable at unity gain, especially when driving capacitive loads.  There seem to be two common configurations to give stability:
(a) Put a capacitor and resistor in series between the opamp inputs, and a larger resistor in series with the input signal
(b) Put a capacitor and resistor in series between the opamp inputs, and a larger resistor in the feedback loop.
Note that both these ways increase the AC noise gain, and decrease the buffer bandwidth.  I won't go into details about how they work since I'd probably describe it wrong - instead see "Useful reading" below.

Now here's a point which I haven't quite worked out yet .. how to get the required current limiting / short circuit protection when using a unity gain buffer.  It seems to me that by using configuration (b) and connecting the output jack to the inverting input of the opamp, then the feedback resistor will act as a current limiter.  However, this sounds a little wrong to me (running an output directly from an input - static discharge could damage input, other reasons I can't think of?) .. probably it's better to use 2 resistors in the feedback loop and connect the output jack to the node between the resistors?  I'd welcome any comments on this ...

In any case, I'm going to have to try it out and see how my design behaves in reality.  As an intuitive guess, I'm going to start with a 330pF capacitor and 250R resistor in series between the opamp inputs, and two 470R resistor in series in the feedback loop.  The output jack will be connected to the junction between the resistors in the feedback loop.  And if I can get it to work stably on breadboard, I'll still have to test it all again once I get the PCB made, since all those stray capacitances in the breadboard are sure to affect things!


Additional points:

(1) Decoupling of opamp's power supply is also important for stability / to prevent oscillation.  The theory is quite involved, but a general rule is to use a 100nF ceramic capacitor as close as possible to each opamp, and a 10-20uF Electrolytic or 2-10uF Tantalum cap for every 5 or so opamp ICs.

(2) If a 1K output resistor is used *outside* the feedback loop then it's possible to passive mix the jack output with other similar outputs.  This is not possible with the (more accurate) configuration of having the output resistor within the feedback loop.

(3) The worst case situation for output protection requirements is connecting one output to another with each giving out voltages on opposite ends of the CV scale (e.g. -7.5V -> 7.5V).  It's actually quite likely this will happen by accident at some point with a modular system, and when it happens it's likely the "short" will last for a number of seconds before the user realises he or she has made a wrong patch.

(4) If short circuits might happen for long periods of time, ensure the resistors used can handle the power without catching fire ;)  A 1/4W 1K resistor is ok, but theoretically (assuming the opamp is able to kick out 150mA) a 2W 100R resistor is needed to sustain continuous short circuit.  Actually, be more worried about how much power the opamp can dissipate!

(5) And the best check of all .. breadboard the circuit: measure the short-circuit current, feel how hot the opamp gets, etc..


Useful reading:

http://www.analog.com/library/applicationNotes/amplifiersLinear/highSpeed/AN257.pdf
(very good document on many aspects of stability of high speed opamps)

http://www.national.com/ms/LB/LB-42.pdf
(deals specifically with unity gain follower stability)

Chapter 8 of "Troubleshooting Analog Circuits" by Bob Pease.
(about opamps)



Hope this summary is helpful for someone .. writing it has certainly been helpful for me!  Please, please respond if you think I got something wrong ;)

Seb