opamp gain stage design and circuit

[Gus]

I decided to draw a simple textbook opamp gain circuit
The gain is x2 to x11
JFET input unity gain stable opamp of your choice
input pair protection R4
output series stability resistor R5
some adjusting points noted
Sim of guitar and cable included to show the high end peak, if "feed" from a resistive source you would not have the high end peak
   NOTE sim with volume at MAX and Tone at MAX treble, things change at different setting
R5 and C8 form a low pass filter if wanted
Supply voltage can adjusted
Max gain can be adjusted up and down with potentiometer value selected for R12
Graph shows about 0uf, .01uf and .022uf for C8

[FiveseveN]

Congrats on a very robust design and simulation. I think this is a good opportunity to raise a challenge that I've been pondering for some time: let's design an amplifier with the lowest noise floor we can, something one could use onboard a guitar par example, to give the best possible S/N ratio.
I for one (would) do a couple of things:
1. instead of the R1/R2/R3 network just use two 1M resistors to bias the opamp (slightly less noise than 1011K) and DC-couple subsequent stages if any. Seems to be plenty current even with BJT-input opamps (didn't do the math, that's my bad, but had no issues thus far). In fact we could probably go as low as 330K but I don't know if Johnson noise is still the biggest issue at that point.
2. R6 is obviously only needed if the whole thing is going to be bypassed.
3. Pretty sure we don't need R10 and if it's a pot we certainly shouldn't use it: never amplify and then attenuate.
4. Could probably go lower than 10K over 1K for the feedback divider.
Again, I'm no expert so take these points with a grain of salt and would appreciate any input. The EMG preamp is probably a useful guideline for what I'm talking about.

[Gus]

I posted this textbook circuit because over the years people have asked about opamp gain stages and buffers.
You can make it a buffer remove R11, R12 R13 and C7 connect the opamp output to the inverting input.
Note TL071 in the screen shot the values selected were for a TL071(or 1/2 of a 72 or LF353 or LF412 etc)
The 47uf C3 bypasses the R1, R2 and R3 node
One can omit C8 if you don't want the output lowpass
9VDC with 10 ohms of series resistance

I added the simple guitar and cable sim to show what can happen at the higher frequencies The guitar is simmed with max treble and max volume.  The opamp circuit has a 1meg input resistance(like some guitar amps)

Again this is textbook circuit for a common opamp like a 1/2 TL072 LF412 LF353.

Note R4 and R5

[Earthscum]

Note R4 and R5

Question: R4 limits the current into the input, correct? Any other function? (RF stop with the cap?)

R5 I understand limits the output to a load of no more than 1.5k if it were shorted to ground, and helps shunt signal to the feedback. Does it set the impedance for the next stage? 1.5k would drive many tone stacks nicely, such as a BMP. Or is there more impedance that the next circuit sees at it's input?

This is the basic circuit I've been looking at using for a pedal I'm building my buddy. It's just an A/B in for his electric and upright (piezo). I assume the piezo input should have it's bias resistor upped to about 2.2M. I'm going to be running a James style stack after it, and another buffer to the output. Basically running 2 preamps and muting or disconnecting the inputs.

[amptramp]

If I were doing this design, I would have a low-value capacitor going from the op amp output to the inverting input pin.  The frequency should be selected so that the phase shift at high frequencies does not become sufficient to cause high-frequency oscillation.  The rising hump at high frequencies should be minimized that way.  If you have stray capacitance to ground at the inverting input, high frequency response will sometimes start to increase with frequency, which can cause a phase shift in a direction towards oscillation.  The feedback lag capacitor should be selected to eliminate it.  It might not necessarily oscillate, but you can get transient response anomalies that cause difficult-to-diagnose problems.

[PRR]

> The rising hump at high frequencies

That's in the pickup, not the amp. (TL072 at gain=10 would hump up around 600KHz, and hardly enough to measure.) It is due to coil and cable reactances resonating.

And that hump is DESIGNED-IN by the pickup coil designer, and selected-to-taste by the guitar-builder or guitar-modifier.

As Gus points out (elsewhere), a 250K volume pot turned-down just 50K pretty much kills the resonance. This is one reason the axe sounds different at "10" than at "7".

Alternatively you can put a hi-Z amp right AT the pickup (zero added capacitance), take a maximum frequency-range signal, and twiddle to taste downstream.

Or you can use a fairly low-Z load, drooping treble, and boost-back.

However the present/customary system has worked OK for guitar-players.

[Projectile]

If I were doing this design, I would have a low-value capacitor going from the op amp output to the inverting input pin.  The frequency should be selected so that the phase shift at high frequencies does not become sufficient to cause high-frequency oscillation.  The rising hump at high frequencies should be minimized that way.  If you have stray capacitance to ground at the inverting input, high frequency response will sometimes start to increase with frequency, which can cause a phase shift in a direction towards oscillation.  The feedback lag capacitor should be selected to eliminate it.  It might not necessarily oscillate, but you can get transient response anomalies that cause difficult-to-diagnose problems.

I don't think this is necessary with modern opamps, since they are usually internally compensated.

[amptramp]

I don't think this is necessary with modern opamps, since they are usually internally compensated.

This is a completely separate issue from compensation and it applies to any op amp whether internally or externally compensated.  If you have enough stray capacitance to ground from the inverting input, you will get a characteristic where gain rises with frequency.  If you let it intersect the naturally falling gain response of the op amp, you will get a large phase shift.  Even if the gain goes below one when the phase shift hits 180° (gain above one at 180° phase shift guarantees oscllation), you can still get transient response problems and there may be additional phase shift at certain output voltages - I have seen many amplifiers that were nominally or marginally stable at low outputs but could break into oscillation during a certain part of a waveform.

Adding a feedback lead capacitor usually tames this response.  If you look at guitar power amplifiers, you sometimes find a feedback loop from the speaker output to the input configured as a resistor with a capacitor across it.  This provides a dominant pole in the response so that the phase shift will not be excessive anywhere.

[Gus]

amptramp
The min gain of the circuit is set at X2 and I do have note in the sim unity gain stable opamp and I do a show a 1.5K series resistor in the output.

I understand you point.   I have had an issue with a power EF circuit I built once (not effect related)  Even EFs can have stability issues.   FWIW the 1K in the collector leg of a wha circuit is there for stability(this is in the wha patent IIRC). 

[Projectile]

I don't think this is necessary with modern opamps, since they are usually internally compensated.

This is a completely separate issue from compensation and it applies to any op amp whether internally or externally compensated.  If you have enough stray capacitance to ground from the inverting input, you will get a characteristic where gain rises with frequency.  If you let it intersect the naturally falling gain response of the op amp, you will get a large phase shift.  Even if the gain goes below one when the phase shift hits 180° (gain above one at 180° phase shift guarantees oscllation), you can still get transient response problems and there may be additional phase shift at certain output voltages - I have seen many amplifiers that were nominally or marginally stable at low outputs but could break into oscillation during a certain part of a waveform.

Adding a feedback lead capacitor usually tames this response.  If you look at guitar power amplifiers, you sometimes find a feedback loop from the speaker output to the input configured as a resistor with a capacitor across it.  This provides a dominant pole in the response so that the phase shift will not be excessive anywhere.

But is this really going to be an issue with this circuit in typical usage? With the current parts values it seems like you would need at least 1000pf of stray capacitance at the inverting input to even start to notice an effect in phase shift from a 20khz audio signal. I realize that signals higher than the audio spectrum present in the signal are an issue, but were still talking about a hell of a lot of stray capacitance before you would start to see problems. Maybe I'm not understanding something.