Feedback/suggestions for OP AMP circuit - Page 1
Forum section: electronics / projects

author: msat
author_rank: Regular Contributor
timestamp: June 01, 2022, 10:22:29 pm
content: So I'm trying to design a simple single cell LiIon-powered guitar preamp circuit that feeds an ADC as follows. Note: None of the component values have been determined yet other than R1 & R2.The feedback network on U1 is selected to provide an input impedance of 1M ohm and 6dB of attenuation. The reason for the attenuation is that a guitar with active electronics might output a signal voltage that is higher than the Op Amp's rail voltage.First question: Does such an attenuation circuit work even though the amp rail voltage may be lower than the signal voltage? It *seems* to me that it would assuming the amp has sufficiently high slew rate.(this reminds me that I forgot to add input protection in the circuit)C2 is there to create a low-pass filter. Is it's placement between R1 & R2 acceptable? I haven't come across any schematics of a low pass filter in this configuration. How is this calculated?Since there's no split power rail, the non-inverting input on U1 is biased mid way between VDD and gnd.U2 provides the variable gain determined by the setting of potentiometer P1.Since U2 is DC coupled to U1, U2 will be biased to whatever voltage U1 is, correct?R5,6,7 & C4,5,7 are all part of a low-pass filter. I don't *think* aliasing will be an issue but I figured I might as well just incorporate a multi-order filter anyway.Should there be a resistor between C7 & C8?Thanks!

author: mawyatt
author_rank: Super Contributor
timestamp: June 02, 2022, 12:20:02 am
content: The junction between R1 and R2 is a virtual ground and C2 should not be there.Best,

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: June 02, 2022, 01:00:03 am
content: Johnson noise of high value resistors may make R1 and R2 a poor choice. You can work it out, probably for a guitar amp it might not matter but those are unusually high resistor values to use.R2 should have a capacitor in parallel, this capacitor rolls off the gain to prevent the op-amp from oscillating at high frequency. You should simulate the AC response in LTSpice and make sure you roll the gain off as early as possible (just above the audio range), there are also proper ways of analysing op-amp stability. That will make the second stage sallen-key filter unnecessary.Also.. R3,R4 can be much higher values. Will reduce battery usage.If you need to protect the op-amp input from overvoltage then there are two different ways. You can either diode-clamp the inputs to Vss/Vdd, or you can use an op-amp with integrated overvoltage protection.https://www.analog.com/en/analog-dialogue/articles/op-amp-input-overvoltage-protection.htmlBear in mind that in either case, feeding the op-amp with too big an input will distort the output. But the op-amp will survive.

author: msat
author_rank: Regular Contributor
timestamp: June 02, 2022, 02:13:47 am
content: Quote from: mawyatt on June 02, 2022, 12:20:02 amThanks. I'm having a hard time grasping the implication of having C2 where it is. Would it be appropriate to split R1 and place the cap between them?Quote from: Geoff-AU on June 02, 2022, 01:00:03 amI read that as a rule of thumb, you generally want to avoid the feedback resistor being more than 1M. It's not that I particularly want to use such high value resistors, but I tried to keep with the other rule of thumb which is that a guitar pre-amp should have an input impedance of 1M.My original idea was to have the input going into a voltage divider and then feeding the noninverting input of a unity gain amp. I might not have a high value resistor in the feedback network, but they'll still need to be there in the bias and divider circuit. Is that any better?QuoteI figured that since the selection of R1 & R2 makes it an attenuator, undesired frequencies will be equally attenuated, whereas if there was a cap in parallel with R2, those frequencies would then be unity gain.QuoteCorrect. None of the component values have been selected other than R1 and R2. all the other values were just the default when placing the components.QuoteI had been doing some reading on input protection, and I intend on adding it. It's just that I forgot when drawing that schematic which I created merely as a reference and for this post.Thanks for the replies

author: TimFox
author_rank: Super Contributor
timestamp: June 02, 2022, 02:57:25 am
content: Splitting R1 into two equal sections and having the capacitor from the mid-point to ground would work as a low-pass filter.The "virtual ground" at the inverting input tries to short out the capacitor, but the op amp will not be happy with the capacitor there.Why do you need such a high input impedance?10 uF and 250 k (from splitting the input into two 500 k) is an absurdly low frequency: 0.064 Hz.What low-pass filter frequency do you need in the first stage?

author: msat
author_rank: Regular Contributor
timestamp: June 02, 2022, 03:53:15 am
content: Quote from: TimFox on June 02, 2022, 02:57:25 amThanks for elaborating on the issue with having C2 where it was. Honestly, I still don't really understand, but I'll take your word for it!Guitar preamps are generally recommended to have a 1M input impedance. I know they can actually be lower, but I figured I'd stick with conventional wisdom.As I mentioned in my OP, I didn't specify any component values other than R1 and R2. All other component values are incorrect and should be ignored. That said, I'm thinking of having my low-pass corner frequency at something like 15kHz.So I'm wondering if the following 1st stage design would be better. This is sort of what I originally had in mind:If I'm not mistaking, assuming an Op Amp with a very high input impedance, the impedance at the node between R2 & R3 is their values in parallel, so 500k. The impedance seen at the input for the passband frequencies is R1 + the parallel values of R2 & R3, so 1M.Also, the combination of resistors sets up a voltage divider of 1/2 seen at the non-inverting input of the amp, is this correct?Thanks!Edit: Having C2 in that location is acceptable for a non-inverting amp, correct? Looking at other schematics online, this appears to be common.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2022, 06:21:00 am
content: The non inverting stage is the better choice, especially for a high input impedance. However it does not really help if a divider is needed at the input.The capacitor at the input is OK than. Overall the circuit has a lot of low pass filtering with a rather low frequency. 1 K * 10 µF gives a crossover of only some 15 Hz - so below the normal audio range. With the large 500 K resistor at the input chances are the cross over would be even lower.So one may have to rethink some capacitor values (e.g. more like 10 nF instead of 10 µF) and maybe also make the filter stage a little higher resistance.Most ADCs are DC coupled, so the capacitor at the output is likely a mistake.

author: msat
author_rank: Regular Contributor
timestamp: June 02, 2022, 06:14:43 pm
content: Quote from: Kleinstein on June 02, 2022, 06:21:00 amThanks for the info. I don't know if I would have caught it, but you're right, the ADC does appear to be DC coupled.