Electronics Calculators
These calculators are useful when configuring the headphone amplifiers discussed on this site.
I Know What Gain I Want. What Resistor Values Do I Need?
This calculator will help you pick a pair of gain resistors that give a desired gain value, within a certain percent tolerance. If you ask for a gain of 5 and a 1% tolerance, you will get resistor values that give a gain between 4.95 and 5.05. The looser the tolerance, the more answers you will get, and the slower the calculator will run.
The calculator uses the standard E-96 values for 1% resistors and the E-24 values for 5% resistors.
To keep the number of answers reasonable, there are some limits on the resistor values the calculator will use. It will only use resistor values in the 1 KΩ to 1 MΩ range, and it will only use the higher values in that range when the gain value is high. If you want to use resistor values that are outside the range that the calculator gives you, scale the values in one of the answers by a factor of 10.
The maximum gain you can calculate with this program is just over 1000.
The first resistor value given in each answer is the one that goes from the inverting input to ground. (R3 in most of the amps on this site.) The second value is the one from the op-amp's output to the inverting input. (R4)
I Know What the Resistor Values Are. What's the Gain?
This calculator takes an existing set of resistor values and calculates the gain. It can do this for either a standard noninverting op-amp gain stage (e.g. a CMoy pocket amp) or a Jung multiloop gain stage, as in the schematics at right.
If you only give values for R3 and R4, it uses the simple gain formula: G = R4÷R3 + 1. If you also give the R5 and R6 values, it uses the Jung multiloop formula derived by Tophu:
You may use power notation: 120K, 1.0M, etc. The power suffix can be uppercase or lowercase.
By the way, another interesting derivation (due to SnoopyRocks this thread) is
where Rg is R3 + R5, ƒo is the op-amp's unity-gain bandwidth, and ƒi is the desired inner loop bandwidth. 100 kHz is a useful starting point for ƒi, but I don't think there's any hard-and-fast reason to stick to this value. We've found that this gives too high a value for R6 in the PPA for stability, if R5 is left at 3.32 KΩ. Just use it as a starting point for investigating your own values, not as gospel.
If you are going to keep R3 and R5 at their default values for PPA and PIMETA, and ƒi constant at 100 kHz, the expression can be approximated as R6 = 0.043 * ƒo.
What Will the Output DC Offset Be?
This calculator estimates the output DC offset of a simple noninverting op-amp gain stage, like the one at right.
The pot value is optional. If you leave it out, the calculator assumes you have a cap in series between the pot and R2, so R2 sets the +IN input impedance. Otherwise, this input impedance is calculated as R2 and the pot in parallel over the pot's range; the calculator uses whichever value gives higher offset.
You have to give at least one of the datasheet values, but not all of them. The calculator will happily give you a partial answer if you just want to know the contribution of one source of offset.
You may use power notation. For resistors, the powers are the standard ones: 120K, 1.0M, etc. For capacitors and current, the powers are for fractions of a farad or ampere: 33p, 10n, 0.1u, 12m etc. Case does not matter with the power suffixes. For current and capacitance, this means that 'm' always stands for 'milli', and never 'mega', even if it is given in upper case.
The equation for the input current part of this calculator comes from Design with Operational Amplifiers and Analog Integrated Circuits 3rd edition by Sergio Franco. It is the modified form of equation 5.11 on page 219. Using this calculator's symbols, it is:
where Rpf is value of the feedback resistors R3 and R4 in parallel.
What Will the Noise Be?
This calculator estimates the output noise of a simple noninverting op-amp gain stage like the one in the schematic above.
This calculator assumes a typical audio application: bandwidth is 20 Hz to 20 kHz, and a full-scale signal is 1 Vrms. The calculator only considers Johnson noise from the resistors surrounding the op-amp, and the inherent voltage and current noise specs of the op-amp itself. If you leave out the op-amp datasheet specs, the calculator uses an ideal, noiseless op-amp. You can also leave out R2 and the pot values, if you will not use these parts.
You may use power notation for the resistors: 120K, 1.0M, etc.
What is the Overall Resistance of Parallel Resistors?
You may use power notation: 120K, 1.0M, etc. The power suffix can be uppercase or lowercase.
What is the Reduction Factor of a Resistor Divider?
RI is the one between the divider's input and the output, and Rg is the one from output to ground.
You may use power notation: 120K, 1.0M, etc. The power suffix can be uppercase or lowercase.