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Op-Amp Bias Calculator: Inverting Amplifiers

For this calculator, Gain is always given as a positive value, even though it’s an inverting topology, and has a negative value. It could be done either way, but this seems more consistent with the other topologies. So if your amplifier has a gain of -10, use the magnitude of the gain, just 10. Also, if your results return a negative value for R4, try using a negative value for your Vsupply. If your circuit does not have a negative voltage supply, then the bias voltage you want is not attainable.

AC Inverting Amplifier

For this amplifier, you will need to know what Gain (G) you want, and the lowest frequency of interest (Fc = Cutoff Frequency). For most audio applications, Fc = 20Hz, the lower threshold of human hearing. You will also need to know the Bias Voltage (Vbias). This the voltage you want the output of the amplifier to rest at, when there is no signal applied. For most single supply applications, this is 1/2 Vsupply, for split supply it’s 0V. R1 and R2 set the gain, and R1 controls the input impedance of the amplifier. R3 and R4 set Vbias. If you’re not certain what to make these values, start with 1k or 10k, and see what the other values become. Smaller resistors have less noise, but draw more power. Your resistor values should lie somewhere between 0.1Kohms and 1,000Kohms. Values outside of this range might cause you problems.

AC Inverting Amplifier

 G (Gain) (Positive value only) R1 (Kohms) R3 (Kohms) Vbias (Volts) Vsupply (Volts) Fc (Hz) R2 (Kohms) R4 (Kohms) C (uF)

DC Inverting Amplifier

For this amplifier, you will need to know some of the same parameters as for the AC amplifier. Instead of inserting Vb, you will need to know an input/output voltage pair (Vi & Vo). This is one known operating point where Vo is the result of a particular Vi. For example, if you want the output to be at 5V for an input of 0V, you would have Vi = 0v & Vo = 5V. You also do not need to enter a Fc, as this is a DC amplifier, and has no lower cutoff frequency. The same resistor rules apply.

DC Inverting Amplifier

 G (Gain) (Positive value only) R1 (Kohms) R3 (Kohms) Vi (Volts) Vo (Volts) Vsupply (Volts) R2 (Kohms) R4 (Kohms)