Inverting OP AMP gain calculator

How to use an inverting OP AMP calculator

To use OP AMP calculator enter three values. The blank field will automatically be calculated. If you fill in all four fields, the last calculated value will be recalculated. You can modify the units using the selectors.

How to use an inverting OP AMP (operational amplifier)

An inverting OP AMP (operational amplifier) allows to achieve a higher or lower output voltage than the input voltage. If 'Rf' is larger than 'Ri', the output voltage will be larger. If the opposite happens, the voltage at the output will be lower. An important thing to keep in mind is that the output voltage has a changed sign compared to the input voltage. If the input is positive, the output will be negative and vice versa. The output voltage of the OPAMP in inverting configuration is limited only by the supply voltages. For example, if the OPAMP is powered at 12[V] and your input voltage is 3[V], by adjusting the resistors you can get any value between 0[V] and 12[V]. The limitation that OPAMPs have is that they cannot deliver too much current. The most common is that its maximum rating is 10-30 mA. Therefore, they are useful for powering low consumption circuits. If you need to handle higher powers, you can use linear regulators or switching power supplies (DC/DC converters). Naturally, you can also use alternating current (AC) at the input of the OPAMP. The equation of the inverting OPAMP is the following:

$$V_{out} = -V_{in}\cdot\left(\dfrac{R_f}{R_i}\right)$$

Concept of Gain in OP AMPs

The gain of an operational amplifier indicates how much the OPAMP amplifies the input signal. In the inverting configuration, the gain depends directly on the ratio between Rf and Ri. This gain can be greater or smaller than one (which is not the case for the non-inverting OPAMP). Inverting operational amplifiers produce a 180° phase shift. They are commonly used in active filters, audio mixers, and signal processing circuits.

Example #1

You have an OPAMP in inverting configuration with 5[V] at the input. 'Rin' is 2k[Ω] and 'Rf' has a value of 4k[Ω]. What will be the value of 'Vout'?

• To calculate the output voltage we use the previous equation and substitute values: $$V_{out} = -V_{in}\cdot\left(\dfrac{R_f}{R_i}\right) = $$
  $$= -5\cdot\left(\dfrac{4000}{2000}\right) = -10[V]$$

Example #2

You want to record audio from a microphone on PC. Your sound card accepts voltages between +5[V] and -5[V], but the output signal of your microphone moves between +15[V] and -15[V], so you have to modify the signal or you could damage the sound card. Thus, you decide to use an OPAMP in inverting configuration. You need to scale the microphone signal by a third. What resistor configuration do you use?

• We can fix one of the two resistances to a value that is convenient for us and calculate the other. For example, we will set 'Rin' to 6k[Ω]. Later we isolate'Rf' from the equation and substitute: $$R_f = R_i\cdot\left(\dfrac{-V_{out}}{V_{in}}\right)= $$ $$= 6000\cdot\left(\dfrac{-(-5)}{15}\right)= 2K[Ω]$$
• Note that 'Vout' is inverted due to the inverting setting of the op amp. Along with the '-' in the equation, we end up with a positive value resistor.

Frequently Asked Questions

• Why is the output voltage negative?
  An inverting amplifier inverts the polarity of the input signal by 180 degrees. If the input is positive, the output is negative, and vice versa. Important: The supply voltages must be sufficient (sufficiently negative) to allow the output to reach the desired negative voltage.
• Can the gain be less than 1?
  Yes, if the feedback resistor (Rf) is smaller than the input resistor (Rin), the gain will be less than 1 (attenuation).
• What determines the input impedance?
  The input impedance is determined primarily by the input resistor (Rin), as the inverting terminal is a virtual ground. This is true as long as the op-amp is operating within its linear range (not saturated).
• What are common applications of inverting OP AMPs?
  Inverting OP AMPs are commonly used in signal processing applications such as active filters, audio mixers, and instrumentation amplifiers due to their ability to provide precise gain control. With inverting op-amps, designers can easily implement functions like summing amplifiers and integrators, making them versatile components in analog circuit design.