Integrators

Integrator

Op-amps can be used as integrators, where the output of the op-amp is the integral of the input voltage, the values of the resistor, and the capacitor. The following example shows the op-amp integrator consisting of R and C connected in an inverting configuration. The V_out to V_in transfer function is also described. The inverting integrator provides an output voltage that is proportional to the time-integral of the input.

By placing a capacitor in the feedback path and a resistor at the inverting input, we obtain the inverting integrator.

If the initial voltage on C (at t=0) is denoted V_initial , then the circuit provides an output voltage that is proportional to the time-integral of the input. RC is the integrator time constant. Where V_in and V_out are functions of time, V_initial is the output voltage of the integrator at time t = 0

Transfer Function

(1)

Vout=∫t0−VinRCdt+Vinitial

Frequency Domain

In the frequency domain, the operation of the op-amp integrator can be described alternatively. The bode plot for the integrator magnitude response can be obtained from |V_out/V_in | = 1/(ωRC). It is a straight line of slope -20dB/decade. This line intercepts the 0 dB line at the frequency that makes | V_out/V_in | = 1, which is the integrator frequency ω = 1/(RC) and is simply the inverse of the integrator time constant.

Example

Design an inverting integrator with an input resistance of 10 kΩ and an integration time constant of 1 ms.

What is the value of the capacitor? What is the gain magnitude and phase angle of this circuit at 10 Hz and at 100 Hz? What is the frequency at which the gain magnitude is unity? In order to limit the DC signal gain of the integrator circuit to 100, a resistor RF is added in parallel with the capacitor. Find the required resistor value.

Answer

Given the transfer function of the integrator: V_o /V_i =-1/(jωRC), ω=2 π f, and the magnitude |V_out/V_in |=1/(ωRC) and phase φ=90°

Integration time constant RC = 10-3s, R = 10kΩ
C = 1m /10 k = 0.1 μf
At f = 10 Hz, |V_out/V_in| = 1/(2 π RC) ≈ 16, φ = +90°
At f = 100 Hz, |V_out/V_in | = 1/(2 π fRC) ≈ 1.6, φ = +90°

Let |V_out/V_in| = 1/(2 π f R C) = 1, we get f ≈ 160 Hz

For DC signal, |V_out/V_in| = R1/R =100, R = 10 kΩ, R1 = 1 MΩ