how to bias a non-inverting op amp - tutorial 3 | applied electronics engineering

# how to bias a non-inverting op amp - tutorial 3

By Applied Electronics - Monday, April 3, 2017 No Comments
In this tutorial we show how to bias a non-inverting op amp. In the last tutorial we showed you how to bias an inverting op amp. See Biasing op amp for output voltage range using straight line method - tutorial 2 for this.

There are couples of ways to bias a non-inverting op-amp(also couple of ways to bias inverting op-amp). Here we show how to bias non-inverting amplifier using voltage divider biasing technique. In the next tutorial we show how to bias a non-inverting op amp using split resistor technique.

While biasing an op-amp with single supply, we need create a DC offset and apply to the input terminal of the op amp. The DC bias or offset voltage must be such that it is the midpoint for the output voltage range. For example regardless of input voltage range, your ADC might only read values from 0.5V to 2.5V. Then the mid point value would be 1.5V. This 1.5V must be fed as DC bias or DC offset to the input terminal of the op-amp.

As said in earlier tutorial, given us the knowledge of the input voltage range and the output voltage range, we can use straight line method to determine the Gain and DC offset. See the tutorial general method to bias op amp - tutorial 1.In terms of straight line equation m, the slope is the Gain and b the intercept is the DC offset.

So for example you have input voltage from -0.2V to 0.2V and output voltage range from 0.5V to 2.5V. Then we obtain a pair of straight line equation as follows.

0.5 = -0.2m + b
or, 0.2m - b = 0. 5   -----------(1)
and,
2.5 = 0.2m +  b
or, 0.2m + b = 2.5   --------------(2)

We can solve these equation to obtain Gain(or slope m) and the DC offset(or the intercept b). We can use program like python(see Download Programming for Computations) to do this. Below is Python program code that solve the simultaneous linear equations.

import numpy as np

a = np.array([[0.2,-1],[0.2,1]])
b = np.array([-0.5,2.5])

soln = np.linalg.solve(a,b)

G = soln[0]
Vbias = soln[1]

This program gives us,

G = 5.0

Vbias = 1.5

Once we have obtained Gain and DC bias value we can determine the resistor value for required gain and DC bias.

### Circuit Diagram to bias a non-inverting op amp

The circuit diagram to bias a non-inverting op-amp is shown below. This is ac coupled non-inverting op amp. The connection at the inverting input is same as in ordinary non-inverting amplifier. This is formed by the resistors R1 and R2 in the figure. The capacitor 2 is decoupling capacitor to reduce noise. At the non-inverting op-amp however, we must apply DC bias source. The DC bias source is formed by the resistors R4 and R5. The resistor R3 is just a parallel resistor to the R5. The main point is that the voltage at the non-inverting terminal is 1.5V, the DC bias voltage. C3 is a decoupling capacitor to reduce noise.

The capacitors C1 is coupling capacitor to block DC from the input source and the capacitor C4 is decoupling/coupling capacitor to block the DC from the the amplifier stage.

### How to determine resistor value to bias a non-inverting op amp

Once you have obtained the Gain and DC offset values you can determine the resistors value in the above circuit.

The Gain is used to calculate the resistors R1 and R2. Here the formula is,

G = 1 + R1/R2

Setting R1 to 1KOhm and that G = 5 we obtain R2 as 250Ohm.

Similarly the DC offset(Vbias = 1.5V) is used to determine the resistors R4 and R5. The resistor R3 is obtained from the resistors R4 and R5 in parallel.

Let's set resistor R4 to 1KOhm. Then R5 is obtained from voltage divider rule.

Vbias = Vs*(R5/R4+R5)

where Vs is 3.3V, the supply voltage.

Solving the above equation we obtain, R5=833Ohm.

The value of resistor R3 is obtained as follows,

R3 = R4*R5/R4+R5

which gives, R3 = 454Ohm

Hence the circuit is solved. Next see the graph of the input signal and output signal.

In this way you have learned how to bias a non-inverting op amp. In the next tutorial we show another way to bias a non-inverting op-amp, the split resistor technique.