How to bias Op Amp for output voltage range | applied electronics engineering


How to bias Op Amp for output voltage range

By Applied Electronics - Thursday, March 30, 2017 No Comments
In this tutorial we show how to bias Op Amp for output voltage range. Many times we require specific output voltage swing from operational amplifiers(op-amps). The process of creating such desired output voltage swing range is called Biasing Op-Amps or creating DC offset. In short it means supplying DC voltage to one of the input terminal so that without any input(from the other input) the output voltage is fixed at some voltage.

Suppose we have an inverting amplifier using an op-amp like LM324 op-amp. The negative input is supplied with your signal but let's assume there is no input signal at this pin. The positive input is then biased or DC offset with some DC voltage. Say that you want the output voltage range to be between 0V and 3V so that the midpoint is 1.5V. So in this case, the positive input terminal of the op-amp is biased or DC offset at 1.5V. With no input at negative terminal the output will be at 1.5V. That is 1.5V acts as our origin or center point. Any input at the negative terminal will then oscillate around the 1.5V. This process of designing the DC offset is called Op-amp biasing.

Such biasing is required for example when you want to see both negative and positive excursion of AC signal. Without proper biasing, the AC signal might get clipped.

First see a normal inverting op-amp amplifier circuit shown below.

The input signal VSINE has amplitude of 100mV and frequency of 1KHz. The supply voltage to the LM324 is 3V. The amplifier gain is 10. Now see the output and input waveform.

As you can see the output signal is clipped.

Now lets say we have center the output voltage at 1.5 volt. For this we need to add a voltage divider biasing circuit that creates 1.5V and apply it to the positive terminal of the op-amp. That is shown below.

The input and output waveform in this case is shown below.

As you can see the output signal is dead, the amplification has been lost. To remedy this we connect the same value of resistor as that used in the voltage divider circuit which is 10KOhm and connect it between the +V supply voltage and the input signal at negative terminal as shown below.

How to bias Op Amp for output voltage range

If we now view the input and output waveform we will see that the output signal is restored and that its center is at 1.5V as desired.

Notice that the output signal is still inverted as expected from inverting amplifier.

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