general method to bias op amp - tutorial 1 | applied electronics engineering

# general method to bias op amp - tutorial 1

By Applied Electronics - Sunday, April 2, 2017 No Comments
Op-Amps are useful ingredients for analog circuits. They are used in RF amplifier, gain stages, instrumentation for sensing analog signals etc. Sometimes the input signal from sensors are DC signals and sometimes the signal are AC signals. The sensed signal have to be connected to ADC(Analog to Digital Converters). Before the signals are to fed to the ADC, amplification of the low amplitude signal from sensors has to be done. This requires one to know the range of amplitude that the ADC can handle. That is the op-amp has to be configured so that the output signal level from the op-amps fits the ADC input voltage range. The process of doing that is called biasing op-amp. That is, when there is no input signal into op-amp, the output signal level must be mapped to the voltage level of the ADC.

In one of previous tutorial How to bias Op Amp for output voltage range we showed one way of biasing an op-amp. Here we show a generalized technique to bias an op-amp.

One of the general method one can apply to bias an op amp is to use the straight line mapping method. In this method, from the knowledge of input voltage level and output voltage level, specific op-amp circuit configuration are used and the resistor values in the circuit are calculated from the knowledge of the voltage level.

Consider an inverting op-amp amplifier(Picture from microelectronics by sedra and smith).

The relationship between the input and output voltage for inverting amplifier is,

If we say that vi is x and vo is y then we have a straight line equation.

y = -mx

where m is the slope of the straight line equation equal to the ratio R2/R1 which is the gain of the amplifier. This straight line passes through the origin in Cartesian coordinate system.

If you remember that the general straight is,

you see that an inverting op amp amplifier does not have b or the y-intercept.

Similarly the non-inverting op-amp circuit diagram is shown below.

The relationship of input and output signal is,

This when compared with straight line equation shows that the slope is 1+R2/R1 and it also does not have any y-intercept.

The y-intercept as we will encounter in the next tutorial leads to the DC offset that is imposed on the Gain(slope) of the operational amplifier.

Thus as we can see from the generalized straight line equation and when compared to op amp characteristics we get a set of four equations as follows.

From given input and output voltage specification, we can design four different op-amp circuits. This will be explained in the next tutorial.