In this post I will discuss the Programmable Gain Amplifier based on the Operational Amplifier. In the previous posts I have discussed about the applications of the Operational Amplifier, ideal attributes of the Operational Amplifier, advantages of using the Operational Amplifier and a number of commonly used configurations of the Operational Amplifier. As mentioned previously that the Operational Amplifier can be configured to perform a large number of operations and the operation that the Operational Amplifier performs depends upon the feedback configuration of the Operational Amplifier. The feedback configuration can be a simple voltage divider network or parallel network of resistors, capacitors or inductors. In this post I will discuss the implementation of the Operational Amplifier as the Programmable Amplifier which utilizes the unique feedback network.
After reading this post you will learn about the basics of the Operational Amplifier, operational amplifier based programmable gain amplifier, applications of the Programmable gain Amplifier. So sit back, keep reading and enjoy learning.
Programmable Gain Amplifier:
Before diving deep into the discussion about the Operational Amplifier based Programmable Gain Amplifier let us first learn what basically is the Programmable Gain Amplifier. As the name implies the Programmable Gain amplifier is the type of amplifier in which the gain factor can be altered according to the requirement using the variable values of the resistors. It is important to note here that the word “Programmable” in the name refers to the fact that this amplifier is Hardware programmable and it implies that the gain of the amplifier can be altered using the range of resistors that can be connected in the feedback network of the Programmable Gain Amplifier. The programmable gain amplifier consists of the analog switch in the feedback network which consists of the resistors of different values and comprises the selection pins through which one of the resistors can be selected to achieve the required gain. The complete circuit and the network description will be discussed in the next section.
Operational Amplifier PGA (Programmable Gain Amplifier):
As mentioned in the above discussion that the Operational Amplifier can be configured in the number of ways to achieve the required operation. One of the configurations is the Programmable gain amplifier. The Programmable Gain Amplifier based on the Operational Amplifier as you will see is basically the non-inverting amplifier with hardware programmable feedback network. The feedback network of the Programmable is just like that of the Inverting Operational Amplifier configuration but with range of resistors to be selected. The feedback network is simply the voltage divider network which feeds the portion of the output signal to the Non-Inverting input terminal. By selecting different resistors in the feedback voltage divider network we basically adjust the amount of the output signal that is to be fed to the Non-Inverting Input terminal and in this way the gain of the Programmable Gain Amplifier is adjusted according to the requirement. The circuit for the Operational Amplifier based Programmable Gain Amplifier is shown in the following figure:
non inverting op amp gain:
As you can see in the figure above that the feedback network has an analog switch with eight switches inside and the 3 bit address bus which determines which of the resistors is to be connected to the in the feedback network. You must have noticed that with eight independent switches in the analog switch and three-bit address line we can have eight different gain factors for the Programmable gain amplifier. The gain factor of the Programmable Gain Amplifier can be understood by the image shown in the following image:
As can be seen in the above image that the formula for calculating the gain is identical to that of the Non-Inverting Operational Amplifier as the Programmable Gain Amplifier is basically the Non-inverting Operational Amplifier with flexibility of choosing the resistor in the feedback network. With different resistances of the resistors in the denominator of the above equation gives different gains accordingly. It must be noted here that the Programmable Gain Amplifier is always used in the Inverting configuration and not the other way round.
You must have figured out by the above discussion that the demonstration of the Programmable Gain Amplifier can be easily made by employing the variable resistor (potentiometer) in the feedback network of the Non-Inverting Operational Amplifier. But this type of circuit is less useful as the gain is to be adjusted manually. The circuit for the Programmable Gain Amplifier is shown in the following figure:
Applications of the Programmable Gain Amplifier:
The Operational Amplifier based Programmable Gain Amplifier finds applications in a number of different areas, for example in some circuits the signal strength of the input signal is sometimes very low and sometimes very high. So in order to maintain the balance in the Output signal so that with weak signal does not get enough amplification to be unable perform the task in hand or the strong signal amplified to such an extent that the amplifier gets saturated itself the Programmable Gain Amplifier is used. The circuit prior to the Programmable Gain Amplifier automatically detects the strength of the input signal and thus calculate which of the resistors needs to be connected in the feedback network to achieve desirable gain factor.
That is all for now I hope this post would be helpful for you. In the next posts I will come up with more interesting applications of the Operational Amplifier. Till then stay connected, keep reading and enjoy learning.