Understanding of Operational Amplifier (1)


Editor : ElectRobot
2016/11/25


 Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.



(Figure 1) Op Amplifier & Chipset


 An Operational Amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. These feedback components determine the resulting function or “operation” of the amplifier and by virtue of the different feedback configurations whether resistive, capacitive or both, the amplifier can perform a variety of different operations, giving rise to its name of “Operational Amplifier”.



(Figure 2) Circuit of Operational Amplifier


 An Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs, one called the Inverting Input, marked with a negative or “minus” sign, and the other one called the Non-inverting Input, marked with a positive or “plus” sign (+).

 The third terminal represents the operational amplifiers output port which can both sink and source either a voltage or a current. In a linear operational amplifier, the output signal is the amplification factor, known as the amplifiers gain (A) multiplied by the value of the input signal and depending on the nature of these input and output signals, there can be four different classifications of operational amplifier gain.


 ㆍ Voltage ? Voltage “in” and Voltage “out”
 ㆍ Current ? Current “in” and Current “out”
 ㆍ Transconductance ? Voltage “in” and Current “out”
 ㆍ Transresistance ? Current “in” and Voltage “out”


1) Voltage Follower
 If we made the feedback resistor, R? equal to zero, (R?=0), and resistor R2 equal to infinity, (R2=∞), then the circuit would have a fixed gain of “1” as all the output voltage would be present on the inverting input terminal (negative feedback). This would then produce a special type of the non-inverting amplifier circuit called a Voltage Follower or also called a “unity gain buffer”.



(Figure 3) Circuit of Voltage Follower


 As the input signal is connected directly to the non-inverting input of the amplifier the output signal is not inverted resulting in the output voltage being equal to the input voltage, Vout=Vin. This then makes the voltage follower circuit ideal as a Unity Gain Buffer circuit because of its isolation properties.

 The advantage of the unity gain voltage follower is that it can be used when impedance matching or circuit isolation is more important than amplification as it maintains the signal voltage. The input impedance of the voltage follower circuit is very high, typically above 1MΩ as it is equal to that of the operational amplifiers input resistance times its gain (RinxAo). Also, its output impedance is very low since an ideal op-amp condition is assumed.


2) Inverting Op Amp
 The basic circuit for the inverting op amp circuit is shown below. It consists of a resistor from the input terminal to the inverting amplifier input of the circuit, and another resistor connected from the output to the inverting input of the op-amp. The non-inverting input is connected to ground.



(Figure 4) Circuit of Inverting Op Amplifier


 In this inverting amplifier circuit the non-inverting input of the operational amplifier is connected to ground. As the gain of the op amp itself is very high and the output from the amplifier is a matter of only a few volts, this means that the difference between the two input terminals is exceedingly small and can be ignored. As the non-inverting input of the operational amplifier is held at ground potential this means that the inverting input must be virtually at earth potential. As a result, this form of amplifier is often known as a virtual earth amplifier.


TOP