An inverting amplifier provides the same function as the common emitter and common-source amplifiers, except that here we are using op-amp. Op-amp that is operational amplifier, before we come to the point what is inverting amplifier we must have to know about fundamentals of Operation amplifier, characteristic of operational amplifier, application of operational amplifier.
What is op-amp?
The term operational amplifier (OP-AMP) refers to a class of high-gain DC coupled amplifiers with two inputs and a single output. The modern integrated circuit version is typified by the famous 741 op-amp. Some of the general characteristics of the IC version are:
• High gain, on the order of a million
• High input impedance, low output impedance
• Used with split supply, usually +/- 15V
• Used with feedback, with gain determined by the feedback network.
The operational amplifier (op-amp) was designed to perform mathematical operations like addition, subtraction, integration, differentiation in low voltage and current application. Although now superseded by the digital computer, op-amps are a common feature of modern analog electronics.
The OPAMP is constructed from four transistor stages, which include
1st stage : known as an input stage made up with dual input balanced output differential amplifier.
2nd stage : known as an gain stage made up with dual input unbalanced amplifier.
3rd stage : known as an level shifting stage made up with emitter follower circuit.
4th stage : known as an output stage made up with class b push pull amplifier.
The differential amplifier consists of a matched pair of bipolar transistors or FETs. The push-pull amplifier transmits a large current to the load and hence has a small output impedance.
The op-amp is a linear amplifier with Vout ∝ Vinp. The DC open-loop voltage gain of a typical op-amp is 103 to 106. The gain is so large that most often feedback is used to obtain a specific transfer function and control the stability.
Cheap IC versions of operational amplifiers are readily available, making their use popular in any analog circuit. The cheap models operate from DC to about 20 kHz, while the high-performance models operate up to 50 MHz. A popular device is the 741 op-amp. It is usually available as an IC in an 8-pin dual, in-line package (DIP).
The schematic diagram for an inverting amplifier is shown in Figure (a). Observe that the offset and D.C. voltages have been left off of these circuits for simplicity. These connections are generally the same for all circuits using the same type of OP-AMP
Inverting amplifier
The input signal is applied to the inverting (minus) input. The (-) input produces a 180o phase shift between input and output signal. The non-inverting (plus) input is grounded and is common to both the input and the output.
Negative feedback (degenerative) is coupled from the output back to the input through the feedback resistor (Rf). The ratio of Ri to Rf will determine, the circuits voltage gain voltage gain for this circuit can be calculated using the formula.
Av = Rf / Ri
This formula can be derive as follows:
The input and feedback current are algebraically added as point G. it is also called summing point. Hence it is assumed to be zero or at ground potential the specific term used for this point is virtual ground.
I1 = Vin / R1
And I2 = -Vout / Rf
Note that negative sign is because of negative feedback current.
By using Kirchhoff’s current law
I1 + (-I2) = 0
Because I1 and I2 are combining at point G.
I1 – I2 = 0
(Vin / R1) – (-Vin / Rf) = 0
(Vin / R1 ) + ( Vout / Rf) = 0
Vin / Rf = Vout Rf
Rf / R1 = Vout / Vin
Vout / Vin = Rf / R1
Voltage gain = Vout / Vin = Rf / R1
Av = Rf / R1
Equation shows that closed loop gain of the inverting amplifier depends on the ratio of two external resistors R1 and Rf. As shown in the figure if we change the value of Rf then voltage gain is also changed. If we increase the value of Rf then gain will increase and vice versa. If we increase the value of R1 then gain will be decrease.
What is op-amp?
The term operational amplifier (OP-AMP) refers to a class of high-gain DC coupled amplifiers with two inputs and a single output. The modern integrated circuit version is typified by the famous 741 op-amp. Some of the general characteristics of the IC version are:
• High gain, on the order of a million
• High input impedance, low output impedance
• Used with split supply, usually +/- 15V
• Used with feedback, with gain determined by the feedback network.
The operational amplifier (op-amp) was designed to perform mathematical operations like addition, subtraction, integration, differentiation in low voltage and current application. Although now superseded by the digital computer, op-amps are a common feature of modern analog electronics.
The OPAMP is constructed from four transistor stages, which include
1st stage : known as an input stage made up with dual input balanced output differential amplifier.
2nd stage : known as an gain stage made up with dual input unbalanced amplifier.
3rd stage : known as an level shifting stage made up with emitter follower circuit.
4th stage : known as an output stage made up with class b push pull amplifier.
The differential amplifier consists of a matched pair of bipolar transistors or FETs. The push-pull amplifier transmits a large current to the load and hence has a small output impedance.
The op-amp is a linear amplifier with Vout ∝ Vinp. The DC open-loop voltage gain of a typical op-amp is 103 to 106. The gain is so large that most often feedback is used to obtain a specific transfer function and control the stability.
Cheap IC versions of operational amplifiers are readily available, making their use popular in any analog circuit. The cheap models operate from DC to about 20 kHz, while the high-performance models operate up to 50 MHz. A popular device is the 741 op-amp. It is usually available as an IC in an 8-pin dual, in-line package (DIP).
The schematic diagram for an inverting amplifier is shown in Figure (a). Observe that the offset and D.C. voltages have been left off of these circuits for simplicity. These connections are generally the same for all circuits using the same type of OP-AMP
Inverting amplifier
The input signal is applied to the inverting (minus) input. The (-) input produces a 180o phase shift between input and output signal. The non-inverting (plus) input is grounded and is common to both the input and the output.
Av = Rf / Ri
This formula can be derive as follows:
The input and feedback current are algebraically added as point G. it is also called summing point. Hence it is assumed to be zero or at ground potential the specific term used for this point is virtual ground.
I1 = Vin / R1
And I2 = -Vout / Rf
Note that negative sign is because of negative feedback current.
By using Kirchhoff’s current law
I1 + (-I2) = 0
Because I1 and I2 are combining at point G.
I1 – I2 = 0
(Vin / R1) – (-Vin / Rf) = 0
(Vin / R1 ) + ( Vout / Rf) = 0
Vin / Rf = Vout Rf
Rf / R1 = Vout / Vin
Vout / Vin = Rf / R1
Voltage gain = Vout / Vin = Rf / R1
Av = Rf / R1
Equation shows that closed loop gain of the inverting amplifier depends on the ratio of two external resistors R1 and Rf. As shown in the figure if we change the value of Rf then voltage gain is also changed. If we increase the value of Rf then gain will increase and vice versa. If we increase the value of R1 then gain will be decrease.
As shown in the figure here input is DC source, value of the resistance is set in such a way that gain is 10. Input is 1v so as per theratiacal calculation output must be 10v.
Here we can get the output with value 9.981 volt practically which is nearly equal to theratical value. The minus sign indicates that output is 180 degree in phase with input.
As shown in the figure we can also connect AC input to the inverting amplifier, value of the resistance is set in a such way that gain is 2. We can observed output in CRO- oscilloscope.
Here as shown in the figure input which is in red colour and output which is blue colour are in 180 degree phase shift with each other, output is double than input.
We can also check that if we set the value of R1 is zero then as per equation gain will be infinite.
In multisim we can check this process but maximum output for opamp is +Vsat and –Vsat. So as shown in the figure by observing the slope of the output signal we can say that output will be maximum.
If you want to do this practical on your own way then you can directly download multisim file from here. To download this file click here.