This article shows how the inverting, noninverting, and differential configurations are useful in such applications as summing, scaling, and averaging amplifiers.
Analog Inverter and Scale Changer
Analog Inverter and Scale Changer
The circuit of analog inverter is shown in Figure 1. It is same as inverting voltage amplifier.
Figure 1
Assuming OP-AMP to be an ideal one, the differential input voltage is zero.
i.e. Vd = 0
Therefore, V1 = V2 = 0
Since input impedance is very high, therefore, input current is zero. OP-AMP do not sink any current.
Therefore; iin = if
Vin / R = - Vo / Rf
Vo = - (Rf / R) Vin
If R = Rf then Vo = - Vin, the circuit behaves like an inverter.
If Rf / R = K (a constant) then the circuit is called inverting amplifier or scale changer voltages.
Inverting Summer
The configuration is shown in Figure 2. With three input voltages Va, Vb & Vc. Depending upon the value of Rf and the input resistors Ra, Rb, Rc the circuit can be used as a summing amplifier, scaling amplifier, or averaging amplifier.
Figure 2
Again, for an ideal OP-AMP, V1 = V2. The current drawn by OP-AMP is zero. Thus, applying KCL at V2 node
This means that the output voltage is equal to the negative sum of all the inputs times the gain of the circuit Rf / R; hence the circuit is called a summing amplifier. When Rf = R then the output voltage is equal to the negative sum of all inputs.
Vo = -(Va + Vb + Vc)
If each input voltage is amplified by a different factor in other words weighted differently at the output, the circuit is called then scaling amplifier.
The circuit can be used as an averaging circuit, in which the output voltage is equal to the average of all the input voltages.
In this case, Ra = Rb = Rc = R and Rf / R = 1 / n, where n is the number of inputs. Here; Rf / R = 1 / 3.
Vo = -(Va + Vb + Vc) / 3
In all these applications input could be either ac or dc.
Non-Inverting Configuration
If the input voltages are connected to non-inverting input through resistors, then the circuit can be used as a summing or averaging amplifier through proper selection of R1, R2, R3 and Rf as shown in Figure 3.
Figure 3
To find the output voltage expression, V1 is required. Applying superposition theorem, the voltage V1 at the non-inverting terminal is given by
Hence the output voltage is
This shows that the output is equal to the average of all input voltages times the gain of the circuit (1 + Rf / R1), hence the name averaging amplifier.
If (1 + Rf / R1) is made equal to 3 then the output voltage becomes sum of all three input voltages.
Vo = Va + Vb + Vc
Hence, the circuit is called summing amplifier.
Differential Amplifier
The basic differential amplifier is shown in Figure 4.
Since there are two inputs superposition theorem can be used to find the output voltage. When Vb = 0, then the circuit becomes inverting amplifier, hence the output due to Va only is
Vo(a) = -(Rf / R1) Va
Similarly when, Va = 0, the configuration is a inverting amplifier having a voltage divided network at the non-inverting input
The basic differential amplifier is shown in Figure 4.
Figure 4
Since there are two inputs superposition theorem can be used to find the output voltage. When Vb = 0, then the circuit becomes inverting amplifier, hence the output due to Va only is
Vo(a) = -(Rf / R1) Va
Similarly when, Va = 0, the configuration is a inverting amplifier having a voltage divided network at the non-inverting input
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