Transcript Summing Amplifier

Summing Amplifier
Introduction
• One of the most common applications for an op amp is to algebraically add
two (or more) signals or voltages to form the sum of those signals. Such a
circuit is known as a summing amplifier, or just as a summer.
• The source of these signals might be anything at all. Common input sources
are another op amp, some kind of sensor circuit, or an initial constant value.
Since we don't have the first two available at this time, we'll use the third
source for this experiment.
• The point of using an op amp to add multiple input signals is to avoid
interaction between them, so that any change in one input voltage will not
have any effect on the other input.
• A summing amplifier is an electrical circuit layout that allows for the
addition of more than one signal, creating a sum of all the circuits
combined.
•
The idea behind it is to incorporate multiple sources of input, while
keeping each source separate to avoid one of the input sources affecting
another. The amplifier allows the user to achieve a voltage sum at output
that is calculated at the same rate mathematically as all of the input sources
added up.
• Essentially, this means the amplifier creates an operational amplifier
because when more than one input voltage is entered into a circuit and
added together through the course of the circuit, the output voltage is a
constant of the sum of all of the input voltages.
Cont..,
•
When a circuit is needed to connect two different microphones to one amplification
source and the user wishes to hear both of the microphones through the same
source, the circuit used as the microphones are plugged into the amplifier is a
simple form of a summing amplifier.
•
The signal coming into the amplifier from both microphones is able to be heard
through the same speaker simultaneously due to the summing amplifier being
created.
•
The use of a summing amplifier is required for any audio mixer machine to
function properly because it must be able to receive signals from multiple sources at
the same time and let them all be output through one source after being mixed by
the user as the signals pass through the circuit.
Summing Amplifier Circuit
• The output voltage, (Vout) now becomes proportional to the sum of the
input voltages, V1, V2, V3 etc. Then we can modify the original equation
for the inverting amplifier to take account of these new inputs thus:
• However, if all the input impedances, (Rin) are equal in value the final
equation for the output voltage is given as:
Summing Action
• The summing action of this circuit is easy to understand if you keep in
mind the main "mission" of the op amp. It's a simple one: keep the potential
of the negative terminal very close to the positive terminal.
•
In this case, keep the negative terminal close to 0V (virtual ground). The
op amp essentially nails one leg of R1, R2 and R3 to a 0V potential. This
makes it easy to write the currents in these resistors.
•
I1 = V1 / R1;
•
So what's the current I flowing in RF? According to our friend Kirchoff, we get
•
I = I1 + I2 + I3
•
Finally, notice that one leg of RF is also kept at 0V. So the output becomes Vo = -
I2 = V2 / R2; I3 = V3 / R3
RF x I. Combining these pieces of information, we have a simple description of the
amplifier
•
•
•
Vo = - RF ( V1 / R1 + V2 / R2 + V3 / R3)
= - ( V1 · RF / R1 + V2 · RF / R2 + V3 · RF / R3 )
As you can see, the gain for each input can be controlled by a single resistor: K1 = RF/R1,
K2 = -RF/R2 and K3 = -RF/R2.
Level Shifter
•
Summing amplifiers make convenient level shifters. What if you have an audio
signal V1= +/-1.0V that needs to be shifted to the input range of an ADC which
is 0 to +2V. You also have a -reference voltage available V2=-5V. You can pass
the audio signal through R1 with gain K1=-1. You can also add in a +1.0V DC
offset using the V2 and R2 .
•
•
•
The gains and resistors are calculated as
Signal Gain K1 = (+2V - 0V) / (-1V - (+1V)) = -1
R1 = -RF / K1 = 10k
Offset Gain K2 = +1 / -5V = -0.2
R2 = -RF / K2 = 50k
Example No1
• Find the output voltage of the following Summing Amplifier
circuit.
• Using the previously found formula for the gain of the circuit
• we can now substitute the values of the resistors in the circuit
as follows,
• we know that the output voltage is the sum of the two
amplified input signals and is calculated as:
• Then the output voltage of the Summing Amplifier circuit
above is given as -45 mV and is negative as its an inverting
amplifier.
The End
…..Thank you….