*Op Amp Summing Amplifier*
CIRCUIT
OPSUM.CIR
Download the
SPICE file
The summing amplifier is a handy circuit enabling you to add several
signals together. What are some examples? If you're measuring temperature,
you can add a negative offset to make the display read "0" at the freezing
point. On a precision amplifier, you may need to add a small voltage to
cancel the offset error of the op amp itself. An audio mixer is another good
example of adding waveforms (sounds) from different channels (vocals, instruments)
together before sending the combined signal to a recorder.
Although, there are many ways to make a summer, this one is nice because
it keeps the interaction between inputs at a minimum. What does that mean
for you the designer? You can change the gain or add another input without
messing with the gains of the other inputs. Just remember that the circuit
also __inverts__ the input signals. Not a big deal. If you need the
opposite polarity, put an inverting
stage before or after the summer.
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; I2 = V2 / R2; I3 = V3 / R3
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 = -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
CIRCUIT ANALYSIS
Set R1=10k. (Initially, remove R2, R3 and V_2 and V_3
from the circuit by placing * in front the of their statements.) Only R1 and
V1 should be in the circuit. Then, run a simulation of OPSUM.CIR. Plot the output V(11) to see the output due to V1 only. The
sine wave should swing positive and negative. Now, add R2
and V2 to the circuit by
removing the * and rerun the simulation. Has the sine been shifted all positive as expected?
HANDS-ON DESIGN
Change V1's amplitude by a factor of 2x. How do you need to
change R1 and R2 to achieve the 0 to 2V input?
SIGNAL MIXER
In this circuit, there's three waveform - sine, square, and triangle- you
can add any way you like. (Initially, remove R2, R3 and V2, V_2 and V_3 from
the circuit by placing * in front the of their statements.). Initially, only R1
and V1 are in the circuit. But, you can add the other resistors by removing the
* at the beginning of the R statement.
CIRCUIT ANALYSIS
Run a simulation of OPSUM.CIR. Plot the output V(11)
to see the output due to V1 only. What is the gain of
V1 input? Because
R1=RF=10k, the gain is - 10k / 10k = -1V/V. Now, add R2 and V_2 to the circuit
by removing the * and rerun the simulation. Can you see the single cycle of
the square wave added to the output V(11)? Add in R3 and V_3 to sum the triangle
wave with the others. To look at the input sources themselves, add traces V(1) sine wave,
V(2) square wave and V(3) triangle wave.
HANDS-ON DESIGN
Suppose you need to add different amounts of each input. For
example, to increase the square wave level by a factor of 10, decrease R2 to 1k making the gain
GAIN_V2 = - RF/R2 = -10k/1k = -10. Check out your new output. Likewise,
increase or decrease R1-R3 to
see their effect on the output
waveform.
CREATE WAVEFORMS WITH THE FOURIER SERIES
With the summing amplifier, you can add a number of sine waves together to
generate various waveforms.
SPICE FILE
Download the file
or copy this netlist into a text file with the *.cir
extention.
OPSUM.CIR - OPAMP SUMMING AMPLIFIER
*
* SINEWAVE 4KHZ
V1 1 0 SIN(0V 1VPEAK 4KHZ)
* DC OFFSET
V2 2 0 DC -5V
* SQUARE WAVE 100HZ
*V_2 2 0 PWL(0MS 1V 5MS 1V 5.01MS -1V 10MS -1V)
* TRIANGLE WAVE 500HZ
*V_3 3 0 PWL(0MS -1V 1MS 1V 2MS -1V 3MS 1V 4MS -1V
+ 5MS 1V 6MS -1V 7MS 1V 8MS -1V 9MS 1V 10MS -1V)
*
* INPUT Rs
R1 1 10 10K
*R2 2 10 10K
*R3 3 10 10K
* FEEDBACK R
RF 11 10 10K
*
* OPAMP
XOP 0 10 11 OPAMP1
*
*
* OPAMP MACRO MODEL, SINGLE-POLE
* connections: non-inverting input
* | inverting input
* | | output
* | | |
.SUBCKT OPAMP1 1 2 6
* INPUT IMPEDANCE
RIN 1 2 10MEG
* GAIN BW PRODUCT = 10MHZ
* DC GAIN (100K) AND POLE 1 (100HZ)
EGAIN 3 0 1 2 100K
RP1 3 4 1K
CP1 4 0 1.5915UF
* OUTPUT BUFFER AND RESISTANCE
EBUFFER 5 0 4 0 1
ROUT 5 6 10
.ENDS
*
* ANALYSIS
.TRAN 0.05MS 10MS
* VIEW RESULTS
.PLOT TRAN V(11)
.PRINT TRAN V(11)
.PROBE
.END
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