Download Mixing Signals

Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
Learning Objectives:
At the end of this topic you will be able to;
 draw a circuit for a mixer based on an op-amp summing amplifier;
 use the gain formula to calculate the output voltage for a given
summing amplifier circuit.
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GCSE Electronics.
Unit E2 : Applications of Electronics
2.4.3
Mixing Signals
A mixer circuit is an important part of many audio systems. For example DJ’s
use a mixer to ‘voice over’ records. Recording studios use mixers to balance
the sound from different voices and instruments.
Summing Amplifier
The basic building block of a mixer is an inverting amplifier, configured as a
summing amplifier in which two or more inputs are used.
Consider the circuit shown below.
The circuit is called a summing amplifier because it adds the voltages at each
of its inputs together.
You will remember for an inverting amplifier:
VOUT
R
 F
VIN
R IN
or VOUT  
RF
 VIN
R IN
For a summing amplifier, this can be rewritten as:
V V 
VOUT  R F  1  2 
 R1 R 2 
If R1 = R2 = RIN then:
VOUT  
2
RF
V1  V2 
R IN
Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
What is the limitation of this summing amplifier if it is to be used as a mixer?
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A two Channel Mixer
The summing amplifier provides the basis for a mixer circuit in which two or
more input signals can be mixed together and faded in or out independently
of one another.
A two channel mixer circuit is shown below.
 Potentiometers VR1 and VR2 are used at the input of each channel to
independently control the fading in or out of each channel. For example
if you consider Signal 1, when the wiper is at the top of VR1, the whole
of signal 1 passes into the summing amplifier. As the wiper moves
towards 0V, less and less of signal 1 appears at V1, and the signal fades
out progressively until the wiper reaches 0V.
 The remainder of the circuit in the blue box is just a summing amplifier
as described on the previous page.
We will now investigate summing amplifiers and mixer circuits.
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GCSE Electronics.
Unit E2 : Applications of Electronics
Activity 1: Using the operational amplifier as a summing amplifier
Information:
We have seen that for the summing amplifier shown below with two equal
input resistors:
VOUT  
1a.
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RF
V1  V2 
RIN
Set up the following circuit, with input resistors of 1kΩ and a feedback
resistor of 10kΩ. You can obtain the variable voltages V1, and V2 by
using potentiometers at inputs A and B. (10kΩ or 47kΩ pots would be
suitable)
Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
1b.
Alternatively if you have access to ‘Livewire’ or ‘Circuit Wizard’, you
can set up the following simplified circuit.
Choose the ‘Input Voltage’ symbol from the power supply menu, double
click on the symbol to display the properties menu and set the voltage
range to 1V for each input.
1c.
1d.
Set V1 to 0.1V, and V2 to 0.2V. Measure the output voltage and record
its value in the table below. Repeat for the other values of input
voltages.
V1
(Volts)
V2
(Volts)
0.1
0.2
0.2
0.3
0.5
0.1
0.5
0.6
0.8
0.7
0.7
0.9
Use the formula:
Measured Value of Theoretical Value of
VOUT (Volts)
VOUT (Volts)
VOUT  
RF
V1  V2 
RIN
to calculate the theoretical value of VOUT in each case.
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GCSE Electronics.
Unit E2 : Applications of Electronics
1e.
Why do the measured and the theoretical values not agree in the last
two cases?
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1f.
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Draw a circuit diagram for a 3-input summing amplifier, using equal
resistors throughout. Set up and test the summing amplifier to check
that it adds up the 3 input voltages
Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
Activity 2:
2a.
Investigating a mixer circuit
Set up the mixer circuit with 2 AC voltage sources connected to 2
potentiometers as shown below.
Note: If you are using ‘Livewire’ or ‘Circuit Wizard’ you can find the AC
voltage source in the power supply menu.
2b.
Check that the circuit is working by setting V1 to maximum and V2 to
zero as shown above.
The output graph should look like this:
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GCSE Electronics.
Unit E2 : Applications of Electronics
2c.
Now reverse the potentiometer settings by changing V1 to zero and V2
to maximum as shown below.
The output should look like this:
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Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
2d.
Now set both the potentiometers to 100% for each signal, and observe
the output now. It should look something like this.
2e.
Investigate the effect of fading each signal in and out at different
settings. Comment on the performance of the mixer circuit.
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Now for some examination style questions.
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GCSE Electronics.
Unit E2 : Applications of Electronics
Examination Style Questions
1.
A mixer needs to be constructed for a disco system. This can be made using a summing amplifier
circuit.
Complete the diagram below to show how a three input mixer can be constructed, (resistor values
are not required)
[3]
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Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
2.
The mixer in a public address system is based on an inverting voltage amplifier circuit.
The diagram shows the circuit of an inverting amplifier.
(a)
Modify it to make a two input mixer, which has the same fixed voltage gain for both
inputs. Give the values of any resistors used in your circuit.
[1]
(b)
The mixer is tested by applying DC signals to the input.
One input of the mixer has a 0.2V signal applied to it.
The other has a 0.3V signal applied to it.
Use the formula VOUT  
RF
V1  V2  to calculate the output voltage produced.
R IN
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[2]
(c)
What additional modification would have to be made to the mixer to allow input signals to
be faded in or out independently.
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[1]
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GCSE Electronics.
Unit E2 : Applications of Electronics
3.
The circuit diagram shows a summing amplifier, with voltages of 0.2V and 1.1V applied to its inputs.
(a)
Use the formula:
V V 
VOUT  R F  1  2 
 R1 R 2 
to calculate the output voltage produced by the summing amplifier.
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[2]
(b)
The DC inputs are replaced with two AC inputs. What other change would be needed so
that the circuit could be used as a mixer.
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[1]
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Topic 2.4 – Analogue Communications.
2.4.3 Mixing Signals.
Self Evaluation Review
Learning Objectives
My personal review of these objectives:



draw a circuit for a mixer based on
an op-amp summing amplifier.
use the gain formula to calculate the
output voltage for a given summing
amplifier circuit.
Targets:
1.
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2.
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