# Summing amplifier help

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#### bungouk

##### Newbie level 3
Could somebody help me with this one, i don't understand it. I have tried to build the circuit in multisim 11 but i'm not getting anywhere.
so if someone could take a look at the attached zip file and perhaps adjust it so at least the first part of the question works and explain what needed to be done.
I have built well i think most of the circuit but not finished as i got stuck

a – Build and test a summing amplifier with four input voltages V1, V2 , V3 & V4 using multisim . The amplifier should have a voltage gain of 1 , 2, 3 & 0.5 for V1, V2,V3 & V4 inputs

1. Set V1, V2, V3 & V4 to the following voltage levels 1, 2, 0.5 & 3. Monitor Vout with the Multimeter and verify that the voltage gain levels are set correctly.
2. Set V1 to 1KHZ at 10mVPP, V2 to 2KHZ at 20mVPP , V3 to 15 mVPP and V4 to 2V D.C. Monitor Vout with the scope and verify that the gain levels are set correctly. Explain your findings.
3. Modify your summing amplifier to convert the binary number 1101 to an equivalent analogue voltage.

#### Attachments

• summing amp 4-input.zip
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• suming amp.jpg
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Last edited:

##### Super Moderator
Staff member
1.

Your input resistors need to be in the correct ratio to the feedback resistor. In your schematic only #2 is correct.

You need to change the values of #1,3,4 so that they yield the specified gain.

Can't hand you the solution, but below shows a rough idea how you'll test it with the square wave.

2.

Check that your supply polarities are correct for the pins they're hooked up to.

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Edited to upload new schematic with the input waveform corrected to 20 mV P-to-P (10mV nominal).

bungouk

### bungouk

Points: 2

#### bungouk

##### Newbie level 3
Thanks BradtheRad, the thing im sturggling with now is find the right resistor combination for getting a gain of 0.5 as well as getting it right for the other gains of 1,2 and 3

##### Super Moderator
Staff member
To obtain a gain less than one, change your input resistor so it is higher ohms than the feedback resistor.

#### FvM

##### Super Moderator
Staff member
I hope you already found out that your resistor combination gives gain of -2 for all inputs. I'm not sure if the exercise problem intends negative or positive gain. If negative gain (inverting amplifier) is O.K., you only need to adjust the resistor values. If you mislayed your circuit design text book, that surely has a formula for inverting amplifier gain, you can get the right result by applying rule of three.

#### LvW

Could somebody help me with this one,
a – Build and test a summing amplifier with four input voltages V1, V2 , V3 & V4 using multisim . The amplifier should have a voltage gain of 1 , 2, 3 & 0.5 for V1, V2,V3 & V4 inputs
As said by FvM already (implicitely), you only have to apply the classic inverter formulas - for each voltage source separately. However, this leads to NEGATIVE gain values (opposite to your task description).
This can be done as long as the opamp can be considered as ideal (virtual ground principle). In this case, the interaction of the input resistors can be neglected - however, the loop gain is decreased considerably.

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