Need help on designing Summing amplifier for a triangle and sine wave

[pavjayt]

Hello,

I am trying to build a summing amplifier circuit using OP27 ICs. I need to sum a saw tooth wave form that has an Vpp of +/-10V and a sine wave that has Vpp of 0-1V. I am using supply voltage of +/-12V which is above the input voltage range. I read somewhere that adding a triangular waveform to another waveform is not the same as adding two sine waveforms, is that true?

Could someone help me with some suggestions or point to some literature that discusses this?

thanks in advance.

 

[crutschow]

I am trying to build a summing amplifier circuit using OP27 ICs. I need to sum a saw tooth wave form that has an Vpp of +/-10V and a sine wave that has Vpp of 0-1V. I am using supply voltage of +/-12V which is above the input voltage range. I read somewhere that adding a triangular waveform to another waveform is not the same as adding two sine waveforms, is that true?

Of course adding a triangular wave to a sine wave is not the same as adding two sine waves. Obviously the output waveform will be different. What do you mean by "not the same"?

But a simple summing amp using the OP27 should work for that. And by using different values of summing resistors you can scale the relative amplitude of the two waveforms being summed, if you like.

 

[pavjayt]

Of course adding a triangular wave to a sine wave is not the same as adding two sine waves. Obviously the output waveform will be different. What do you mean by "not the same"?

But a simple summing amp using the OP27 should work for that. And by using different values of summing resistors you can scale the relative amplitude of the two waveforms being summed, if you like.

Thanks for your quick reply. I am little confused on using the summing resistors to adjust relative amplitude. I want the actual values to be summed together, for example if my saw tooth is at 10V and my second waveform is at 1V, I would like to see 11V out. Can it be achieved using a simple summing circuit?

My normal op-amp circuit that I generally use has LM6181 with 820Ohm feedback resistor connected to 1K pot to gnd on inverting input. I connect input with a 50Ohm resistor to gnd and output to 50Ohm resistor with supply voltage at +/-12V.

With the above circuit as base, I removed the 50Ohm resistor to gnd on input side and connected both inputs directly and set my inverting input gnd resistor to 820Ohm so that I can have unity gain. With the circuit that I have put together, the input waveforms are at 150KHz (sawtooth waveform at +/-5Vpp and sine wave at 0-1Vpp), the output waveform comes out to be a triangular wave at 7.4V, the output voltage changes if I reduce the frequency of sawtooth waveform going upto 18V in some cases.

It would be great if someone can draw a basic schematic to start with... I am confused on the input resistor values to start with.

thanks

 

[crutschow]

The LM6181 is not a typical op amp. It operates in a current feedback mode, not a voltage feedback mode. It's advantage is that is generally has a higher frequency response than the usual voltage feedback op amp. But it may not operate properly in some of the typical op amp circuit configurations.

That being said, it should work OK if you use it as an inverting summing amp. (You tried to connect it up as a non-inverting amp which is not a true summer) For this connect the positive (non-inverting) input to ground and leave the 820Ω feed back resistor from the output to the minus (inverting) input. Then add two 820Ω resistors, also to the minus input. Connect the input signals to each of these resistors. That will give you a summing amp with a gain of -1 for each input.

(Note that summing a ±5V triangle wave with 0-1V sine wave of the same frequency will just give a distorted appearing triangle wave, the shape depending upon the phase between the triangle wave and the sine wave.)

You aren't trying to build a modulator are you, where you want the product of the two waveforms, not the sum?

 

[pavjayt]

The LM6181 is not a typical op amp. It operates in a current feedback mode, not a voltage feedback mode. It's advantage is that is generally has a higher frequency response than the usual voltage feedback op amp. But it may not operate properly in some of the typical op amp circuit configurations.

That being said, it should work OK if you use it as an inverting summing amp. (You tried to connect it up as a non-inverting amp which is not a true summer) For this connect the positive (non-inverting) input to ground and leave the 820Ω feed back resistor from the output to the minus (inverting) input. Then add two 820Ω resistors, also to the minus input. Connect the input signals to each of these resistors. That will give you a summing amp with a gain of -1 for each input.

(Note that summing a ±5V triangle wave with 0-1V sine wave of the same frequency will just give a distorted appearing triangle wave, the shape depending upon the phase between the triangle wave and the sine wave.)

You aren't trying to build a modulator are you, where you want the product of the two waveforms, not the sum?

Thanks for your reply. I am trying to build a pure summing amplifier (10+1 = 11), not even an averaging amplifier, in which I read that we need to use equal resistances connected to input signals.

I guess if I use inverting summing amplifier, I need to use another amplifier to make it non-inverting with respect to the original inputs, correct?

Actually I am using the same circuit that I used for LM6181 and replaced LM with OP27. I will test your suggestion and see how it works.

 

[crutschow]

Yes, you will need to add a unity-gain inverter if you want the output in phase with the inputs.

 

[Winterstone]

pavanbabut,

if you need a non-inverting summing amplifier with equal gain values for both inputs - why don`t you use a single opamp in non-inverting configuration?
In this case, both input signals are connected via two equal-valued resistors to the non-inv. input and the feedback to the inv. input determines the gain.

 

[crutschow]

pavanbabut,

if you need a non-inverting summing amplifier with equal gain values for both inputs - why don`t you use a single opamp in non-inverting configuration?
In this case, both input signals are connected via two equal-valued resistors to the non-inv. input and the feedback to the inv. input determines the gain.

That will work as long as both voltage sources have a low output impedance since there is crosstalk of the input currents to the opposite input.

 

[Winterstone]

Hi chrutschow, yes - agreed. My only point was: One should know the options available before fixing the design.