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Operational Amplifier for a Voltage Summer

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flames4791

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Hi, I am trying to design a voltage summer and all of them require an opamp. I am having trouble understanding how the OpAmp can help the summer, especially when the sum of inputs is less than the voltage required to put the input transistors in saturation. I have designed a two stage single ended OpAmp with gain of about 40dB but when I connect it to the summer circuit it doesn't work. But when I put an ideal opamp from the Cadence library into the summer circuit with a gain of 100, the circuit works though. If anyone could give me some advice for how I can realize the summer, that would be great. Thank you!
 

A summer is a mixer. A mixer is an opamp with a voltage gain of a few hundred thousand as an inverting low gain circuit. Each input has a series resistor to the inverting input of the opamp and the output has the same value resistor to the inverting input to make the gain 1. Nothing is supposed to saturate because it is linear.

This mixer circuit needs to have a plus and minus supply:
 

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Audioguru said:
A summer is a mixer. A mixer is an opamp with a voltage gain of a few hundred thousand as an inverting low gain circuit. Each input has a series resistor to the inverting input of the opamp and the output has the same value resistor to the inverting input to make the gain 1. Nothing is supposed to saturate because it is linear.

This mixer circuit needs to have a plus and minus supply:
Click to expand...

Okay so my opamp design architecture is something like this

Picture3_3.png.scaled500.png

You are saying that my input gates should be biased in linear region then? And I have been measuring the AC gain of Vout/Vin, is that what should be equal to 1? Thanks for your response.
 

The dynamic range (maximum and minimum output voltage range) of the opamp being used must be such that none of the transistors go to linear region at any voltage level.

In the above diagram, the output of the summer is so low/high that M6/M7 is going to linear. Calculate the possible dynamic range of the opamp. In case of common source output stage, it will be Vdd-Vdsatpmmos-Vdsatnmos. If you want to get an output voltage which is out of that range, it wont be possible.
 

So my summer is taking inputs between -1v and 1v and the output is somewhere in that range as well. Am I not able to use this architecture then?
 

LvW said:
...a mixer?
I don`t think so. The classical task of a mixer is to multiply signals.
Click to expand...
No, he is right, a mixer is a summing amplifier.
That is exactly its job, to add signals.
 

Purposeless fighting.

In audio applications, a signal summer is called a "mixer" (e.g. studio or stage audio mixing deck). In RF range, the term "mixer" is dedicated to non-linear circuits that are generating sums and differences of frequencies, e.g. a double balanced mixer which is a kind of multiplier.
 

Most opamps have DC and low frequency voltage gain 2000 times more than yours.
Your opamp is shown without any negative feedback and its (+) input has no DC reference voltage.
Since your input is between -1V and +1V and you want the output to be the same then Vdd must be a positive power supply voltage, Vss must be a negative power supply voltage and their junction 0V must be the input and output grounds.
 

flames4791 said:
So my summer is taking inputs between -1v and 1v and the output is somewhere in that range as well. Am I not able to use this architecture then?
Click to expand...

You can't achieve the full swing of Vdd at the output since the transistors always eat up some headroom. If your supplies are 1v and -1V, you probably can achieve a swing of about 1.7-1.8 V with two transistors stacked up (i.e the output stage). So, if your output falls anywhere within about +0.85V and -0.85V, you should be ok. But if you are seeing the output transistors just entering linear, you could try 1) increasing the size to reduce Vdsat and 2) make sure the output common mode is set to the right value.
 

Most opamps have a low impedance push-pull output. Yours doesn't.
The supply current in most opamps does not increase as they heat up. Yours will, which makes it hotter which makes more current which makes it hotter which makes more current and it will have thermal runaway.
 

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Audioguru said:
Most opamps have DC and low frequency voltage gain 2000 times more than yours.
Your opamp is shown without any negative feedback and its (+) input has no DC reference voltage.
Since your input is between -1V and +1V and you want the output to be the same then Vdd must be a positive power supply voltage, Vss must be a negative power supply voltage and their junction 0V must be the input and output grounds.
Click to expand...

So would a 2v supply fix the swing issue? And what region should I be biasing the transistors? And am I looking for the highest possible AC gain?
 
Last edited:

Since your output is a class-A Mosfet feeding a current source then it can pull strongly positively but not negatively. Therefore its output resistance is high and its output swing is determined by its current and its load resistance. Why doesn't your opamp have a push pull class-AB output like most other opamps?
 

Audioguru said:
Since your output is a class-A Mosfet feeding a current source then it can pull strongly positively but not negatively. Therefore its output resistance is high and its output swing is determined by its current and its load resistance. Why doesn't your opamp have a push pull class-AB output like most other opamps?
Click to expand...

I'm sorry, how would I implement the push pull?
 

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