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Consider the case that I have a standard inverting summing amplifier with two inputs: 0.5V DC and 1V DC. Therefore, the output of the amplifier should be -1.5V related to GND. However, If I connect the power supply pins of the amplifier to 5V and GND, can the amplifier still generate negative voltage? Or, I must have a negative voltage source in order to make the summing amplifier work?

If I need a negative voltage source to make it work, what is the simplest way to generate a negative voltage source from a positive power supply? Would the method described in the following webpage work?

https://www.analog.com/en/technical-articles/amplifier-ic-generates-a-negative-voltage-reference-with-the-fewest-parts-and-a-single-supply-rail.html#:~:text=A%20common%20way%20to%20generate,to%20generate%20the%20negative%20output.

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  • \$\begingroup\$ Please draw a schematic with the included editor or add the closest one to your design. It helps greatly to us answering the questions and to those who come across the questions to understand and learn more. \$\endgroup\$ Apr 17, 2023 at 1:50
  • \$\begingroup\$ Also, you need to specify in your first sentence that you are talking about an INVERTING summing amplifier. \$\endgroup\$ Apr 17, 2023 at 3:40
  • \$\begingroup\$ An alternative solution is to connect the positive input pin of the opamp to some other reference voltage than 0 V. Then the output will be summed in relation to that voltage instead. \$\endgroup\$
    – jpa
    Apr 17, 2023 at 17:08

4 Answers 4

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Simplest way to get a few mA at negative voltage is to use a charge pump IC such as the ICL7660 or its many clones such as TC7660.

enter image description here

There are also solutions that require an inductor, suitable for higher currents, but for a simple op-amp circuit this should be adequate and it is relatively low-noise.

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    \$\begingroup\$ Thank you, a charge pump IC looks good to me. Also, the MAX889 series charge pump IC can output up to 200 mA. Which is sufficient for most circuit projects. \$\endgroup\$
    – Aurora7979
    Apr 17, 2023 at 15:04
  • \$\begingroup\$ MAX889 is a fine chip, however 10-20x the price of the 7660 and single-sourced. \$\endgroup\$ Apr 17, 2023 at 15:19
  • \$\begingroup\$ Is there any disadvantage for a single-sourced IC? \$\endgroup\$
    – Aurora7979
    Apr 17, 2023 at 15:32
  • \$\begingroup\$ @Aurora7979 Mostly availability (and often price) can be inferior. On the plus side they may be something you can't easily do any other way. In the recent chip crunch the "jellybean" multiple-sourced devices have generally been easy to get. Single sourced parts like MCUs, there might be a year or more lead time for any quantity. \$\endgroup\$ Apr 17, 2023 at 15:36
  • \$\begingroup\$ @Aurora7979 How would you continue producing a product that is designed using a MAX889 in the case Maxim decides that they don't feel like producing those chips anymore unless someone orders at least 100.000 of them, when you are just selling 5000 of them per year? \$\endgroup\$ Apr 17, 2023 at 21:16
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Using a standard operational amplifier for a summing amp you would not be able to get a negative output with a single supply, in the case where the inputs would mathematically drive the output negative it will simply limit it to something near zero volts.

The amplifier (MAX9820) used in the linked article is a headphone amplifier IC that has an internal charge pump that generates a negative voltage, it's designed to be able to output negative voltage with a single supply.

You could use that circuit to generate a negative supply for a summing amplifier, it's voltage range is rather limited though, the article shows -3 V, I don't know if it could do more than that. There are other ways to generate a negative voltage, such as a 555 timer running as an oscillator driving a negative output diode voltage doubler as seen here.

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  1. "If I connect the power supply pins of the amplifier to 5V and GND, can the amplifier still generate negative voltage?"
  • Of course not: an amplifier cannot output a voltage range outside of the supplied voltage range UNLESS it has an additional circuitry, whether built-in or externally added, to provide such voltages.
  1. "Or, I must have a negative voltage source in order to make the summing amplifier work?"
  • If you expect or need voltages outside of the supplied voltage range, you need a way to provide/supply the maximum expected/needed voltage range.
  • If you are summing AC voltages/signals (like audio), a virtual ground as a center point can be created with a simple resistive voltage divider, halfway between the positive and the negative supply voltage.
  • If you are summing DC voltages with a non-inverting summing amplifier and they are all above GND (or between GND and positive supply voltage), you don't need a negative supply.
  • But, if you are summing DC voltages with an INVERTING summing amplifier, you definitely need a negative supply, which is most easily done with the classic diode-capacitor combination as below (taken from a Maker Pro page):
    enter image description here

The square wave can be generated with a 555 oscillator (image from All About Circuits), as below: ![enter image description here
Or it could be generated using logic gates, as below (taken from the same page as above): enter image description here

NOTE: To keep quiescent current consumption low, use a CMOS version of the 555 timer, like LMC555, TLC555, TS555, ICM7555 or ICL7555.

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  • \$\begingroup\$ Thank you. I also read this article. But in the end a charge pump IC is simpler to implement since it requires less components. \$\endgroup\$
    – Aurora7979
    Apr 17, 2023 at 15:09
  • \$\begingroup\$ @Aurora7979 Yes, always go for a simpler/faster/easier solution if it's available or you can wait for specific parts. I was going for the solutions that are normally already in our possession or easily found on old PCBs. Many places and people in the world don't have the luxury of getting specific components quickly or easily. Specific solutions are sometimes hard to find, taking more time then the available old solutions, and specific components can more easily disappear from the market. \$\endgroup\$ Apr 17, 2023 at 15:23
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As a general rule for every op-amp circuit, the output voltage is never able to go outside the bounds of the supply voltage rails connected to the opamp.

In the OP case, you do need a negative power supply rail. Of at least the most negative voltage you ever expect to see at the output. (E.g. if you never expect the voltage to be lower than -1.5V, and a rail-to-rail opamp is used, then a -1.5V supply rail should be sufficient.)

In some cases, the voltage of the opamp output can never reach the supply rails of the opamp, and will be clipped (constrained) a volt or two below the positive rail, or above the negative rail. So the usable output voltage is smaller than the power supply rail's range.

This is especially common with cheap, jellybean op-amps like the LM358 and LM324.

You can overcome this by buying (usually slightly more expensive) op-amps which are specified as "rail-to-rail" opamps, meaning the opamp output voltage is able to swing to the entire range of the supply rails, without any dead range that it can't reach close to the supply voltages.

But any opamp, even a rail-to-rail opamp, can never have an output voltage swing that goes outside the supply rails.

You also need to check that the opamp you select is compatible with the supply voltage rails that you want to use. Some opamps may not be specified for (positive or negative) supply rails that are too high.

And you will need to check the usual AC characteristics of the opamp, like gain-bandwidth product and slew rate, to ensure it is fast enough for your signal, if relevant.

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