Difference Amplifer Design with Single Supply Voltage

In summary, the individual is working on a difference amplifier design that needs to operate from a positive voltage supply only. They have tried using a single supply voltage, but the amplitude decreases dramatically and the inputs do not hit the rail voltage. They have also tried using different op-amps, but have not found success. They are currently using the LMH6609 op-amp, but need an op-amp with a high slew rate. They have found a solution by adding a DC bias to the inputs, which ensures they do not hit the negative rail. However, this may introduce non-linear distortion due to varying input impedance. The individual has solved their initial problem, but is still considering the implications of using the arrangement for signals down to DC.
  • #1
akhurash
25
0
Hello, I need some help with a difference amplifier design I'm working on. The amplifier has to be work from a positive voltage supply only. I'm having a hard time getting it work with single supply voltage (it works when I use both negative and positive supply voltages). Both the inputs have a peak-to-peak voltage of about 2.2V (with the 2.5V DC bias). Also, the input voltages is not hitting the rail voltage (5V).

When I use a single supply the curve below 2.5V is railed and the positive voltage is not railed BUT the amplitude decreases dramatically. I think the single supply isn’t working because when the subtraction takes take it negates the DC bias voltage. Am I correct in this assumption? If so, how can I design a difference amplifier with just a single supply?

Attached is the difference amplifier circuit design.
 

Attachments

  • Difference_Amplifier.gif
    Difference_Amplifier.gif
    14.9 KB · Views: 941
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  • #2
akhurash said:
Hello, I need some help with a difference amplifier design I'm working on. The amplifier has to be work from a positive voltage supply only. I'm having a hard time getting it work with single supply voltage (it works when I use both negative and positive supply voltages). Both the inputs have a peak-to-peak voltage of about 2.2V (with the 2.5V DC bias). Also, the input voltages is not hitting the rail voltage (5V).

When I use a single supply the curve below 2.5V is railed and the positive voltage is not railed BUT the amplitude decreases dramatically. I think the single supply isn’t working because when the subtraction takes take it negates the DC bias voltage. Am I correct in this assumption? If so, how can I design a difference amplifier with just a single supply?

Attached is the difference amplifier circuit design.

The TL071 opamp is not designed for use as a single-supply opamp. Use a different opamp -- one specifically designed to be used with a single-supply. What do you find when you use Google to search for such an opamp...?
 
  • #3
I asked about single supply op-amps here actually and I was told TL071 would be a good start. I tried other op-amps and when I googled for them I was lead to TI and Analog websites. I used the part search on Digikey to try few other single supply op-amps, such as LMH6609, LT1006, and TL084. I also tried a difference amplifer (can't remember the name) and that also didn't work when using single supply; although I don't think the difference amplifer was meant for single supply applications.
 
  • #4
Right now I'm using the LMH6609 because I need an op-amp with high slew rate.
 
  • #5
akhurash said:
I asked about single supply op-amps here actually and I was told TL071 would be a good start. I tried other op-amps and when I googled for them I was lead to TI and Analog websites. I used the part search on Digikey to try few other single supply op-amps, such as LMH6609, LT1006, and TL084. I also tried a difference amplifer (can't remember the name) and that also didn't work when using single supply; although I don't think the difference amplifer was meant for single supply applications.

akhurash said:
Right now I'm using the LMH6609 because I need an op-amp with high slew rate.

I'm not sure why anybody would suggest the TL071 opamp for single supply operation -- it is meant for low-noise split supply operation.

The LM324 is the classic single supply opamp, although there are others. It is not all that fast, though, so it may not fit your application.

You can look at CMOS rail-to-rail opamps... They work fine for single supply operation, although they are a bit more expensive than the classic LM324.
 
  • #6
If anyone is interested in knowing how I got this to work please see the attachment. I'm sorry but I don't' remember where I got that info from or else I would've given credit but that is how to do it.

You have add a DC bias to the inputs and in your gain you will always have a positive component which will ensure you are never hitting the negative rail (in my case the negative rail is 0V and I used a DC bias of 2.5V).
 

Attachments

  • Diff_Amp_Biased.PNG
    Diff_Amp_Biased.PNG
    34.8 KB · Views: 532
  • #7
It's not clear — have you solved what you were having trouble with? Are you wanting the diff amp to operate right down to DC?

Your second attachment looks like it should work, but be aware that it doesn't present a fixed input impedance; Zin varies with the amplitude of the output, so this potentially introduces some non-linear distortion.
 
  • #8
I have solved what I was having trobule with. But I didn't think about the input impedance, :(. Although this shouldn't matter as much for my application.
 
  • #9
akhurash said:
I have solved what I was having trobule with.
So you've discarded the arrangement in your opening post, it being something that won't work?

If using down to DC, be aware that whereas your input signal sources might be designed to, say, source current at 1.8v, when connected to your biasing arrangement, that signal source will find itself needing to sink current when delivering 1.8v.
 

1. What is the purpose of a difference amplifier with a single supply voltage?

A difference amplifier with a single supply voltage is used to amplify the difference between two input signals while operating with a single power supply instead of a dual power supply. This makes it more cost-effective and compact for certain applications.

2. How does a difference amplifier with single supply voltage work?

A difference amplifier with single supply voltage works by using a voltage divider circuit to create a virtual ground at half the supply voltage. This allows for amplification of the difference between the two input signals without the need for a negative power supply.

3. What are the advantages of using a difference amplifier with single supply voltage?

The main advantages of using a difference amplifier with single supply voltage are cost-effectiveness and compactness. It also eliminates the need for a negative power supply, making it more convenient for certain applications. Additionally, it can provide better common-mode rejection, reducing noise in the output signal.

4. Are there any limitations to using a difference amplifier with single supply voltage?

One limitation of using a difference amplifier with single supply voltage is that the output signal can only swing as high or as low as the supply voltage. This can limit the dynamic range of the amplifier. Additionally, the virtual ground created by the voltage divider circuit may not be as stable as a true ground, leading to potential noise and offset in the output signal.

5. How do I choose the appropriate single supply voltage for my difference amplifier?

The appropriate single supply voltage for a difference amplifier depends on the input signal range and the desired output signal swing. It is important to ensure that the supply voltage is high enough to accommodate the input signals and provide enough headroom for amplification. However, choosing too high of a supply voltage can lead to excess heat dissipation and potential damage to the amplifier. It is recommended to consult the amplifier's datasheet for guidance on choosing the appropriate supply voltage.

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