Differential Amplifier output shifting

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
Hi,
I am giving a 2.5V amplitude sign wave to my differential amplifier circuit as input and getting the output of 3V as you can see in the below snapshot. but if I remove the -VCC(negative supply) of the amplifier and connect it to the ground then I am getting a shift of 1V in output.
The output starts from 1V to 3V, instead of 0-3V. I don't want to use the negative supply in my amplifier to reduce the circuit cost. Please help me get output 0-3V without giving a negative supply to the amplifier.
 

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Ian0

Joined Aug 7, 2020
5,839
I would say that the offset was already on your input signal.
You could try AC coupling, but putting capacitors in series with R1 and R4
 

ericgibbs

Joined Jan 29, 2010
16,002
hi ashad,
With that type of OPA designed for dual supply operation, you will get a positive voltage offset above zero volts on the Vout, for zero Vin, when using a Single supply voltage.
E
Clip from Datasheet.
The TLE207x are fully specified at +/-15 V and +/-5 V. For operation in low-voltage and/or single-supply systems,


Because BiFET operational amplifiers are designed for use with dual power supplies, care must be taken to
observe common-mode input voltage limits and output swing when operating from a single supply. DC biasing

of the input signal is required and loads should be terminated to a virtual ground node at mid-supply. Texas
Instruments TLE2426 integrated virtual ground generator is useful when operating BiFET amplifiers from single
supplies.
 

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
hi ashad,
With that type of OPA designed for dual supply operation, you will get a positive voltage offset above zero volts on the Vout, for zero Vin, when using a Single supply voltage.
E
Clip from Datasheet.
The TLE207x are fully specified at +/-15 V and +/-5 V. For operation in low-voltage and/or single-supply systems,


Because BiFET operational amplifiers are designed for use with dual power supplies, care must be taken to
observe common-mode input voltage limits and output swing when operating from a single supply. DC biasing

of the input signal is required and loads should be terminated to a virtual ground node at mid-supply. Texas
Instruments TLE2426 integrated virtual ground generator is useful when operating BiFET amplifiers from single
supplies.
Dual supply is the feature of TLE207x but it should work with a single supply also with few changes.
 

Ian0

Joined Aug 7, 2020
5,839
You also need a rail-to-rail op-amp as @ericgibbs said.
look at the common mode input and negative output voltages in the datasheet.
The TLE2072 really doesn’t do rail-to-rail on either input or output.
 

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
It works with a single supply provided that the input never goes below 4V and the output is never required to go below 1V
My circuit satisfies both the conditions that you are suggesting, input is +-2.5V and output should be 0-3V.but still output is starting from 1-3V.It is rail to rail aopam at V+
1659691634780.png
 

ericgibbs

Joined Jan 29, 2010
16,002
Dual supply is the feature of TLE207x but it should work with a single supply also with few changes.
hi ashad,
You will not get 0V, Vout for Vin=0 with a single supply, this what the d/s states.
E
EG 1299.png

observe common-mode input voltage limits and output swing when operating from a single supply. DC biasing
 

DickCappels

Joined Aug 21, 2008
9,078
This is the wrong opamp for this kind of operation. If this is said enough time you might start believing it. It is true.

Total supply is the total voltage as measured between the Vcc+ and Vcc- pins.

The common mode range is the minimum and maximum voltage on the input pins with respect to the power supply (Vcc+ and Vcc-) pins when both input pins have about the same voltage on them. Some amplifiers do funny things like invert the signal or die when the common mode range is exceeded.

Correction: my output is going below 1V.
But how do you conclude that output will not go below 1V?
This is an analog circuit. It is very common for components to be able to operate outside their specified limits, this is necessary so that the manufacturer can have acceptable yields and so that the customer's circuit work. The fact that a device seems to work outside of its specified parameters does not necessarily mean that it will work when conditions change (age, temperature, supply voltage, etc.) so using parts other than specified on a datasheet is poor practice.

Some say that it is the difference between a circuit that was designed and one that was discovered.

If you describe in detail what your input signal is and what output signal you want somebody here will be able to show you exactly how to get what you want.
 

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
This is the wrong opamp for this kind of operation. If this is said enough time you might start believing it. It is true.

Total supply is the total voltage as measured between the Vcc+ and Vcc- pins.

The common mode range is the minimum and maximum voltage on the input pins with respect to the power supply (Vcc+ and Vcc-) pins when both input pins have about the same voltage on them. Some amplifiers do funny things like invert the signal or die when the common mode range is exceeded.



This is an analog circuit. It is very common for components to be able to operate outside their specified limits, this is necessary so that the manufacturer can have acceptable yields and so that the customer's circuit work. The fact that a device seems to work outside of its specified parameters does not necessarily mean that it will work when conditions change (age, temperature, supply voltage, etc.) so using parts other than specified on a datasheet is poor practice.

Some say that it is the difference between a circuit that was designed and one that was discovered.

If you describe in detail what your input signal is and what output signal you want somebody here will be able to show you exactly how to get what you want.
I want a circuit having a +-2.5V input signal (differential input) and output of 0-3V, by using a single supply in opam.
 

DickCappels

Joined Aug 21, 2008
9,078
Up to what frequency?

You will not be sure of being able to get all the way to zero without a negative power supply. There are some opamps that come close.

There are 8 pin IC's that can provide a low output voltage if lightly loaded. You can get your circuit to work both above and below ground with a single power supply if you add a voltage converter chip. I bought a lot of an unbranded version of the ICL7660 for about U.S. $0.05 each and they work fine.

1659697666924.png

Name-brand versions are available from Intersil (Now Renesas ICL7660), Maxim (MAX1044/ICL7660), Texas Instruments (LMC7660) and probably several I don't know about.
 

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
Up to what frequency?

You will not be sure of being able to get all the way to zero without a negative power supply. There are some opamps that come close.

There are 8 pin IC's that can provide a low output voltage if lightly loaded. You can get your circuit to work both above and below ground with a single power supply if you add a voltage converter chip. I bought a lot of an unbranded version of the ICL7660 for about U.S. $0.05 each and they work fine.

View attachment 273170

Name-brand versions are available from Intersil (Now Renesas ICL7660), Maxim (MAX1044/ICL7660), Texas Instruments (LMC7660) and probably several I don't know about.
Thanks for your important suggestion
 

Thread Starter

ashadkhan.m

Joined Jun 23, 2021
15
Thank you guys for sharing your experience and knowledge. I understand the problem because of you people and I tried with other opam and its working now
 

MisterBill2

Joined Jan 23, 2018
12,697
If the inputsignal is bi-polar and the amplifier is DC coupled then the power supply must also be bipolar. If the input was not direct coupled then an offsetting voltage could be added to have the signal all positive.
BUT it would no longer accurately reflect the input signal.
 
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