TL072 unit gain buffer shows DC bias.

Mohammad Mohiuddin

Other Parts Discussed in Thread: TL072, TL972, LM258, OPA365

Hello All,

I wanted to build this circuit with TL972, but Tina model/macro is not available. So I thought I will use TL072. The circuit I build in Tina, a simple unity gain buffer, displays a 1.55V DC at its output.

I wanted to attach Tina schematic but forum does not allow it. The circuit I built is a unity gain amplifier with 3.3V supply voltage,a voltage generator with 250kHz sinewave input.

Can anyone comment what may be wrong here?

Thanks,

M Mohiuddin

over 14 years ago

0 Ron Michallick over 14 years ago

TI__Guru**** 152966 points

Hello M Mohiuddin,

The TL072 needs a higher supply voltage and its common mode range is (V-) + 4V to (V+).

Try setting the TL072 supply voltage to +5V & -5V.

The TL972 has a rail to rail output, but the common mode range is (V-) + 1.15V to (V+) - 1.15V.

Regards,
Ron M.

0 Mohammad Mohiuddin over 14 years ago in reply to Ron Michallick

Prodigy 230 points

Hi Ron,

It took me a while to digest common-mode-voltage and come back and ask this question.

I decided to go with LM258 which is cheaper opamp and may do the job. I have included tinypic link that shows my TINA schematic below. Hoping you can access it.

Now question I have is: In TINA simulation, LM258 is not amplifying the input signal as theoretical calculations suggest. I am trying to rectify and amplify a 240kHz input signal. Any suggestions?

Thanks,

M Mohiuddin

0 Ron Michallick over 14 years ago in reply to Mohammad Mohiuddin

TI__Guru**** 152966 points

Try the OPA365 instead, it is high bandwidth, has rail to rail input and output, and likes 3.3V supply

Pspice and TINA models are available using part number OPAy365.

The input signal must not cause the non inverting input to be less than -0.5V (if this is possible use a shunt Schottky diode.)

Regards,
Ron M.

0 Mohammad Mohiuddin over 14 years ago in reply to Ron Michallick

Prodigy 230 points

Hi Ron,

Even though OPA365 is a bit more expensive, it did the trick and simulation shows desired result.

I am a bit confused about the need for such an opamp. Input signal is only 240kHz; why a higher GBW (50 MHz in this case) component is needed.

Can an opamp with much lower, say 1MHz GBW, do the job and in eventuality save some cost?

Thanks again for your time and efforts.

M Mohiuddin

0 Ron Michallick over 14 years ago in reply to Mohammad Mohiuddin

TI__Guru**** 152966 points

The minimum op-amp bandwidth needed is voltage gain * 250kHz..
Since gain bandwidth is a typical spec, pick a device at least double the minimum.
To get a reliable gain set by the feedback, double it again; otherwise the gain will a little lower than expected.

To support a large scale output signal, the slew rate is factor, too.
The minimum slew rate needs to be greater than VOUTp-p * Pi * Freq (in V/S, divide by 1,000,000 to get V/uS)

Ron M.

0 Mohammad Mohiuddin over 14 years ago in reply to Ron Michallick

Prodigy 230 points

I have simulated the circuit in TINA; it shows expected results - except the range of gain, output never reaches the 3.3V rail does not matter what gain is set.

To have a better understanding for GBW requirements, is there a technical document available that you can suggest or share with us.

Thanks,

M Mohiuddin

0 Ron Michallick over 14 years ago in reply to Mohammad Mohiuddin

TI__Guru**** 152966 points

Mohammed,

Which output does not hit 3.3V, the opamp or the final output?
The diode will drop some voltage.

Op Amps for Everyone Design Guide (Rev. B) Chapter 6 covers GBW limitations.

http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slod006b

0 Mohammad Mohiuddin over 14 years ago in reply to Ron Michallick

Prodigy 230 points

Hi Ron,

You questioned it right. Prior to diode and filter drops, output does reach rail.

Thanks for all your help and pointing to the literature for GBW.

M Mohiuddin

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TL072 unit gain buffer shows DC bias.