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TL441: Is the Figure 15 on the datasheet actually reproducible?

user5200856
Prodigy 10 points
Part Number: TL441
Other Parts Discussed in Thread: LOG112, LOG114, TLC271

Hi there,

I built the same circuit shown on Fig 15 but couldn’t exhibit such a good linearity (except for between 0.01V and 0.5V DC) so i got two questions:

1. Would you give me some examples for variable resistors parameters?

2. Is this datasheet actually reproducible? I doubt such a good linearity of this shown figure because I couldn’t reproduce it.

Is anyone use TL441 like this?
Thanks in advance.

  • 0
    TI__Guru* 93105 points
    Hello user5200856,

    The TL441 is a legacy log amplifier that was introduced by TI over 40 years ago. All indications are that the Figure 15 circuit was developed, characterized and presented in the first datasheet published in 1976. I cannot find any additional information about the circuit and its stated performance, but don't have a reason to doubt the original measurements.

    Do note that Figure 15 states these Notes that mus be observed:
    A. Inputs are limited by reducing the supply voltages for the input amplifiers to ±4 V.
    B. The gains of the input amplifiers are adjusted to achieve smooth transitions.

    Would you give me some examples for variable resistors parameters?
    Resistance value: 5 k-Ohm
    Resistance tolerance: 5 %
    Power rating: 1/2 W

    See: www.mouser.com/.../T93XA502JT20

    I imagine adjusting the potentiometers for decade to decade conformity is not an easy task. Have you seen the datasheets for modern log amplifiers such as the LOG112 and LOG114?

    www.ti.com/.../log114.pdf

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0
    Guru 140200 points

    Hi user,

    the OPAmp circuits in figure 15 must provide a gain of 30dB. Input offset voltages of OPAmps (TLC271) must be adjusted. And keep in mind that the input impedance of the circuit is very low (500R).

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points
    Hi Kai,

    It looks like the two input potentiometers would be adjusted for about 3.06 kilohms, for ideal 100 Ohm resistors, to set the gain to 30 dB. The output op amp is a difference amplifier, and since the other three resistors are 2 kilohms the slope potentiometer would be set to about that value as well.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Guru 140200 points
    Hi Thomas,

    yes, I see it like you.

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Hi user 5200856

    We haven't heard back from you so we assume this resolved your issue. If not, post a reply below, or create a new thread if this one has timed out.

    Thanks

    Dennis