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I'm playing with an antique LM308 http://web.mit.edu/6.301/www/LM108.pdf For some simple tests I've wired it as a voltage follower.

schematic

simulate this circuit – Schematic created using CircuitLab

Now I'm trying to understand my measures :

  • 0V <= Vin <= 600mV Vout = 600mV
  • 600mV < Vin <= 4.57V Vout = Vin
  • Vin > 4.57V Vout ~ 4.6V

One important thing is that I got Vout(peak-peak) at an high level, for these antique op amp, adding a compensating capacitor between 1 and 8 pin is needed.

From datasheet with Vs=+-15

  • common mode min input range = +-14V
  • Min output swing = +-13V

So with 0-5V supply, does that mean :

  • input range = 1-4V
  • output range = 2-3V ? Why is 600mV measured?

An another op amp http://www.analog.com/media/en/technical-documentation/data-sheets/OP270.pdf, I've these results :

  • 0V <= Vin <= 750mV Vout = 0V
  • 750mV < Vin <= 5V Vout = Vin + 900mV How can I explain results?

After this first experiment, I wired a non inverting amplifier with 2 gain (R1 = R2 = 1k)

  • 0V <= Vin <= 600mV \ Vout = 600mV
  • 600mV < Vin <= 1.9V Vout = 2 x Vin
  • Vin > 1.9V Vout = 3.8V

As in previous setup, op amp not active when Vin <=600mV. When Vin > 1.9V Vout reaches a max value of 3.8V, why does this value differ from previous setup?

Thanks for your explanations. lah

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2 Answers 2

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The LM308 is not a rail-to-rail opamp; the 600 mV minimum input voltage you see is the voltage drop across the output transistor inside the opamp (the output on the negative side is actually a cascade(*) configuration). The same applies to the positive side.

output cascade of LM308

The 3.8 Volt is probably because your operating voltage is too low and some internal transistor goes into saturation. Increase the power supply to 10 Volt and you should get better results.

(*) Can't come up with the proper term right now.

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So with 0-5V supply, does that mean :

It means nothing!

The supply voltage range for this op-amp is +/-5V to +/-20V. You can't run it from a single 5V power supply. Read page 5 of your data sheet link, look at the graphs.

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  • \$\begingroup\$ The databook from National Semiconductors that I have here lists a minimum supply voltage of +/- 2V, so it might work on 5 Volt single phase, but barely. \$\endgroup\$
    – JvO
    Mar 21, 2016 at 15:48
  • \$\begingroup\$ You are true, I did not take care about minimum operating supply range. It is suggested from graphs and note 4. \$\endgroup\$
    – rem
    Mar 21, 2016 at 15:58
  • \$\begingroup\$ @JvO Note 4 in the DS says this: "These specifications apply for +/-5V <= VS <= +/-20V". Running it at 5V or +/-4V means the whole of the numbers in the data sheet are invalidated and you cannot make any sensible assumptions about the device. \$\endgroup\$
    – Andy aka
    Mar 21, 2016 at 15:58
  • \$\begingroup\$ @Andyaka Yups, makes you wonder why they even bother writing (quote) "The device operates with supply voltages from +- 2V to +- 15V [...]" \$\endgroup\$
    – JvO
    Mar 21, 2016 at 16:05
  • \$\begingroup\$ = page 1 DS BS! \$\endgroup\$
    – Andy aka
    Mar 21, 2016 at 17:32

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