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circuit

Hi, I'm using an emitter follower op-amp buffer with LM6132 and PN2222A. I'm expecting almost an exact follow?, as simulated with multisim. However, I'm getting an output offset of around 50mV at zero input. Reading LM6132 I should be able to swing to 0 with +/-5v supply. I'm wondering if it's an op amp input offset problem, I'll be testing this when I have access to an oscilloscope. Also, I'm using a DC-DC converter for my two rails, specifically 102-3333-1-ND, and I have yet to add input/output capacitors to the converter.

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  • \$\begingroup\$ I think you have +5 V for each supply ! \$\endgroup\$ – jp314 Nov 23 '15 at 4:23
  • \$\begingroup\$ It is negative, but it isn't labelled clearly. \$\endgroup\$ – Sams Nov 23 '15 at 4:25
  • \$\begingroup\$ It should work OK then. If you don't use the emitter follower, what offset do you get ? Put a 0.7 V source in series with the base of the NPN to approximately cancel the VBE voltage -- not that that's practical in real life, but you can see if it would work then also. \$\endgroup\$ – jp314 Nov 23 '15 at 4:36
  • \$\begingroup\$ How would VBE affect 0V input? I will be able to test tomorrow. \$\endgroup\$ – Sams Nov 23 '15 at 4:47
  • \$\begingroup\$ it shouldn't affect it, but it's not working either -- just an idea to help debug. \$\endgroup\$ – jp314 Nov 23 '15 at 5:32
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The emitter follower can only source current, it cannot sink it*. The LM6132 has an input bias current of (typically) 110nA at room temperature. That current flows out of the op-amp input.

If you connect it to a 500K load to ground, you'll get around 50mV of output offset. To make this go away you could add an E-B resistor- something like 1K should work very well.

*If you had a -10V supply it would probably appear to work since the transistor E-B junction would break down. This is not a good thing. Even with your +/-5V supplies it would be a good idea to add a reverse-biased diode such as a 1N4148 across the E-B junction (in parallel with the resistor I mentioned above).

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    \$\begingroup\$ That is exactly the problem I was experiencing. Thank you for the explanation Spehro. \$\endgroup\$ – Sams Nov 23 '15 at 23:11

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