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In my circuit (picture below), ATMEGA32 output is connected to a RC filter (R=100K, C=10uF 63V) to non-inverting terminal of OpAmp (LT1006). OpAmp is configured as voltage follower (Buffer Amplifier). Voltage in non-inverting terminal varies (0 to 5 V) by changing values in OCR1 register of ATmega32.

OCR1 of

Theoretically, in a buffer amplifier configuration, output will be the same with respect to input of an OpAmp but in this case the output go high rather than input (eg: input=1.45 V, output=2.68 V).

I connected different voltage from other source to non-inverting terminal output will be the same so the mistake is happening while connecting via RC filter.

Would anyone kindly give any solution for this?

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  • \$\begingroup\$ We have a circuit editor built into the site. It makes schematics that are a lot easier to read than a hand drawing. \$\endgroup\$
    – Greg d'Eon
    Commented Jan 28, 2015 at 11:30
  • \$\begingroup\$ Even if you fixed the problem you appear to have, the input range for the LT1006 is limited to between 0V and 3.5 volts (on a 5V power rail). \$\endgroup\$
    – Andy aka
    Commented Jan 28, 2015 at 11:38
  • \$\begingroup\$ Anyway you can remove the 3.3mOhm resitor ? Does the same happen thing happen ? Could be the output the along with the low resitance + BE is over driving the opamp sourcing capabilities. \$\endgroup\$
    – efox29
    Commented Jan 28, 2015 at 12:42
  • \$\begingroup\$ Input of RC filter is PWM from microcontroller RC filter act as a DAC and RC filter Could not drive The Transistor directly hence I connected via a buffer amplifier. I also checked the output of opamp with out load (Input range 0v to 2.5v)the same result i got. \$\endgroup\$
    – KRISHNACHANDRANC CHANDRASEKHAR
    Commented Jan 29, 2015 at 6:17
  • \$\begingroup\$ Finally I got the result by changing RC filter values, R replaced by 10k and C replaced by 100uf, thank you every one for the great support ....see u later..... \$\endgroup\$
    – KRISHNACHANDRANC CHANDRASEKHAR
    Commented Feb 5, 2015 at 4:33

2 Answers 2

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The OPAMP is totally unsuited to what you want with a 5V power supply.

  • It has an input voltage range of 0V to about 3.5V
  • It has an output voltage range that might reach down to 1V and probably won't rise above 4V. It's guaranteed to swing only to 2V of either supply rail on a +/-15V rail.

Fix this then re-examine the problem you are having by disconnecting the output transistor (just to make sure this isn't causing a problem due to mis-wiring the device.

Try an AD8605 for the op-amp - it has rail-to-rail input and output capability but still remember that getting it's output down to 0V when the most negative rail is 0V isn't going to happen - you might get the output down to 50mV or a bit less.

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  • \$\begingroup\$ Why he can not connect the uC output directly to the transistor. Is the filter necessary? I assume he wants a switch. \$\endgroup\$
    – Nazar
    Commented Jan 28, 2015 at 13:02
  • \$\begingroup\$ @Naz - this should be a comment under the OP's question. \$\endgroup\$
    – Andy aka
    Commented Jan 28, 2015 at 13:04
  • \$\begingroup\$ True. I was wondering if it was possible to eliminate the OpAmp and the filter, but it seems like because of RC t=1s, he wants the switch to work in "slow" mode. Probably dimming lights instead of instantaneously shutting them off. \$\endgroup\$
    – Nazar
    Commented Jan 28, 2015 at 13:35
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Input of RC filter is PWM from microcontroller RC filter act as a DAC and RC filter Could not drive The Transistor directly hence I connected via a buffer amplifier. I also checked the output of opamp with out load (Input range 0v to 2.5v)the same result i got.

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