I would like to clarify a concept of single supply voltage operation on OP-AMP.

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Aug 26, 2015.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015

    Please see the attached image first,

    The image shows how to construct non-inverting negative feedback OP-AMP network with single supply voltage, which is preferred by me.

    As my red note on the image tells, Single supply voltage operation can be compared to dual supply mode to understand operation.

    Vcc/2 is virtual ground which is system ground when it is in dual supply mode. Similarly ground here is negative supply voltage in dual mode.

    By comparing this circuit to dual mode which operation is well known, I got the following equation.

    Vout = (1 + R2/R1)(Vin - Vcc/2) and this Vcc is referenced to Vcc/2, virtual ground of this circuit.

    Thus desired output voltage with respect to system ground is Vout' = Vout + Vcc/2.

    Could you please tell me my approach to understand single supply operation on OP-AMP is right?

    And Can I change Vcc/2 to any value between Vcc and system ground, 0?

    And in addition, I'm now trying to amplify 5V single square pulse of 10 us in pulse duration to double without seriously distorting signal to trigger some measurement instrument in my physic experiment. Is there other wave to achieve this more simply than using OP-AMP?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    The diagram has a huge omission! Look at the non-inverting input. There must be a DC path to Vcc/2 for the amplifier's input bias current.
    Dong-gyu Jang likes this.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    A couple of things. Your approach is basically correct, with a few corrections. As above, the non-inverting input must have a DC path to somewhere, or the input bias current will eventually charge up the coupling capacitor until the input saturates. This plays into another correction: your statement about input impedance for minimum bias current error should read "input resistance". It is the two input resistances to GND that must be equal to minimize errors. At DC, a capacitor has an infinite resistance; this is why you need a resistor on the + input. Note that the resistor will form a highpass filter with the input coupling capacitor.

    Another note - your circuit performance varies greatly depending on the specific opamp used. All opamps can be biased for single-supply operation, but most can not operate with the inputs or outputs near the negative rail. Many have an input range that can not swing to within 1 or 2 volts of the rails. Something to watch out for.

    As for amplifying a pulse - if you want a linear amplifier because you need to exactly double the input amplitude including its variations, then a fast rail-to-rail opamp will work. But if this is a simple pulse amplifier, then a switching transistor will do the job. For example, if you need a 10 V output when the input is anything between 3 and 6 volts, and the output is driving a medium or high impedance load, then an NPN transistor switch will work.

    Dong-gyu Jang likes this.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    Here is an example of what Mike and AK are talking about. R8/R9 creates the Vcc/2. C4/C5 lower the AC impedance of Vcc/2. R2 biases the non-inverting input to Vcc/2.