Amplify an AC signal with a DC offset

Discussion in 'General Electronics Chat' started by Dritech, Apr 20, 2016.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    I have an AC signal which has a 2V DC offset. How can I amplify the AC component while keeping the DC offset at 2V?
    The op-amp to be used for the amplifier has to be supplied using a single rail.
    Any suggestions?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,766
    1,101
    Here's one way:
    AmpWithOffset.PNG
     
    Dritech and RichardO like this.
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply.
    Did you use fc= 1/(2πRC) to determine the values for C2 and R2?
    Also, I am going to use a 1st order BPF as shown below. Is it possible to use this configuration as a filter and for amplification instead of using two op-amps?
    I know that the gain can be achieved by changing the resistor values, but that would also very the cut-offs frequencies of the BPF.

    [​IMG]
     
  4. MrAl

    Well-Known Member

    Jun 17, 2014
    2,418
    488
    Hi,

    How much do you need to amplify the AC part? If it results in an amplitude with a peak greater than 2v then you need a negative supply unless you can handle more offset at the output. I assume you still want 2vdc at the output along with the AC.
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    @MrAl, I need an amplification of around 20 since the AC component at the input will be 0.1Vp-p max.

    Yes I need to keep the 2Vdc offset at the output of the amplifier/filter.
     
  6. MrAl

    Well-Known Member

    Jun 17, 2014
    2,418
    488
    Hello again,

    Ok just to be clear then, your output signal will be +1v to +3v right?
    0.1/2=0.05 peak
    0.05*20=1.00 peak
    Riding on 2v offset, that means -1 to +1 on top of 2, which comes out to +1 to +3.
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    If you need to keep the 2V offset than apply 2Vdc to the plus (+) input of the op amp in the circuit of post #3.
     
  8. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    The main problem is to get the gain without changing the cut-off frequences of the BPF.
    Something I forgot to mention is that the gain needs to be ajdusted (using a potentiometer). If I replace the feedback resistor of the BPF, I will obtain the required adjustable gain. The problem is that I will also very the cut-off frequency. Is there a way to change the gain with no effect on the BPF performance?
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,766
    1,101
    Not explicitly. I used an arbitrary value for R2 and made C2 'big enough' to have negligible reactance (relative to the R2 resistance) at the arbitrary 1kHz signal frequency.
     
Loading...