# Amplify an AC signal with a DC offset

Thread Starter

#### Dritech

Joined Sep 21, 2011
901
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?

#### Alec_t

Joined Sep 17, 2013
13,801
Here's one way:

Thread Starter

#### Dritech

Joined Sep 21, 2011
901
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.

#### MrAl

Joined Jun 17, 2014
10,621
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.

Thread Starter

#### Dritech

Joined Sep 21, 2011
901
@MrAl, I need an amplification of around 20 since the AC component at the input will be 0.1Vp-p max.

I assume you still want 2vdc at the output along with the AC.
Yes I need to keep the 2Vdc offset at the output of the amplifier/filter.

#### MrAl

Joined Jun 17, 2014
10,621
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.

#### crutschow

Joined Mar 14, 2008
32,952
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.

Thread Starter

#### Dritech

Joined Sep 21, 2011
901
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?

#### Alec_t

Joined Sep 17, 2013
13,801
Did you use fc= 1/(2πRC) to determine the values for C2 and R2?
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.