# Need help AC coupling push pull amplifier with op amp

#### brianmullins222

Joined Nov 12, 2020
3
I have an AC input of 0-5V that i need to go from -2.5 to 2.5V. this will then go through an op amp and then trough a push pull amplifier configuration. I just cant seem to find a value for the capacitor that will work, so if someone could explain where im going wrong that would be greatly appreciated. The input frequency is 1Hz.

#### Martin_R

Joined Aug 28, 2019
90
Hi, you need a resistor from the + I put of the opamp to find to bias the opamp input.

#### Martin_R

Joined Aug 28, 2019
90
+ I put should read + input of the opamp. Try 10k for starters.

#### ericgibbs

Joined Jan 29, 2010
13,197
hi 222,
A 1n0 input cap for 1Hz is much too low a value.
Try a 1uF.
E

#### ericgibbs

Joined Jan 29, 2010
13,197
hi brian,
Is this a College/Homework assignment project.?
E

#### brianmullins222

Joined Nov 12, 2020
3
yes i just couldnt find any formulas for the problem. i think i got it working fairly well at 10u and 100k

#### Marc Sugrue

Joined Jan 19, 2018
216
I have an AC input of 0-5V that i need to go from -2.5 to 2.5V. this will then go through an op amp and then trough a push pull amplifier configuration. I just cant seem to find a value for the capacitor that will work, so if someone could explain where im going wrong that would be greatly appreciated. The input frequency is 1Hz.
View attachment 224842
C1 Needs to be a Lot Bigger, at 1Hz
yes i just couldnt find any formulas for the problem. i think i got it working fairly well at 10u and 100k
The capacitive reactance of the capactor in Ohms (Xc) is 1/(2*pi*F*C). Any capacitor will do DC blocking but to allow 1Hz to pass a 10uF would approximate a 16k Ohms impedance.

#### crutschow

Joined Mar 14, 2008
27,392
to allow 1Hz to pass a 10uF would approximate a 16k Ohms impedance.
The signal will be reduced by 3dB (.707) at 1Hz with those values.
A value of 10μF and 50k will reduce the amplitude about 0.4dB (.95) at 1Hz.

#### MrChips

Joined Oct 2, 2009
23,849
You still need DC bias on the input of the op-amp.
The input resistance of TL072 is 10^12Ω.

#### Martin_R

Joined Aug 28, 2019
90
You still need DC bias on the input of the op-amp.
The input resistance of TL072 is 10^12Ω.

#### Audioguru again

Joined Oct 21, 2019
3,296
1Hz is almost DC. The signal generator is grounded so if it has no output coupling capacitor then its DC output is 0V and the input capacitor feeding the opamp is not needed.

#### Marc Sugrue

Joined Jan 19, 2018
216
The signal will be reduced by 3dB (.707) at 1Hz with those values.
A value of 10μF and 50k will reduce the amplitude about 0.4dB (.95) at 1Hz.
The values were just based on his values stated as acceptable in post #6, i missed the 100k resistor