# Non-Inverting Amplifier (AC Coupled) : How to solve this circuit with lower and high frequency?

#### Dragnovith

Joined Apr 16, 2021
16
I need to design this circuit with a gain of 26 for signals where the frequency varies from 800Hz to 8kHz and I also need to find out the current on the RL load. (VA = 100mV)
Will C1 behave like an open circuit in ac analysis?
I am getting to lower the frequency (800Hz) but I am not able to project to the frequency of 8kHz. Regarding the current I have a little doubt if it will be only 2.5 V divided by the RL.

#### Papabravo

Joined Feb 24, 2006
16,773
It looks like you hooked up the power backwards &
The components don't seem to have values
For AC analysis use AC amplitude 1 so the output will be properly scaled in dB

#### Jony130

Joined Feb 17, 2009
5,243
All the capacitors you have now in your circuit will only influence the overall lower cut-off frequency. Because all the capacitors you have now form high pass filters only.
Thus the higher cut-off frequency will be determined by the opamp gain-bandwidth product (GBP) and the mid-frequency gain Av = (1 + R2/R1).
Thus the 3dB higher cut-off frequency is equal to Fh = GBP/Av

#### crutschow

Joined Mar 14, 2008
27,697
Will C1 behave like an open circuit in ac analysis?
No.
Why would you think that?
It behaves as a capacitor, with its reactive impedance varying inversely with frequency.

#### Dragnovith

Joined Apr 16, 2021
16
All the capacitors you have now in your circuit will only influence the overall lower cut-off frequency. Because all the capacitors you have now form high pass filters only.
Thus the higher cut-off frequency will be determined by the opamp gain-bandwidth product (GBP) and the mid-frequency gain Av = (1 + R2/R1).
Thus the 3dB higher cut-off frequency is equal to Fh = GBP/Av
Okay, I was in doubt with the highest frequency. Thanks

#### Dragnovith

Joined Apr 16, 2021
16
No.
Why would you think that?
It behaves as a capacitor, with its reactive impedance varying inversely with frequency.
I was referring when you do the dc analysis.

#### crutschow

Joined Mar 14, 2008
27,697
I was referring when you do the dc analysis.
You said AC analysis.
For DC analysis the capacitor is, of course, considered an open circuit.

#### Dragnovith

Joined Apr 16, 2021
16
You said AC analysis.
For DC analysis the capacitor is, of course, considered an open circuit.
My mistake, sorry.

#### Dragnovith

Joined Apr 16, 2021
16
I'm still making a mistake, can you tell me which one? Using R1 = 1k, R = 5k and RL = 1k.

OpAmp Configuration:

#### crutschow

Joined Mar 14, 2008
27,697
You model has only a 208KHz gain-bandwidth so the -3dB rolloff is 208kHz divided by the circuit gain.