Hi,
I am in the process of setting up an amplifier circuit as illustrated below. Despite feeding a 200mV 25kHz sinusoidal signal, I am encountering signal distortion at the output. My suspicion lies with the variable capacitor at the input. However, due to constraints, I must work with these designated values. What strategies could I employ to achieve a clear amplified output signal?

 

However, due to constraints, I must work with these designated values.

Is this homework?

 

Is this homework?

No, I am building this circuit with SMT components.

 

Why on earth do you have resistors in series with the supplies? You say that are 221 Ω, but that is not a standard value. Is that the marking on them? If so, they are nominally 220 Ω. Not much difference in this case, but it's something you want to make sure you understand or else it will bite you big time at some point.

Why are you using resistors in the hundreds of kilohm and many megohm range? That's asking for noise issues.

 

No, I am building this circuit with SMT components.

How does that relate to whether or not it is homework?

 

Why on earth do you have resistors in series with the supplies? You say that are 221 Ω, but that is not a standard value. Is that the marking on them? If so, they are nominally 220 Ω. Not much difference in this case, but it's something you want to make sure you understand or else it will bite you big time at some point.

Why are you using resistors in the hundreds of kilohm and many megohm range? That's asking for noise issues.

221ohm resistors are available.
https://www.digikey.com/en/products/detail/vishay-dale/CRCW0402221RFKEDC/7925880.
This constitutes a small section of a larger circuit. I'm currently examining this specific part of the circuit.
My primary concern is the excessive reactance across the capacitor, which is preventing the signal from passing due to the presence of a 0-ohm resistor right after the capacitor.
As a result, the signal is being hindered from proper transmission.
Unfortunately, there is no signal detected at the second terminal of the capacitor.

 

My primary concern is the excessive reactance across the capacitor,

Yes, 5pF has over 1MΩ if reactance at 25kHz.
Why are you using such a small capacitor?

due to constraints, I must work with these designated values.

What constraints?
I'm familiar with the "Fix it but don't change it" school of design, but you won't get the circuit to work if you can't use appropriate component values.

 

Yes, 5pF has over 1MΩ if reactance at 25kHz.
Why are you using such a small capacitor?
What constraints?
I'm familiar with the "Fix it but don't change it" school of design, but you won't get the circuit to work if you can't use appropriate component values.

Hi, thank you for the answer! I appreciate your help.
Does using a braided wire to connect the second terminal of the variable capacitor instead of a regular wire lead to a reduction in reactance?

 

221ohm resistors are available.
https://www.digikey.com/en/products/detail/vishay-dale/CRCW0402221RFKEDC/7925880.
This constitutes a small section of a larger circuit. I'm currently examining this specific part of the circuit.
My primary concern is the excessive reactance across the capacitor, which is preventing the signal from passing due to the presence of a 0-ohm resistor right after the capacitor.
As a result, the signal is being hindered from proper transmission.
Unfortunately, there is no signal detected at the second terminal of the capacitor.

Why are you using 1% resistors in that spot? Why are you using resistors at all in that spot. That is going to completely trash your power supply rejection.

But be that as it may, the presence of a 0 Ω resistor isn't your problem -- it's just a wire. At 25 kHz the reactance of of a 5 pF capacitor is about -1.3 MΩ. With the 100 kΩ resistor in parallel with the 560 kΩ resistor, the attenuation is going to be over 90% and you can't expect to see more than about 13 mV of your 200 mV signal on that node.

 

Does using a braided wire to connect the second terminal of the variable capacitor instead of a regular wire lead to a reduction in reactance?

Not for the capacitive reactance, and negligibly for the inductive reactance.
You are beating a dead horse here.

 

Constraints that guarantee the circuit cannot work are unusual, to put it mildly.

What constraints apply to restrict you to such high resistance values?

I'm curious to know the thinking behind the 5pf cap that is shorted by the 0 ohm resistor.

 

Hi
gain is set to 10, so you expect 2V at the output. Supply is +/- 6V. Voltage drop on 221 Ohm resistors is about 1 V ( 5mA*221 Ohm), so only +/- 5V on the pins of opamp. As shown on the datasheet, the output voltage is about 3V below supply voltage. The circuit work on margin...
Replace 221 Ohm to 10-20 Ohm or elevate supply voltage.
Can you replace 100 kOhm at the input to 1 MOhm at least to minimize attenuation?

 

Remove the 100k resistor and increase your 5pF capacitor to something like 47pF