Look at the circuit, and see if you can answer it. Hint: it does not require ANY circuit analysis.

Bob

 

Look at the circuit, and see if you can answer it. Hint: it does not require ANY circuit analysis.

Bob

Yes, I have seen the circuit. The current passing through parallel resistor and capacitor and the voltage will measured across them. From this configuration, I understand that the parallel RC circuit filtering the current signal only. If I'm wrong please correct me.

From post #26: LvW posted the formula as Wo=1/RC. Then, I have posted the derivation on post #27. I don't have any reply either right or wrong, the derivation which I have posted.

Since first post, Im just asking two formulas to design the circuit to filter the input and feedback signal of differential amplifiers. I thing I have a formula to calculate the cut-of frequency of the feedback signal of the differential amplifiers which is posted on post # 35 (2nd figure). But, still I haven't get the answer or I couldn't find the formula to design the circuit to filter the input signal of the differential amplifier which is posted on the post #35 (1st figure).

Below, you can see where I want to add the capacitors (C1 and C2) to do the filtering. For that, I want to have formulas to calculate the cut-of frequency of the signal which is passing through on input stage and feedback stage.

In order to calculate the cut-off-frequency for the circuit (R3//C2), as I know the formula is , Correct me If I'm wrong?.

I don't know how to calculate the cut-of-frequency for the circuit (R4 // C1), so please, could you provide me the formula for this?.

I hope you understand. Thank you very much.

 

If R1=R2, R3=R4 and C1=C2, then the -3dB frequency is just
f=1/(2πR3.C2)
and the gain is
R3/R1

 

If R1=R2, R3=R4 and C1=C2, then the -3dB frequency is just
f=1/(2πR3.C2)
and the gain is
R3/R1

Yes, That is typo.. Thank you for the correction.

Then, what is the formula for R4//C1?. Since more than 20 posts Im asking this formula. Please could anyone provide me?.

 

Yes, That is typo.. Thank you for the correction.

Then, what is the formula for R4//C1?. Since more than 20 posts Im asking this formula. Please could anyone provide me?.

R4=R3 and C1=C2
The inverting and non-inverting signal paths must have the same impedances. That’s the whole point of a differential circuit.

 

Yes, That is typo.. Thank you for the correction.

Then, what is the formula for R4//C1?. Since more than 20 posts Im asking this formula. Please could anyone provide me?.

Perhaps you did not get a satisfying answer to this question up to now because nobody can understand this question?
A formula for R4||C1 ? Do you want the expression for the resulting impedance - or what?

 

Perhaps you did not get a satisfying answer to this question up to now because nobody can understand this question?
A formula for R4||C1 ? Do you want the expression for the resulting impedance - or what?

I want to add the C1 capacitor, parallel to the resistor R4 to filter the input signal of the amplifier.

 

I want to add the C1 capacitor, parallel to the resistor R4 to filter the input signal of the amplifier.

the -3dB frequency is just f=1/(2πR3.C2)
and the gain is R3/R1
R1=R2, R4=R3 and C1=C2
The inverting and non-inverting signal paths must have the same impedances. That’s the whole point of a differential circuit.

 

I want to add the C1 capacitor, parallel to the resistor R4 to filter the input signal of the amplifier.

hi P,
That is an ambiguous question, you MUST try harder to explain in detail what you are querying.

E

 

I want to add the C1 capacitor, parallel to the resistor R4 to filter the input signal of the amplifier.

As far as I can see, you have two input signals. Hence, the circuit has two transfer functions.
What do you want? The output signal as a function of the two inputs or both transfer functions?
Please, spend some time and effort to ask an understandable question.

 

the -3dB frequency is just f=1/(2πR3.C2)
and the gain is R3/R1
R1=R2, R4=R3 and C1=C2
The inverting and non-inverting signal paths must have the same impedances. That’s the whole point of a differential circuit.

As you said that in the above post, R1=R2, R4=R3 and C1=C2, and also the inverting and non-inverting signal paths must have the same impedance. Thus, the C1 and C2 should have the same value of the capacitor to keep/match the same impedance on the both inputs (non-inverting and inverting) of the amplifier. I think I understand now, Am I correct?.

 

As you said that in the above post, R1=R2, R4=R3 and C1=C2, and also the inverting and non-inverting signal paths must have the same impedance. Thus, the C1 and C2 should have the same value of the capacitor to keep/match the same impedance on the both inputs (non-inverting and inverting) of the amplifier. I think I understand now, Am I correct?.

Yes.
I am assuming that V1 and V2 are the inverting and non-inverting parts of a differential signal, not two separate signals.

 

Yes.
I am assuming that V1 and V2 are the inverting and non-inverting parts of a differential signal, not two separate signals.

Yes, the V1 and V2 are the inverting and non-inverting signals.
But, I have little confusion here, when you mention that not separate signals, what you mean by separate signals?.

I think the separate signals only we are applying to the input terminals of the differential amplifiers. We are not applying the same signals to the differential amplifiers, Right?.

 

But, I have little confusion here, when you mention that not separate signals, what you mean by separate signals?.

To take an audio example:
V1 could be from a CD player, and V2 from a radio. That’s what I meant by separate.
Differential signals, although there are two signals, they are both from the same source, such as a balanced line microphone.

 

To take an audio example:
V1 could be from a CD player, and V2 from a radio. That’s what I meant by separate.
Differential signals, although there are two signals, they are both from the same source, such as a balanced line microphone.

Ohh.. Ok ok. I understand now and thank you for the explanation.

Yes, Both V1 and V2 are from the same source.