What is the gain of the low pass filter amplifier?

xqh123

What is the gain of the low pass filter amplifier? [Copy link]

 

If R2 is removed from the figure, is it still a second-order filtering amplification? How do we calculate the amplification factor?

maychang

"If R2 is removed from the figure, will it still be a second-order filter amplifier?" "Remove" has two meanings: one is that the resistor is open-circuited, and the other is that the resistor is short-circuited.

xqh123

maychang posted on 2018-7-18 10:55 "If R2 is removed from the figure, will it still be a second-order filter amplification?" "Remove" has two meanings: one is that the resistor is open-circuited, and the other is that the resistor is short-circuited.

If the resistor is short-circuited, or it is a second-order filter, how do you calculate the amplification factor?

chunyang

xqh123 posted on 2018-7-18 14:05 How to calculate the amplification factor when the resistor is short-circuited or there is a secondary filter?

The DC gain has nothing to do with R2.

maychang

xqh123 posted on 2018-7-18 14:05 How to calculate the gain when the resistor is short-circuited or the filter is secondary?

You have already given the calculation formula in the first post The gain is a function of frequency. To calculate the gain when R2=0, just substitute R2=0 in the above formulas. The DC gain is the gain when the frequency tends to zero, which is the numerator in the frequency response calculation formula, which is Rf/R1 and has nothing to do with R2.

xiaxingxing

maychang posted on 2018-7-18 10:55 "If R2 is removed from the figure, will it still be a second-order filter amplifier?" "Remove" has two meanings: one is that the resistor is open circuit, and the other is that the resistor is short circuit.

Hello, what is the difference between connecting the right end of C2 to the ground and connecting it to the output terminal? Please guide, thank you!

maychang

xiaxingxing posted on 2018-7-19 17:13 Hello, what is the difference between the right end of C2 being grounded and connected to the output? Please guide, thank you!

"What is the difference between the right end of C2 being grounded and connected to the output?" The difference is that when the right end of C2 is grounded, the potential of the right end is fixed to zero. When the right end is connected to the output of the op amp, the potential of the right end of C2 moves up and down with the output potential of the op amp. In the first post, the left end of C2 is connected to the inverting input of the op amp, so the output of the op amp must move in the opposite direction of the inverting input of the op amp. When the potential of the left end of C2 decreases (after the input Vi is filtered by R1C1), the potential of the output end of the op amp, that is, the right end of C2, increases. When the potential of the left end of C2 increases, the potential of the right end of C2 decreases. In this case, the current flowing through C2 is greater than the current flowing through C2 when the right end of C2 is grounded (because the potential of the right end of C2 changes in reverse phase). From the outside of C2, it seems that the capacity of C2 (compared with the right end of C2 being grounded) has increased.

maychang

xiaxingxing posted on 2018-7-19 17:13 Hello, what is the difference between the right end of C2 being grounded and connected to the output end? Please guide, thank you!

If op amp A is replaced by a vacuum tube, bipolar transistor or MOS tube, and C2 is the inter-electrode capacitance (distributed capacitance) of the vacuum tube, bipolar transistor or MOS tube, then C2 seems to have a larger capacity from the external circuit. This effect is called the Miller effect. Now the vacuum tube, bipolar transistor or MOS tube is replaced by an op amp, and the original inter-electrode capacitance is replaced by C2. From the external circuit, it still seems that the capacity of C2 is larger than that of the right end of C2 being grounded (possibly many times larger). This phenomenon is still a manifestation of the Miller effect, but it is not usually called that.

xiaxingxing

maychang posted on 2018-7-19 17:44 "What is the difference between the right end of C2 being grounded and connected to the output terminal?" The difference is that when the right end of C2 is grounded, the potential of the right end is fixed to zero, and when the right end is connected to the op amp, the output...

Following your logic, when the right end of C2 is connected to the output terminal, its corresponding cutoff frequency (compared with when the right end of C2 is grounded) is smaller. Is this the right understanding? ? ? In addition, is it more advantageous to connect the right end of C2 to the output terminal? Or is it more advantageous to ground the right end of C2? Please guide, thank you!

maychang

xiaxingxing posted on 2018-7-22 21:51 Following your logic, when the right end of C2 is connected to the output terminal, the corresponding cutoff frequency (compared with when the right end of C2 is grounded) is smaller. Is this how you understand it? ...

Following your logic, when the right end of C2 is connected to the output terminal, the corresponding cutoff frequency (compared with when the right end of C2 is grounded) is smaller. Is this how you understand it? ? ? Not only is the corresponding cutoff frequency lower when the right end of C2 is connected to the output terminal of the op amp, but the shape of the amplitude-frequency curve is also slightly different when the right end of C2 is connected to the output terminal of the op amp compared to when it is grounded.

maychang

xiaxingxing posted on 2018-7-22 21:51 Following your logic, when the right end of C2 is connected to the output end, the corresponding cutoff frequency (compared with when the right end of C2 is grounded) is smaller. Is this the right understanding? ...

In addition, is it more advantageous to connect the right end of C2 to the output end? Or is it more advantageous to ground the right end of C2? This is hard to say. Whether there is an "advantage" depends on your needs.