Hi! i want to calculate the Rinput and Routput of this Active Filter.
How can i do that?
The input and output impedances will of course be frequency dependent. The easiest way to calculate them is to simulate the filter using LTSpice, which is free and easy to use.
Hi and many thanks for your reply! I will give more information about my question on the schematic.jremington:
The input and output impedances will of course be frequency dependent. The easiest way to calculate them is to simulate the filter using LTSpice, which is free and easy to use.
So the frequency bandwidth is DC to 17khz (audio)! Before this filter there is a double balanced ring diode mixer that downconvert a signal of 30MHz to a baseband DC to 17khz. The mixer’s output (ideally) is 50ohm.
RF front…—Mixer-----Active Filter–Demodulator (DSP). I want to calculate the total gain of the stages of receiver but in the filter i don’t know the Zinput and Zoutput to convert the voltage gain=20log(Vout/Vin) to power gain. I need to know what is the Zout and Zin. After the Active filter there is a DSP kit i think it’s Zinput is about 10k maybe not sure…
The input impedance of the unity-gain Sallen-Key filter at band center is high, probably a bit less than 50K ohms (which is the impedance of the parallel combo of 100K resistors). How much less depends on the bootstrapping, which is not easy to guess. The output impedance of the posted circuit is very low, probably less than 1 ohm, but nonlinear depending on the load because the output current could be limited by the output circuitry of the LM358 op amp.
Neither input nor output impedance is a good match for 50 ohms.
jremington:
The input impedance of the unity-gain Sallen-Key filter at band center is high, probably a bit less than 50K ohms (which is the impedance of the parallel combo of 100K resistors). How much less depends on the bootstrapping, which is not easy to guess. The output impedance of the posted circuit is very low, probably less than 1 ohm, but nonlinear depending on the load because the output current could be limited by the output circuitry of the LM358 op amp.Neither input nor output impedance is a good match for 50 ohms.
thanks for the reply ! “at the band center”. what do you mean?
the Sallen-key is a unity gain amp Voltage Gain=1. About the Amplifier Stage there is a jumper of 2 option. First option’s Gain is =R11/R13=10K/3,3K=3 and Second OPTion’s gain is VARIABLE ( i have a potensiometer). So, let’s say that is Gain=3. Reject the potensiometer. Maybe i can use KCL to find my Zin?
what can i do with the capasitors. they have Zc=1/jωC …How can i add this with the Resistors? I am a begginer sorry about that
