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Fovakis
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Hi! i want to calculate the Rinput and Routput of this Active Filter.
How can i do that?
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How can i do that?
berkeman said:I believe you mean Zin and Zout, right?
Do you want to calculate those impedances, or measure them with your simulator? What frequency range do you want this circuit to operate over?
Fovakis said:yeah exactly this ! :)
Hi and many thanks for your reply! I will give more information about my question on the schematic.
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...
AlephZero said:A practical way to find Zin (which will be frequency-dependent, in general) is:
1. Disconnect the filter input from the rest of the circuit
2. Short circuit the filter output
3. Connect an ideal voltage source (i.e. a sine wave) ito the input and measure the input current.
If the output of the filter is the output of an op-amp (U1B in your diagram?) then for practical purposes the output impedance is the same as the op-amp output impedance, unless the filter's feedback loop has a very low impedance for some strange reason. Check the op-amp datasheet if you really need the value, but unless the filter is driving a low impedance input (which is probably a bad design idea anyway) the exact value isn't important.
EDIT: or do what Berkeman said - the two posts had exactly the same time stamp before I edited this one!
Fovakis said:Hi AlephZero ! thanks! i am a begginer so please more simple the explanations if you can :(
First of all i understand that Zout is Zoutput of Operation Amplifier + pluse Zof capacitor=1/jωC where C=10^(-6)Farad. (basically i have a dual fet OPA2608 not the LM358 that is at the schematic).
What do you mean the filter input? what components consisting that?
berkeman said:The input to the filter is the SMA connector on the left...
NascentOxygen said:@Fovakis. The input impedance is not a fixed value, it varies with frequency following a complicated equation. What do you need to know about it? What is the important consideration? It would be much easier to ask your simulator to "measure" the input impedance at various frequencies to allow you to plot a graph of impedance over the range of frequencies of interest.
NascentOxygen said:BTW, you won't get any DC through this filter with those 3uF caps in the way. Are you wanting its passband to extend right down to DC (i.e., are you wanting a low-pass filter)?
I presumed the KiCad that you drew the schematic on?Fovakis said:yes you right about that. what simulator can i use for that?
I had concluded that it was beyond your ability, at this stage.Also, if i want to calculate the Zin for f=20khz. it would be much better to do the maths i guess.
Zin will be a different complex number for every frequency. The equation is very complicated.I am starting, first a Capacitor with Zc=1/j2πfC1..8/j Farads. Is it right to do with capacitors? then i must add this with the voltage diviver ? i will have a complex number for Zin right?
So what modifications are you going to make to it, to turn it into a filter that passes DC? It has just ratcheted up a notch in complexity, because the circuit is shown operating off a single power supply. It won't do well preserving the DC component of the input.Fovakis said:Yes it is a low pass filter the first stage. the second is an amp.
NascentOxygen said:So what modifications are you going to make to it, to turn it into a filter that passes DC? It has just ratcheted up a notch in complexity, because the circuit is shown operating off a single power supply. It won't do well preserving the DC component of the input.
NascentOxygen said:@berkeman. I see no virtual Earth's in this schematic.
An active filter is an electronic circuit that uses active components, such as transistors and op-amps, to filter out unwanted frequencies from a signal.
Rinput and Routput refer to the input and output resistors, respectively, of an active filter. These resistors are used to control the flow of current and voltage in the circuit.
Calculating Rinput and Routput allows us to determine the gain of the active filter and ensure that it is functioning as desired. It also helps us to select appropriate values for these resistors to achieve the desired filter characteristics.
Rinput and Routput can be calculated using Ohm's law, which states that resistance is equal to voltage divided by current. In an active filter, these values can be determined by measuring the voltage and current at the input and output of the circuit.
The values of Rinput and Routput can be affected by the type of active components used, such as the type of transistor or op-amp, as well as the design and layout of the circuit. Temperature and other external factors can also impact these values.