What do I need to consider (or learn) in order to design a low pass filter to follow a BJT buffer?

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talikarng

Joined May 27, 2017
20
I've learned how to bias a BJT, but designing a low pass filter to connect to the buffer is beyond "Dick Smith's Fun Way into Electronics".
The circuit will run at 170MHz and I plan to use it to provide the sine wave that will be passed through a low pass filter (with a variable capacitor) and then I will read the output from a diode detector to get a rough idea of the value of the variable capacitor.

Screenshot from 2018-07-05 20-37-03.png

I'm at the "unknown unknowns" stage and not sure what else I need to know to design the low pass filter. I won't think about the diode detector yet... one thing at a time. (WHAT I THINK I KNOW): The low pass filter will be an LC filter or an RC filter (with perhaps only a 10ohm resistor) because capacitors (unless only a few pF) have very low reactance at 170MHz.

As far as low pass filters, I have only seen three component Butterworth and Chebyshev filters which I think are designed to match the impedance of... some other part of the circuit?

Can I just "connect" a 10 ohm resistor and a variable capacitor to C3 (removing R7)? Do I have to think about the output impedance of the buffer circuit? If so, how do you do that?

If there is something else I am missing, please let me know (I'm a mental health nurse doing this for a hobby, not even a electrical engineering student)
 

Papabravo

Joined Feb 24, 2006
12,704
I've learned how to bias a BJT, but designing a low pass filter to connect to the buffer is beyond "Dick Smith's Fun Way into Electronics".
The circuit will run at 170MHz and I plan to use it to provide the sine wave that will be passed through a low pass filter (with a variable capacitor) and then I will read the output from a diode detector to get a rough idea of the value of the variable capacitor.

View attachment 155646

I'm at the "unknown unknowns" stage and not sure what else I need to know to design the low pass filter. I won't think about the diode detector yet... one thing at a time. (WHAT I THINK I KNOW): The low pass filter will be an LC filter or an RC filter (with perhaps only a 10ohm resistor) because capacitors (unless only a few pF) have very low reactance at 170MHz.

As far as low pass filters, I have only seen three component Butterworth and Chebyshev filters which I think are designed to match the impedance of... some other part of the circuit?

Can I just "connect" a 10 ohm resistor and a variable capacitor to C3 (removing R7)? Do I have to think about the output impedance of the buffer circuit? If so, how do you do that?

If there is something else I am missing, please let me know (I'm a mental health nurse doing this for a hobby, not even a electrical engineering student)
You have seen the can of worms and appreciate the problems. Here it is in a nutshell. Each of the components interacts with each of the other components. Then the components of the filter interact with the components in the buffer, specifically with the the transistor at the frequency of interest. The filter will also interact with the components of the source. AC circuits and analysis of complex variables is generally a junior level courses for most electrical engineers. That said, I have tremendous respect for the things that you learn in a nursing curriculum; my grandaughter is in one right now. It won't happen overnight, but you can get there with a divide an conquer approach.

I would start with the algebra of complex numbers. They are useful in dealing with problems that are dependent on the frequency of the signals being processed. You check out this site for the fundamentals of AC circuits including what happens when there are imepdance mismatches between a signal source and a load. This might not be quick but you can get there. Come back and ask questions.
 
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